WBBSE Solutions For Class 10 Physical Science And Environment Chapter 5 Light

WBBSE Class 10 Physical Science Question Answer In English

Chapter 5 Light MCQs

Question 1. The formation of the spectrum is due to:

1. Reflection
2. Refraction
3. Dispersion of light

Answer: 3. Dispersion of light.

Question 2. A person uses the spectacle of power + 2D. He is suffering from:

1. Presbyopia
2. Astigmatism
3. Long-sightedness of hypermetropia
4. Short-sightedness of myopia

Answer: 3. Long-sightedness of hypermetropia.

Question 3. A person cannot see objects clearly beyond 2m. The power of the lens required to correct his vision will be :

1. + 2D
2. -1D
3. + ID
4. 0.5D

Answer: 4. 0.5D.

Question 4. Myopia is due to:

1. Older age
2. Shorting of eyeball
3. Irregular change in focal length
4. Elongation of the eyeball.

Answer: 4. elongation of the eyeball.

Read And Learn More: WBBSE Solutions For Class 10 Physical Science And Environment

Question 5. Rainbow is formed due to a combination of 

1. Refraction and absorption
2. Refraction and scattering
3. Dispersion and total internal reflection
4. Dispersion and focussing

Answer: 3. Dispersion and total internal reflection.

Question 6. Which of the prism is used to see the infrared spectrum of light?

1. Rock salt
2. Flint
3. Nicol
4. Crown

Answer: 1. Rock salt.

WBBSE Class 10 Physical Science Question Answer In English

Question 7. Which of the following colours suffers maximum deviation in a prism?

1. Yellow
2. Green
3. Blue
4. Orange

Answer: 2. Blue.

Question 8. The splitting of white light into several colours on passing through a glass prism is due to:

1. Reflection
2. Diffraction
3. Interference
4. Refraction

Answer: 4. Refraction.

Question 9. A convex lens will become less convergent in

1. Oil
2. Water
3. Both
4. None

Answer: 2. water.

Question 10. The source of ultraviolet light is :

1. Electric bulb
2. Carbon arc lamp
3. Red hot iron bulb
4. Sodium vapour lamp

Answer: 2. carbon arc lamp.

Question 11. Which of the following materials cannot be used to make a lens?

1. Water
2. Plastic
3. Clay
4. Glass

Answer: 2. Clay.

WBBSE Class 10 Physical Science Question Answer In English

Question 12. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be :

1. Plane only
2. Concave only
3. Convex only
4. Either plane or convex

Answer: 4. Either plane or convex.

Question 13. A ray of light suffers refraction through an equilateral prism. The deviation produced by the prism does not depend on the:

1. Angle of incidence
2. Size of the prism
3. colour of light
4. material of the prism

Answer: 3. size of the prism.

Question 14. A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from:

1. its second focus
2. its optical centre
3. its first focus
4. the centre of curvature of its second surface

Answer: 1. its second focus

Question 15. What is the value of W₁/f₁+ W₂/f₂

1. 2
2. 3
3. 5
4. 0

Answer: 4. 0

Question 16. The terminal colours of the pure spectrum of white light are :

1. Blue and violet
2. Red and orange
3. Red and violet

Answer: 3. Red and violet.

Question 17. Yellow colour is called :

1. Primary
2. Middle colour
3. Secondary colour
4. Top colour

Answer: 2. Secondary colour

WBBSE Class 10 Physical Science Question Answer In English

Question 18. The___________ colours can be seen distinctly in pure spectrum.

1. Six
2. Eight
3. Seven
4. Nine

Answer: 3. Seven.

Question 19. The dispersion of white light was first observed by:

1. Sir Issac Newton
2. Boyle
3. Max Well

Answer: 1. Sir Issac Newton.

Question 20. The arrangement of the seven colours of spectrum is:

1. VIBGYOR
2. RAINBOW
3. VBIGOYR

Answer: 1. VIBGYOR.

Question 21. The object is placed in between ‘f’ and ‘2f’ in convex lens, the size of image will be

1. Greater in size the object
2. Equal in size of the object
3. Greater in size the object.
4. Smaller in size than the object

Answer: 1. Greater in size the object

Question 22. To for m virtual image on the same side of the object in case of a convex lens, the object should be placed :

1. In between the lens and the focus
2. In between ‘f’ and ‘2f’
3. At ‘2f’ from the lens
4. At a distance greater than ‘2f’ from the lens

Answer: 1. In between the lens and the focus.

Question 23. Linear magnification is equal to :

1. (Length of image)/(Length of the object).
2. (Length of image) × (Length of the object).
3. (Length of image) + (Length of the object).
4. (Length of image) – (Length of the object).

Answer: 2. (Length of image)×(Length of the object)

WBBSE Class 10 Physical Science Question Answer In English

Question 24. A mirror produces a magnified erect image of an object. The nature of the mirror is

1. Convex
2. Concave
3. None of these
4. Plane

Answer: 2. Concave.

