# WBBSE Class 10 Physical Science Solutions Chapter 5 Light

Detailed explanations in West Bengal Board Class 10 Physical Science Book Solutions Chapter 5 Light offer valuable context and analysis.

## WBBSE Class 10 Physical Science Chapter 5 Question Answer – Light

Very Short Answer Type Questions :

Question 1
On normal incidence of a ray of light on a plane mirror, what are the angle of incidence and angle of reflection?
∠ i = ∠ r = 0°

Question 2.
What is the linear magnification of image in a plane mirror ?
Magnification, m = $$\frac{I}{O}$$ = pm 1 as size of image, I = size of object, O

Question 3.
Name the type of mirror which has focal length equal to infinity.
A plane mirror.

Question 4.
What is leteral inversion of image?
The side of an object appears as left in the image of the object and vice versa in an image formed in a plane mirror. This is known as lateral inversion of image.

Question 5.
What is the radius of curvature and focal length of a plane mirror?
In a plane mirror both radius of curvature and focal length of a plane mirror are infinity.

Question 6.
A man approaches a vertical plane mirror at a speed u. At what speed does he approach his image?
The image is formed at the same distance behind the plane mirror as the object is infront of it. Thus, the man will approach it image with a speed = u-(-u) = 2 u.

Question 7.
Define principal axis of a spherical mirror.
The straight line passing through the pole and the centre of curvature is called principal axis of the mirror.

Question 8.
Why is a convex mirror used as driver’s mirror?
As a convex mirror always produces an erect image of diminished size providing a wide field of view.

Question 9.
Which mirror is convergent, concave or convex?
Concave mirror.

Question 10.
Can we cast the image formed by a convex mirror on a screen?
The image of real object cannot be cast, but the image of a virtual object can be cast.

Question 11.
Can a convex mirror form a magnified image?
No.

Question 12.
When does a concave mirror form a virtual image?
When the object lies between the pole and focus of the mirror.

Question 13.
What is the relation between f and r of a spherical mirror?
f = $$\frac{r}{2}$$

Question 14.
Can a virtual image be photographed?
Yes, it can be photographed with the help of the lens of the camera.

Question 15.
What is the number of images of an object held between two parallel plane mirror?
n = $$\left(\frac{360}{\theta}-1\right)$$ = α when θ = 0°

Question 16.
What is the effect of the size of a mirror on the nature of the image?
Nature of the image does not depend on the size of the mirror.

Question 17.
Does the focal length of a concave mirror change if it be immersed in water?
No.

Question 18.
Which of the following does not change when light goes from one medium to other frequency, wavelength speed and intensity?
Frequency

Question 19.
What is the absolute refractive index of vacuum?

Question 20.
For which medium is refractive index (i) minimum (ii) maximum?
(i) μ min for vacuum (=1) and (ii) μ max for diamond (= 2.42)

Question 21.
Can total internal reflection occur when light travels from a rarer to a denser medium?
No.

Question 22.
Does critical angle depend on the colour of light?
Yes, as μ is different for different colour of light.

Question 23.
What is the cause of refraction of light?
Different velocity of light is different media.

Question 24.
Give the relation between critical angle and refractive index.
θc = sin-1 $$\left(\frac{1}{\mu}\right)$$

Question 25.
Can light travelling from air to glass suffer total internal reflection?
No.

Question 26.
What is the principal of optical fibre?
Total internal reflection of light.

Question 27.
What do you mean by refraction of light ?
The phenomenon of bending of light rays from its original path while passing from one medium to another is called refraction of light.

Question 28.
Define refractive index of the material.
For a given pair of media and a colour of light, refractive index,
$$\mu=\frac{\sin i}{\sin r}=\frac{C}{C^{\prime}}$$
where i and r and angles of incidence and refraction respectively and C and C’ are velocities of light in vacuum and in the medium respectively.

Question 29.
What is the basic cause of refraction of light?
The speed of light is different in different media.

Question 30.
Can absolute refractive index of light of any material be loss than one? Why?
No, because μ = $$\frac{c}{v}$$ and c is always greater than v.

Question 31.
What is the main use of optical fibres?
It can transmit light from one place to another without any loss of intensity of light.

Question 32.
What will be the focal length and power of a plane glass plate?
f = α ; power = 0

Question 33.
An object is placed at the focus of concave lens where will be the image?
At the midpoint of focus and optical centre of the concave lens.

Question 34.
Define power of a lens.
The reciprocal of the focal length of the lens is called its power.

Question 35.
How can a convex lens behave like a diverging lens ?
It can behave a diverging lens, if it placed in a medium of refractive index greater than the refractive index of the material of the lens.

Question 36.
An object is placed at the focus of a concave lens, where will be its image formed?