Question 25. A plane mirror produces a magnification of:

1. -1
2. + 1
3. Zero
4. Between 0 and + a

Answer: 2. + 1.

Question 26. Focal length of a convex lens will be maximum for:

1. Blue light
2. Red light
3. Greenlight
4. Yellow light

Answer: 3. Red light.

Question 27. The number of images observable between two parallel plane mirror is:

1. 2
2. 4
3. 11
4. Infinite

Answer: 4. Infinite.

Class 10 Physical Science WBBSE

Question 28. When a mirror is rotated through an angle 0, the reflected ray from it turns through an angle of :

1. θ
2. θ/2
3. 0
4. 2θ

Answer: 4. 2θ

Question 29. A ray is incident at an angle 38° with a mirror. The angle between normal and deflected ray is :

1. 90°
2. 75
3. 38°
4. 52°

Answer: 4 52°

Question 30. The focal length f of a speherical mirror of radius of curvature R is :

1. \(\frac{R}{2}\)
2. R
3. \(\frac{3}{2}\) R
4. 2R

Answer: \(\frac{R}{2}\)

Question 31. A concave mirror of focal length f (in air) is immersed in water (μ = 3). The focal length of the mirror in water will be :

1. \(\frac{4}{3}\) f
2. \(\frac{9}{5}\) f
3. \(\frac{3}{5}\) f
4.  f

Answer: 4. f

Class 10 Physical Science WBBSE

Question 32. A plane mirror is approaching a person at a speed of 5 cm S-1. At what speed will his image approach him?

1. 10 cms^-1
2. 20 cm S^-1
3. 5cms^-1
4. 15cm S^-1

Answer: 1. 10 cm S^-1

Question 33. If an object is 30 cm away from a concave mirror of focal length 15 cm, the image will be :

1. Erect
2. Reflection of light
3. Diminished
4. Of same size

Answer: 4. Of the same size

Question 34. Mirage is a phenomenon due to:

1. Reflection of light
2. Refraction of light
3. Diffraction of light
4. Total internal reflection of light

Answer: 4. Total internal reflection of light.

Question 35. The principle behind optical fibres 

1. Total internal reflection
2. Total external reflection
3. Diffraction
4. Both (1) and (2)

Answer: 1. Total internal deflection

Question 36. A convex lens is dipped in a liquid whose refractive index is euqal to the refractive index of the lens. Then its local length will:

1. Become zero
2. Become infinite
3. Increase
4. Reduce

Answer: 2. Become infinite.

Class 10 Physical Science WBBSE

Question 37. What is the value of the refractive index of a diamond?

1. 2.47
2. 3.47
3. 1.47
4. 0.47

Answer: 1/. 2.47.

Question 38. Write an example of an isotropic medium.

1. Air
2. Water
3. Iron
4. Fire

Answer: 1. Air.

Question 39. Write the value of speed of light.

1. 3 x 10⁶ m/s
2. 3 x 10⁸ m/s
3. 3 x 10⁷ m/s
4. 3 x 10⁹ m/s

Answer: 3. 3 x 10⁸ m/s.

Question 40. What is the value of refractive index of alcohol?

1. 1.37
2. 2.37
3. 4.37
4. 0.37

Answer: 1. 1.37.

WB Class 10 Physical Science Question Answer Chapter 5 Light Very Short Answer Type Questions

Question 1. What is the far point of a normal eye?
Answer: Infinity.

Question 2. Which colour of light travels faster in vacuum?
Answer: Light rays of all colour travel with same speed in vacuum.

Question 3. What is the least distance of distinct vision?
Answer: 25 cm.

Question 4. How is deviation produced in a prism related to the angle of the prism and angles of incident and emergency?
Answer: δ= i₁+i₂ – A

Question 5. Is dispersion of white light in a vacuum possible?
Answer: No.

Question 6. Define the power of a lens.
Answer: The reciprocal of the focal length of the lens is called its power.

Question 7. What will be the focal length and power of a plane glass plate?
Answer:  f = α; Power = 0

Question 8. Why is a night-angled prism a better reflector than a plane mirror?
Answer: As in the prism no part of the light refracted.

Question 9. Does a beam of light give a spectrum on passing through a hollow prism containing air, explain?
Answer: No, because all colours of light pass through vacuum or air same speed and there is no dispersion.

Question 10. What kind of lens one should use to correct myopic eye?
Answer: Concave lens.

Question 11. What kind of lens one should use to correct the astigmatic eye?
Answer: Cylindrical lens.

Question 12. What is monochromatic light?
Answer: The light which consists of only one colour is known as monochromatic light.

Question 13. What is a double convex lens?
Answer: If both the surfaces of a convex lens are convex then the lens is called a double convex lens. It is used in camera, telescopes etc.

Question 14. Will the focal length of a convex lens vary for refraction of light of different colours through it?
Answer: For different colours of light the convex lens will have different focal length.

Question 15. What is the edge of a prism?
Answer: The intersection of the refracting surfaces is called the edge of the prism.

Class 10 Physical Science Solution WBBSE

Question 16. An object is placed at the focus of a concave lens, where will be its image formed?
Answer:

u = f; f = –  ve

So, \(\frac{1}{v}\)– \(\frac{1}{u}\) = \(\frac{1}{f}\) Or,

\(\frac{1}{v}\)=  \(\frac{1}{f}\)+ \(\frac{1}{u}\) Or,

\(\frac{1}{v}\)=  \(\frac{1}{f}\)+ \(\frac{1}{f}\)

= 0

∴ v= α

Question 17. Name the type of mirror which has focal length equal to infinity.
Answer: A plane mirror.

Question 18. What are the types of reflection of light?
Answer:

The types of reflection of light are:

1. Regular reflection
2. Irregular reflection

Question 19. Write the expression of Newton’s Equation.
Answer: The expression of Newton’s Equation xy = f².

Question 20. What is the mathematical expression of linear magnification?
Answer: The mathematical expression of linear magnification is m=\(\frac{v}{u}\)

Question 21. Write two classifications of image.
Answer: The two classifications of image are real and virtual.

Question 22. Write an example of opaque substance.
Answer: An example of opaque substance is iron.

Question 23. What will be the nature of the lens for u-v graph.
Answer: The nature of the graph is rectangular hyperbola.

Class 10 Physical Science Solution WBBSE

Question 24. Write the expression of dispersive power of lens materil.
Answer:

W = \(\frac{δμ}{μ- 1}\)

Question 25. Write the mathematical expression of generalised snell’s law.
Answer: The mathematical expression of generalised snell’s law :

⇒ μ₂= sin i1

= μ sin i 2

Question 26. Write a characteristic of the image formed by a plane mirror.
Answer: The image is erect but laterally inverted.

Question 27. What is the value of refractive index of Benzene?
Answer: The value of refractive index of Benzene is 1.501.

Question 28. What are the two types of lens?
Answer: The two types of lens are convex and concave.

Question 29. Write the mathematical expression of lens maker’s formula.
Answer:

\(\frac{1}{f}\)= (μ – 1) = (1/r₁ – 1/r₂)

Question 30. Write the mathematical expression of the refraction index.
Answer:

⇒ \(\frac{sin r}{sin i}\) =  2μ₁

WBBSE Solutions Guide Class 10 Chapter 5 Light Fill In The Blanks

Question 1. The light which consists of only one colour is called_____ light.
Answer: Monochromatic.

Question 2. Yellow colour is a _______colour.
Answer: Secondary.

Question 3. A transparent body absorbs all the colours of white light except the light of colour of its_________
Answer: Own.

Question 4. The elementary colours of white light are arranged in order of _______
Answer: Wavelength.

Question 5. _______First observed the dispersion of white light.
Answer: Sir Issac Newton.

Question 6. Lenses are generally of two types, concave and _______
Answer: Convex.

Question 7. The _________colours can be seen distinctly in pure spectrum.
Answer: Seven.

Question 8. A convex lens is called a _______lens.
Answer: Converging.

Question 9. The middle colour is _______
Answer: Yellow.

Question 10. Red flower appears _______green light.
Answer: Black.

Question 11. Yellow and blue colours are _________colours colours of each other. 
Answer: Complementary.

Question 12. A beam of rays of light parallel to the principal axis of a convex lens, will passe through the______focus after refraction.
Answer: Principal.

Question 13. Magenta colour is made by mixing red and ______colour.
Answer: Blue.

Question 14. During dispersion of light, light is splitting up into________ colours
Answer: Seven.

Question 15. A ________ lens is used as a magnifying glass.
Answer: Convex.

Question 16. The arrangement of seven colours of the spectrum is ________
Answer: VIBGYOR.

Question 17. A plane mirror produces a magnification of ________
Answer: +1.

Question 18. The focal length f of a spherical mirror of radius of curvature R is.
Answer:

⇒ \(\frac{R}{2}\)

Question 19. Mirage is a phenomenon due to ________.
Answer: Total internal reflection of light

Question 20. The band of different colours obtained due to dispersion of white light is called the________.
Answer: Spectrum

Question 21. The lens which is thinner at the centre and wider at the two edges is known as a ________ lens.
Answer: Convex

WB Class 10 Physical Science Question Answer

Question 22. If a ray be incident on a convex lens parallel to its principal axis, then the refracted ray through the lens passes through the ________  of the lens.
Answer: Focus

Question 23. The straight line that joins the centres of the spherical surfaces forming the lens is called the  _______ axis of the lens.
Answer: Principal

Question 24. The distance of the principal focus from the optical centre of a lens is called its principal _______ length.
Answer: Focal

Question 25. An object place perpendicular to the principal axis of a convex lens will have its image _______ to the principal axis.
Answer: Perpendicular

Question 26. The focal length of a convex lens will be maximum for _______
Answer: Red light

Question 27. The number of images observable between two parallel plane mirror is_______
Answer: Infinite

Question 28. The formation of the spectrum is due to _______
Answer: Dispersion

Question 29. The terminal colours of pure spectrum of white light are
Answer: Red and violet

Question 30. _________of the prism is used to see infrared spectrum of light.
Answer: Rocksalt.