Question 37.
How does the power of a concave lens vary, if the incident red light is replaced by violet light?
As the focal length is minimum for violet light, so power increases.

Question 38.
What is deviation produced in a prism?
The angle between the emergent ray and the direction of incident ray is called angle of deviation or the deviation produced in a prism.

Question 39.
How is angle of incidence at the first face of a prism related to the angle of emergence at the other face at minimum deviation position?
i1 = i2

Question 40.
What is meant by dispersion of light?
The phenomenon of splitting of white light into its constituent colours is called dispersion of light.

Question 41.
For which colour, m of the material of the prism is (i) minimum (ii) maximum?
(i) red
(ii) violet.

Question 42.
Which colour of light deviate (i) most (ii) least on passing through a prism?
(i) violet
(ii) red

Question 43.
Is dispersion of white light in vacuum possible?
No.

Question 44.
How is deviation produced in a prism related to the angle of the prism and angles of incident and emergency?
δ = i1 + i2A

Question 45.
Why is a angled prism a better reflector than a plane mirror?
As in the prism no part of the light is refracted.

Question 46.
Does a beam of light give spectrum on passing through a hollow prism containing air, explain?
No, because all colours of light passes through vacuum or air same speed and their is no dispersion.

Question 47.
What is principal section of prism ?
Any section of the prism by a plane at angle to the refracting edge is called a principal section of a prism.

Question 48.
What is the edge of a prism ?
The intersection of the refracting surfaces is called the edge of the prism.

Question 49.
Which of the colours has (i) maximum velocity and (ii) minimum velocity in a glass prism ?
(i) red (ii) violet.

Question 50.
What is the cause of dispersion?
The difference in the deviations suffered by the two extreme colours (i.e. red and violet) of white light in passing through a prism is called angular dispersion.

Question 51.
What is the cause of dispersion?
The deviations of different colours of light through a prism are different.

Question 52.
What is the unit of angular dispersion?
Degrees.

Question 53.
What is the dispersive power of a prism?
The dispersive power of a prism is the ratio of angular dispersion to the mean deviation produced by the prism.

Question 54.
Does dispersive power of a prism depend on the angle of the prism?
No, it depends on the nature of the material of the prism.

Question 55.
What is the essential condition for observing rainbow?
The sun should be at the back of the observer after rainfall.

Question 56.
Can a short sighted person read books without spectacles?
Yes, A short sighted person can see objects at short distance.

Question 57.
What kind of lens one should use to correct myopic eye?
Concave lens.

Question 58.
What kind of lens one should use to correct astigmatic eye?
Cylindrical lens.

Question 59.
What is the least distance of distinct vision?
25 cm.

Question 60.
What is the far point of a normal eye?
Infinity.

Question 61.
Which colour of light travels faster in vacuum?
Light rays of all colour travel with same speed in vacuum.

Question 62.
How many colours of light mix to produce white light?
Seven colours of light mix to produce white light.

Question 63.
Will focal length of a convex lens vary for refraction of light of different colours through it?
For different colours of light the convex lens will have different focal length.

Question 64.
What will be the position and nature of the image formed with a convex lens when the object is situated at a distance twice the focal length of the lens?
The image will be at a distance twice the focal length on the other side of the lens and the nature of the image will be real, inverted and same size as that of the object.

Question 65.
What is a convex lens?
If the lens is thicker at its centre than at its edges, it is a convex or converging lens.

Question 66.
Where would you place an object before a convex lens to get a virtual and magnified image?
If the object is at a distance less than the focal length of the lens, the image becomes virtual, erect and magnified.

Question 67.
Who discovered first the dispersion of light?
Dispersion of light was first discovered by Sir Issac Newton in 1666.

Question 68.
What is dispersion of light?
The splitting up of white light into seven colours on passing through a refracting medium like prism is known as dispersion of light.

Question 69.
What is a spectrum?
The band of different colours obtained due to dispersion of white light is known as a spectrum.

Question 70.
What is a lens?
A lens is a piece of transparent refracting substance bounded by two spherical or one spherical and another plane surfaces.

Question 71.
What is a concave lens?
The lens which is thinner of the centre and wider at the two edges is called a concave lens.

Question 72.
What is prism?
A homogenous transparent refracting medium enclosed by three rectangular and two triangular surfaces is known as a prism.

Question 73.
What is the principle section of the prism?
The surface which intersects the refracting surface of the prism perpendicularly is known as the principle section of the prism.

Question 74.
What is the refracting angle of the prism?
The angle made by the two refracting surface of the prism is known as the refracting angle of the prism.

Question 75.
What is polychromatic light?
The light which is composed of more than one colour is known as polychromatic light.

Question 76.
What is monochromatic light?
The light which consists of only one colour is known as monochromatic light.