WB Class 10 Physical Science Question Answer Chapter 5 Light Short Answer Type Questions

Question 1. What is a centre of curvature of a lens?
Answer:

Centre of curvature: The spherical surface of a lens is a part of sphere. The centre of this sphere is known as centre of curvature of the surface of lens.

Question 2. What is Focal plane?
Answer:

Focal plane:  The plane passing through the principal focus of a lens and perpendicular to the principal axis of the lens is known as the focal plane of the lens.

Question 3. What is Radius of curvature?
Answer:

The radius of curvature: The radius of curvature of a lens is the radius of the glass sphere from which the surfaces of the lens are cut.

Question 4. What is Focal length?
Answer:

Focal length: The distance between the optical centre and the focus is known as focal length.

Question 5. What do you mean by the principal axis of a convex lens? 
Answer:

Principal axis of a convex lens joining the centres of curvature of the two spherical surfaces of a convex lens is called its principal axis.

Question 6. What is optical centre?
Answer:

Optical centre: If a ray of light strikes the surface of a lens in such a way that the emergent ray from the other surface is parallel to it then the corresponding refracted ray passes through a definite point on the principal axis. This point (A) is the optical centre of the lens.

Question 7. What does the term ‘thin’ signify when it is related to a convex lens? Do all rays of light suffer deviation while crossing through a thin convex lens?
Answer:

A thin convex lens is one, the thickness at the middle of which is extremely small compared to the radii of curvature of its surfaces. Rays directed towards the optical centre do not suffer deviation while crossing through a convex lens.

WB Class 10 Physical Science Question Answer

Question 8. What is a spectrum?
Answer:

Spectrum: The band of different colours obtained due to dispersion of white light is known as a spectrum.

Question 9. A concave mirror of a small aperture forms a sharper image why?
Answer: It is because such a mirror is free from the defect due to spherical aberration.

Question 10. What are luminous and non-luminous object?
Answer:

Bodies which emit light of themselves are luminous sources. Bodies which cannot emit light of themselves are non-luminous sources.

Question 11. State the principle of reversibility of light path.
Answer:

If a ray starting from a point reaches a second point after suffering any number of reflections and refractions and then be reversed in direction, it will retrace its path and reach the first point provided that all other conditions under which reflections and refractions took place remain unaltered.

Question 12. Define real image.
Answer:  If reflected or refracted rays actually converge to a point, the point is called a real image.

Question 13. Define virtual image.
Answer: If the reflected or refracted rays only appear to diverge from a point, the point is called a virtual image.

Question 14. Write the differences between real and virtual image.
Answer:

• A real image is formed when a reflected or refracted ray actually converges to it, but in case of a virtual image, the reflected or refracted ray appear to diverge from it.
• A real image can be seen directly with eye. It can also be cast on a screen. But a virtual image can only be seen with eye but it cannot be cast on a screen.

Question 15. Define image.
Answer:

It the rays diverging from a paint undergo changes in direction by reflection, refraction of by both, ultimately they either converge to a point or appear to diverge from a point.

Question 16. What is incident ray and reflected ray?
Answer:

• The light ray striking a reflecting surface is called the incident ray.
• The light ray obtained after reflection from the surface, in the same medium in which the incident ray is travelling, is called the reflected ray.

Question 17. What is plane of incidence and reflection?
Answer:

The plane containing the reflected ray and normal, is called the plane of reflection. The plane containing the reflected ray and the normal, is called the plane of reflection.

Physics Class 10 WBBSE Question 18. Difine Lateral inversion.
Answer: The interchange of the left and right sides in the image of an object in a plane mirror is called the lateral inversion.

19. Write two uses of plane mirror.
Answer:

• In a kaleidoscope, three plane mirrors inched with each other at 60° are used.
• In the solar heating devices such as solar cooker, etc plane mirror is used to reflect the light rays incident from sun. The substance to be heated.

Question 20. What is the principle of simple periscope.
Answer: This instrument is used for looking over an obstacle which blocks the path of light from an object to the eyes of an observer.

Question 21. Define

1. Centre of curvature
2. Redius of curvature
3. Principle axis

Answer:

• The centre of curvature of a mirror is the centre of the sphere of which the mirror is a apart.
• The radius of the sphere of which the spherical mirror is a part, is called the radius of curvature of the mirror.
• It is the straight line joining the pole of the mirror to its centre of curvature.

Question 22. Define

1. Pole
2. Aperture.

Answer:

• The geometric centre of the spherical surface of the mirror is called the pole of the mirror.
• The part of the mirror which is exposed to the incident light is called the aperture of the mirror. Thus it is the surface of the mirror from which reflection occurs.

Physics Class 10 WBBSE Question 23. Define focus and focal length.
Answer:

• Focus-The focus of a concave mirror is a point on the principle axis through which the light rays incident parallel to the principal axis pass after reflection from the mirror.
• The focus of a convex mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, appear to meet after reflection from the mirror.
• Focal length-The distance of focus from the pole of the mirror is called the focal of the mirror.

Question 24. Write the differences between convex mirror and concave mirror.
Answer:

Concave mirror And Convex mirror:

Question 25. Write two characteristics of the image formed by a plane mirror.
Answer:

• The dista nces of the object and its image is measured from the mirror are equal.
• The line joining the object and its image intersects the mirror perpendicularly.

Question 26. Define

1. First principal focus.
2. Second principal focus

Answer:

• Rays either diverging from or directed through the first principal focus of a spherical surface, after refraction at the surface becoms parallel.
• Rays moving parallel to the principal axis after refraction at a spherical surface either coverge to or appear to diverge from a point on the principal axis, known as second principal focus of the surface.

Question 27. What is Myopia?
Answer:

Myopia: The defect where for point is less than infinity is known as myopia. The defect is also called short Sightedness.

Question 28. How does the ray passes through the prism in the minimum deviation position?
Answer: In the minimum deviation position, the ray passes symmetrically through the prism i.e. the incident ray and emergent ray are equally inclined to the respective faces of prism.

Physics Class 10 WBBSE Question 29. On what factors does the angular dispersion depend?
Answer:

The angular dispersion depends on :

1. Refraction angle of the prism
2. Nature of the material of the prism
3. Wavelength of the incident light

Question 30. Why does a convex lens of glass = 1.5 behave as a diverging lens when immersed in a liquid of “μ = 1.65?
Answer:

The refractive index of glass with respect to the liquid [/latex]\frac{1.5}{1.65}[/latex]) is less than 1.

Question 31. Why does a diamond sparkle with great brilliance?
Answer: Multiple total internal reflections of light occur within the diamond. Diamond cutter uses this fact.

Question 32. What type of mirror would you use as shaving mirror?
Answer: A concave mirror of larger focal length is used for shaving purpose. Because it produces a magnified erect image of the face when placed within the focus of the mirror.

Question 33. The refractive index of diamond is much greater than the ordinary glass. Is this fact of some use to a diamond-cutter?
Answer: A critical angle [sin^-1(\(\frac{1}{μ}\))] reflection takes place easily. of a diamond is much smaller and total internal

Question 34. How would you determine whether a mirror is plane, concave or convex?
Answer:

A plane mirror produces an erect image of the same size as that of the object. A concave mirror produces an erect and magnified image of an object when placed within its focus. A convex mirror produces always an erect and diminished image of an object.

Physics Class 10 WBBSE Question 35. What is a Linear magnification of lens.
Answer: Linear magnification of lens: The ratio of the length of the image and that of the object is called the linear magnification.

Question 36. What is Focus of a convex lens?
Answer:

Focus of a convex lens: If a beam of parallel rays, travelling parallel to the principal axis of a convex lens are refracted by the lens, the rays become converg- ing and intersect each other at a particular point on the axis. The point is known as the focus of the convex lens.

Class 10 Physical Science WBBSE Question 37. Which rays should be considered n drawing ray diagrams for images formed by a convex lens?
Answer:

The following points should be considered :

• A beam of rays parallel to the principal axis after refraction through the lens pass through the principal focus.
• A beam of rays passing through principal focus emerge parallel to the principal axis, after refraction through the lens.
• A beam of rays through the optical centre pass out undeviated.

Question 38. What is a pure spectrum?
Answer:

Pure spectrum: The spectrum in which the constituent colours do not overlap on each other and are seperated distinctly into elementary colours is known as a pure spectrum.

39. What is an impure spectrum?
Answer:

Impure spectrum: The spectrum in which the constituent colours partially superpose on each other and are not seperated distinctly into elementary colours is known as an impure spectrum.

40. Is dispersion possible without refraction?
Answer:

Question Explanation: Dispersion without refraction is not possible. In an optical medium different colours travel with different speed and so deviate with different magnitudes. Thus, the constituent colours of a poly chromatic light deviate by different amounts i.e. they are dispersed after refraction.

41. Why blue is used after washing white shirts?
Answer:

Explanation Blue: Whitens yellow or orange colour. So, white shirts after washing may have some yellow or orange stain. To convert such stains to white, blue is used.

Class 10 Physical Science WBBSE Question 42. What is the optical plane?
Answer:

Optical plane: It is an imaginary vertical plane that cuts the principal axis perpendicularly and passes through the optical centre of a lens.