Question 77.
What are primary colours ?
Red, green and blue are called primary colc ars.

Question 78.
Give the name of a secondary colour.
Yellow is called a secondary colour.

Question 79.
What is linear magnification ?
The ratio of the length of the image and that of the object is called the linear magnification.

Question 80.
What is focal plane of the lens?
The plane passing through principal focus of a lens and perpendicular to the principal axis of the lens is known as the focal plane of the lens.

Question 81.
How does a green cloth appear in red light?
The green cloth appears black in red light.

Question 82.
What is a double convex lens ?
If both the surfaces of convex lens are convex then the lens is called double convex lens. It is used in camera, telescope etc.

Question 83.
What is a plano convex lens ?
If one surface of convex lens is plane and the other is convex then it is called plano convex lens. e.g. eye piece of telescope.

Question 84.
What is concavo convex lens?
If one surface of convex lens is convex and the other is concave then it is called concavo convex lens. It is used in spectacles.

Question 85.
What colour of light is used for danger signal ?
Red.

Question 86.
Name a phenomenon occuring in nature due to dispersion of light.
Rainbow.

Question 87.
What type of reflection is used in torch?
A torch uses concave reflector with the bulb placed very near to the focus of the reflector.

Question 88.
When a point object is placed in front of a concave mirror in between centre of curvature and focus, where will the image formed?
The image will be formed at a distance greater than the radius of curvature.

Question 89.
What type of mirror is represented by the front side of a shining stainless spoon?
The front side of a shining stainless steel spoon acts like a concave mirror.

Question 90.
What type of mirror is represented by the backside of a shining stainless steel spoon?
The backside of a shining stainless steel spoon acts like a convex mirror.

Question 91.
Refractive Index is 2.41. What is meant by that?
Refractive Index is 2.41 – it means that light travels 2.41 times faster in air (vacuum) than in diamond.

Question 92.
What do you mean by refracting faces or refracting surfaces of a prism?
The two rectangular surfaces through which light can enter the prism or come out from the prism are called refracting surfaces of a prism.

Question 93.
What are the factors on which the lateral shift depends?
Lateral shift depends on
(a) thickness of the glass slab
(b) angle of incidence
(c) refractive index of the slab.

Question 94.
What shall be the angle of refraction when angle of incidence is 90° ?
There is no refraction taking place in this case and the ray of light passes along the line of separation of the two media.

Question 95.
If the speed of light in two different transparent media is same, will a ray of light suffer refraction?
If the speed of light in both the media is same, the ray of light will not be refracted.

Question 96.
Why is a normal eye unable to clearly see objects placed closer than 25 cm ?
The least distance of distinct vision for a normal eye is 25 cm. So, a normal eye is not able to clearly see objects, placed closer than 25 cm.

Question 97.
Your friend can read a book properly but cannot see the writings on the blackboard from the last bench. What type of defect of viston he/she has?
He/She is suffering from myopia or short-sightedness.

Question 98.
Your friend has difficulty in reading a book properly but can see the writings on the blackboard from the last bench clearly. What type of defect of vision he/she has?
He/She is suffering from hypermetropia.

Question 99.
What is the focal length of a parallel sided glass slab ?
A parallel sides glass slab has infinite focal length and radius of curvature.

Question 100.
What is the nature of image formed by eye lens-real or virtual?
The convex eye lens forms and inverted, real image of the object on the retina.

Question 101.
Is dispersion possible without refraction?
No, dispersion is not possible without refraction. In an optical medium rays of different colour travel with different speeds and so, they deviate as per their infrangibility. Hence a polychromatic light is dispersed only after refraction.

Question 102.
What is monochromatic light?
Rays of light which do not undergo splitting into different colours due to dispersion are known as monochromatic light. Each colour of the spectrum is monochromatic.

Question 103.
Does dispersion of white light take place in vacuum?
In vacuum, rays of different colours travel with same speed. So, travelling through vacuum, they will not be separated from each other and hence no dispersion will take place.

Question 104.
Hod does the refractive index of glass with respect to air change with change in wavelength of light?
The refractive index of glass with respect to air increases with the decrease in the wavelength of light.

Question 105.
What do you mean by vistble spectrum?
The part of the spectrum which is visible to human eye is known as visible spectrum. The wave length of visible spectrum ranges from 400 nm to 700 nm.

Question 106.
Whick ray has the highest frequency?
Gamma-ray has the highest frequency.

Question 107.
What would have been the colour of the sky if there were no atmosphere?
If the earth did not have any atmosphere, there would have been no particles to scatter the sunlight and the sky would have appeared black.

Question 108.
What is the nature of light wave?
Light wave is transverse electromagnetic waves.