Question 43. Draw a neat diagram of the formation of image of an object at infinity by a convex lens.
Answer:

Formation of the image of an object at infinity by a convex lens :

• Distance of object: At infinity.
• The image formed: On the focal plane.
• Nature of image: Real and inverted.
• Size of image: Very much smaller than the object.

Question 44. The critical angle for a material c is 30°. Find its refractive index.
Answer:

⇒ μ=1/ sinc=1/sin 30% = 2.0

Question 45. What is the focal length of a concave mirror of radius of curvature of 16.0 cm?
Answer:

The radius of curvature = 16cm. Focal length.

= ½ × Radius of curvature

= (\(\frac{1}{2}\) x 6) cm

= 8 cm.

Class 10 Physical Science WBBSE

Question 46. A lens has a power of -2.5D. What is the focal length and the nature of the lens?
Answer: P = – 2.5 D

Focal length =\(\frac{1}{P}\)

= \(\frac{1}{-2.5}\)

= -0.4m

= 40 sin.

Question 47. A lens has a focal length of -25cm. What is the power of the lens and what is the nature?
Answer:

Power = P = \(\frac{1}{f in m}\)

= \(\frac{1}{-0.25}\)

= – 4D.

Question 48. If the refractive index of glass in 1.5 and Zi=55, find the angle of refraction of glass.
Answer:

μ= \(\frac{sini}{sin r}\)

sin r = \(\frac{sin i}{μ}\)

=\(\frac{sin 55°}{1.5}\)

= 33.6°.

Physics Class 10 WBBSE Chapter 5 Light Broad Answer Type Questions

Question 1. Draw a labelled diagram to show refraction of a light ray through a prism.
Or
Path of a ray of light through a prism
Answer:

Path of a ray of light through a prism:

• Path of a ray of light through a prism Let ABC be the principal section of a prism AB and AC are two refracting surfaces. BC is the base. PQRS is the path of a ray. It is to be noticed that in i a prism.
• The refracted ray and the emer- gent ray bend towards the base of the prism, when the surrounding medium is optically rarer than the material of the prisms.
• But, if the medium outside the prism be optically denser than the prism material, the refracted and emergent rays bend oppositely i.e they bend towards the angle of the prism.

Question 2. What are the conditions of formation of pure spectrum?
Or
Production of pure spectrum
Answer:

Conditions of formatin of pure spectrum:

• Slit should be very narrow to allow, rays of light to pass as minimum as possible.
• The slit should be placed at the focus so that the emergent light rays may be Parallel.
• Prism should be placed at the minimum deviation position of spec- trum i.e. at the mean deviation of yellow spectrum.
• The second lens (L₂ L₂) should be placed in such a position so that the screen may be in the focal plane of the lens.

Question 3. How will you explain twinkling of stars?
Answer:

Twinkling of stars:

The light coming from stars is refracted continuously by different layers of the atmosphere before coming to us. Due to there repeated refractions, The apparent position of a star is different from its actual position. The temperature and density of air in the atmosphere continuously changing and so apparent position of the star also changes continuously. Due to this continuous change in the apparent position of a star, it appears to be twinkling.

Question 4. A ray of light shows no dispersion on emerging from a glass slab. Explain why?
Answer:

A rectangular glass slab cut diagonally into two pieces behaves like a pair of prisms placed side by side in reverse order. So the light dispersed by the first prism, recombines by the second prism. Thus there is no dispersion of light in a rectangular glass slab, bulb it is only laterally displaced.

Question 5. State the laws of reflection.
Answer:

The laws of reflection:

• The incident ray, the refracted ray and the normal to the refracting Surface at the point of incidence lie in the same plane.
• 2. For a given pair of media and for a given colour of light, sine of the angle of incidence bears a constant ratio to the sine of the angle of refraction.
• Thus if i and r be the angles of incidence and of refraction respectively when the ray passes from medium 1 to medium 2.

⇒\(\frac{sini}{sin r}\) = constant

= 1^μ2

Physics Class 10 WBBSE Question 6. Deseribe about the action of periscope.
Answer:

The mirror M, forms a virtual image I₁, of the object O and this image ₁, acts as virtual object-for the mirror M₂, which forms an image I₂, which is seen by the eye the final image F₂, is also virtual. Since light rays from the object are reflected by two mirrors, the lateral inversion caused by the first mirror is reversed by the second mirror. This males the final image to appear with on lateral invession.

Question 7. State the laws of refraction.
Or
What is snell’s law?
Answer: 

1. Laws of refraction: The incident ray, the reflected ray and the normal to the refracting. Surface at the paint of incidence lie in the same plane.
2. For a given pair of media and for a given colour of light, the sine of the angle of incidence bears a constant ratio in the sine of the angle of refraction.

Thus, it i and r be the angles of incidence and of refraction respectively when the ray passes from medium 1 to medium 2.

\(\frac{sini}{sinr}\)= Constant

= 1^μ2

The second law of refraction was devolped by W, Snell and is know a Snell’s law.

Question 8. Establish the relation between 1μ₂ and 2μ₁.
Answer:

From Snell’s law:  sin¹/sin^r = 2μ₁

Where 2 u₁, is the R. I. of medium 1 w.r.t. medium combining equations we get

1μ₂×2μ₁= \(\frac{sin1}{sinr}\)×\(\frac{sinr}{sin1}\) = 1

Or

1μ₂X = 1/2μ₁

Thus R.I. of medium 2 w.r.t. medium 1 is the reciprocal of R.I of medium 1 w.r.t. medum 2

Physics Class 10 WBBSE Question 9. State generalised Snell’s law.
Answer: If i₁ and i₂ be the angle of incidence and of refraction respectively then we have from Snell’s law

sini₁/sini₂ =1, μ₂

We can write

1, μ₂ =μ₂ /μ₁

⇒μ₂/μ₁

∴ sini₁/sini₂= μ₂/μ₁

Or,μ₁ sini₁ = μ₂ sini₂ → Generalised Snell’s Law

Question 10. How far from a concave mirror of radius 2m would you place the object to get an image magnified 3 times?
Answer:

Given m = ± 3 \(\frac{v}{u}\) i.e.u= 7.3u

For real image v = -3u, r = -2m and u is negetive.

So, from relation \(\frac{1}{v}\)+ \(\frac{1}{u}\)= \(\frac{2}{r}\)

Or, – \(\frac{1}{3u}\) – \(\frac{1}{u}\) = – \(\frac{2}{2}\)

For virtual image u =+ 3u, r = -2m and u is negative

So =-1 or, u= = 0.67m

= \(\frac{1}{3u}\) – \(\frac{1}{u}\) = – \(\frac{2}{2}\)  Or \(\frac{1-3}{3u}\) = -1

u= \(\frac{2}{3}\)

= 0.67 m

WBBSE Class 10 Physical Science Question Answer In English Question 11. A concave mirror forms an image of 20 cm high object on a screen placed 5m away from the mirror. The height of the image is 50 cm. Find the focal lenght of the mirror and the distance between mirror and the object.
Answer: Given size of object = 20cm.

Size of the image = 50cm.

u = -5m.

= \(\frac{-10}{7}\)

= −1.43m

And distance between the mirror and the object = 2m.

Question 12. Light falls from glass of refractive index 1.5 to air. Find the angle of incidence for which the angle of deviation is 90°.
Answer:

Refractive index of glass is 1.5. So, the critical angle of glass to air medium is given by

⇒ − q= sin^-1(\(\frac{1}{u}\)) sin^-1(\(\frac{1}{1.5}\))

= 41.8°

So, reflection in the glas medium takes only when the angle of incidence exceeds  41.8°. Now the angle of deviation. Would be 90° when the angle between the incident ray and the reflected ray becomes 90° i.e. the angle of incidence be equal to 45°.

Question 13. Where should an object be placed from a converging lens of focal length 20cm, so as to obtain a real image of magnification 2 ? u =2, or, v = + 2u, f= + 20cm
Answer:

Given m =\(\frac{u}{u}\)= 2 or  , v= +2u, f= +20cm

∴ \(\frac{1}{v}\)– \(\frac{1}{u}\) = \(\frac{1}{f}\) or,

=  \(\frac{1}{2u}\) – \(\frac{1}{u}\)

= \(\frac{1}{20}\) or,

= \(\frac{1-2}{2u}\)

∴ u= \(\frac{− 20×1}{2}\)

= – 10 cm

i.e. object to be placed 10 cm in fornt of the lens.

WBBSE Class 10 Physical Science Question Answer In English

Question 14. Two thin lens of focal lengths 15cm. and 30cm. respectively are kept in contact with each other. What is the power of the combined system?
=
Answer: Given f₁= 15cm, f₂ = 30cm.

Then focal length F of the equivalent lens

\(\frac{1}{F}\)= 1/f₁+1/f₂

\(\frac{1}{15}\) + \(\frac{1}{30}\)

= \(\frac{2+1}{30}\)

= \(\frac{1}{10}\)

Power of the combination = \(\frac{100}{F}\)

= \(\frac{100}{10}\)

= +10

Question 15. The dew drops deposited on hairy leaves appear glittering way?
Answer:

The hairs out the leaf and the surface tension of water create an air gap under the dew drop. Hence some of the rays trying to pass from the drop to the side air gap suffer total internal reflection. Such reflected rays may reach our eyes after refraction through the upper surface of the drop and the drop appears glittering.

16. Show that, for a spherical mirror the focal length is half of its radius of curvature.
Answer:

P = Pole

F= Focus of the mirror.

PF = f

C = Centre of curvature.

∠BP’C = ∠P’CF (alternate angles.) and

∠BP’C = ∠CP’F (law of reflections i = r)

Hence ∠’P’CF = ∠CP’F

ΔFP’ C is isosceles. Hence P’F = FC

If the aperture of the mirror is small, the point P i very close to the point P, then P’F = PF

∴ PF=  FC or, PF + PF = PF + FC

or , 2PF = PC or, PF =  ½ PC

or, f = ½ R.

Question 17. Establish relation between the angle of deviation (Dm) and the refractive index of the material of the prism (μ).
Answer: Let, the angle of deviation be D, the refractive index of the material of the prism be μ.

For minimum deviation, we have ¡=i’

Question 18.

1. Write Cauchy’s equation 
2. Define-angular dispersion.

Answer:

1. The Cauchy’s equation: = А+B/ λ2

A, B = Cauchy’s constant

2. The angular dispersion is defined as the difference between the angles of deviation of the extreme colours, red and violet of the spectrum.

WBBSE Class 10 Physical Science Question Answer In English

Question 19. 

1. What is the condition for dispersion with deviation and deviation without dispersion?
2. What is the pure and impure spectrum?

Answer:

1. Condition for dispersion with deviation

⇒(μ_v-1)A = (μ’_y-1)A’

The condition for devotion without dispersion :

⇒ δ_y -δ_r (μ_y-1)A(w-w’)

2. 

• Pure spectrum: A spectrum in which the colours present do not overlap one another so that different colours occupy distinct and separate positions is called pure spectrum.
• Impure spectrum: A spectrum in which the colours present overlap one another and hence can not be seen distinctly separated from one another is called an impure spectrum.

Question 20. Light of two colours (say X, Y) are sent through a prism. Suppose X bents more than Y. Which colour travels more slowly in the material of the prism?
Answer:

We know, D, (μ_x-1)A

D1 = (μ_y-1)A

⇒ D_y>D_y

⇒ μ _x> μ_y

C/ V_x>C/ V_y

∴ V_y > V_x

The colour X travels more slowly in the material of the prism.

Question 21. Draw a ray diagram for the formation of a concave mirror when the object is at focus F.
Answer:

AB is an object placed at the focus F. The ray AD is incident on the mirror parallel axis and it gets reflected along DF, passing through the focus f. AE appears to be coming from the centre of curvature C, so it gets reflected back along EA. The two reflected rays DF and EA are parallel to each other.

The image is formed at infinity (very far from the mirror) which is real, inverted and highly magnified.

When the object is at focus F.

Thus when the object is at the focus F, the image is at infinity. It is

1.  Real
2. Inverted, and
3. Highly magnified.

Question 22. How the angle of minimum deviation of a glass prism changes when it is immersed in a liquid of R.I. greater than I? sin A+D m
Answer:

Minimum deviation of a glass prism:

Hence the angle of minimum deviation decreases.

Question 23. Define-Absorption Spectrum.
Answer:

Absorption Spectrum:

The missing wavelengths provide information about the absorbing material. Such a spectrum is called an absorption spectrum. The absorption spectrum may be of two types depending on the absorbing material and the conditions such as temperature.

Question 24 . An object 4 cm. high is placed at a distance of 10cm from a convex lens of focal length 20cm. Find the position, nature and size of the image.
Answer:

O= 4cm, u=-10 cm, f= + 20cm.

\(\frac{1}{V}\) = \(\frac{1}{f}\) +\(\frac{1}{u}\)

= \(\frac{1}{20}\) – \(\frac{1}{10}\)

= – \(\frac{1}{20}\)

V= -20 cm

\(\frac{1}{o}\) – \(\frac{V}{u}\)

I = \(\frac{V}{u}\) × 4

= \(\frac{-20}{-10}\) ×4

= 8cm

Question 25. Two lenses one of focal length 20 cm. and another focal length -15cm, are placed in contact. What is the focal length and power of the combination?
Answer: f, 20cm=0.2m f15cm – 0.15m

Power of the first lens, P₁ = \(\frac{1}{0.2}\) = 5D

Power of the second lens, P₂ = \(\frac{-1}{0.2}\)

= 6.67 D

Power of combination, P = P₁ + P₂ = 5 – 6.67 = -1.67D

Focal length, f= \(\frac{1}{1.67}\)  0.599m

The -ve sign indicates that the combination acts as a concave lens.

Question 26. An object 4 cm. high is placed at a distance of 10cm from a convex lens of focal length 20cm. Find the position nature and size of the image.
Answer:

0=  4cm, u = -10cm, f= + 20cm,

\(\frac{1}{V}\) =\(\frac{1}{F}\)+ \(\frac{1}{u}\)

= \(\frac{1}{20}\) – \(\frac{1}{10}\)

= – \(\frac{1}{20}\)

V= – 20 cm

\(\frac{I}{O}\)= \(\frac{V}{u}\) or, \(\frac{V}{u}\)×0

= \(\frac{-20}{-10}\)×4

= 8cm