Short answer type questions :

Question 1.
What is centre of curvature of a lens ?
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 do you mean by the principal axis of a convex lens?
Principal axis of a convex lens : The line joing the centres of curvature of the two spherical surfaces of a convex lens is called its principal axis.

Question 3.
What is radius of curvature ?
Radius of Curvature : 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 focus of a convex lens ?
Focus of a convex lens : If a beam of parallel rays, travelling parallel to the principal axis of a cc the lens, the rays become converging and intersect each other at a particular point on the axis. The point is known as the focus of the convex lens.

Question 5.
What is optical centre ?
Optical Centre : If a ray of light strikes on 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 6.
What is focal length ?
Focal length : The distance between the optical centre and the focus is known as focal length.

Question 7.
What is Focal plane ?
Focal plane : The plane passing through principal focus of a lens and perpendicular to the principal axis of the lens is known as the focal plane of the lens.

Question 8.
What is optical plane?
Optical plane : It is an imaginary vertical plane that cuts the principal axis perpendicularly and passes through the optical centre of a lens.

Question 9.
What is linear magnification of lens ?
Linear magnification of lens : The ratio of the length of the image and that of the object is called the linear magnification.

Question 10.
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?
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.

Question 11.
Which rays should be considered in drawing ray diagrams for images formed by a convex lens ?
The following points should be considered :

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

Question 12.
Draw a neat diagram of the formation of image of an object at infinity by a convex lens.

Formation of image of an object at infinity by a convex lens : Distance of object : at infinity Image formed: On the focal plane. Nature of image : real and inverted. Size of image : very much smaller than the object.

Question 13.
Draw a neat diagram of the formation of image of an object which is placed in between infinity and 2f from the lens.
Object is between infinity and 2 f from the lens:

• Distance of object : between infinity and 2 f from the lens.
• Image formed: On the opposite side of the object and is situated between ‘f and 2f,
• Nature of image : real and inverted.
• Size of image : smaller than the object in size.

Question 14.
Draw a neat diagram of the formation of image of an object at 2 f from the lens.
Object is at 2f from the lens :

• Distance of object : at 2f from the lens.
• Image formed : at a distance 2 f from the lens.
• Nature of image : real and inverted.
• Size image : equal to the size of the object.

Question 15.
Draw a neat diagram of the formation of image of an object which is placed in between ‘f’ and 2f
Object is between ‘f’ and 2 f from the lens :

• Distance of object: between f and 2 f from the lens.
• Image formed: Or, the opposite side of the object at a distance greater than 2f. Nature of image : real and inverted.
• Size of image: Greater in size than the object.

Question 16.
Draw a neat diagram of the formation of image of a object which is at the focus of the lens.
Object is at the focus of the lens :

• Distance of object : at the focus.
• Image formed : at infinity.
• Nature of image : real and inverted.
• Size of image : Highly magnified.

Question 17.
Draw a neat diagram of the formation
Fig. for Question 16 of image of a object which is situated between the lens and the focus.
Object is within the focal length of the lens :

• Distance of object : Within the focal length.
• Image formed : On the same side of the object.
• Nature of image : virtual and erect.
• Size of image : Magnified.

Question 18.
(i) Draw a neat diagram to explain the principle of a magnifying glass
(ii) To read the tiny scripts of a book with ease, where should be book be kept in front of a magnifying glass ?

(i) Nature and size of image : An erect, virtual and magnified image is formed on the same side of the object. This property of a convex lens is working principle of magnifying glass.
(ii) To read tiny scripts of a book, the book must be placed within the focal length of the lens.

Question 19.
What is a spectrum ?

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

Question 20.
What is a pure specturm ?
Pure spectrum : The spectrum in which the constituent colours do not overlap on each other and are separated distinctly into elementary colours is known as a pure spectrum.

Question 21.
What is an impure spectrum ?
Impure spectrum : The spectrum in which the constituent colours partially superpose on each other and are not separated distinctly into elementary colours is known as an impure spectrum.

Question 22.
What are the differences between Pure and Impure spectrum ?
Distinction between Pure and impure spectruin :

 Pure spectrum Impure spectrum (i) In a pure spectrum all the seven colours occupy distinct and different positions on a screen. (i) In an impure spectrum all the seven colours do not occupy different and distinct positions on a screen. (ii) In a pure spectrum, monochromatic colours presents in white light, are placed in order of refrangibility. (ii) In an impure spectrum, monochromatic colours are not placed in order of refrangibility.

Question 23.
Is dispersion possible without refraction?
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 polychromatic light deviate by different amounts i.e. they are dispersed after refraction.

Question 24.
Why blue is used after washing white shirts ?
Explanation : Blue whitens yellow or orange colour. So, white shirts after wash may have some yellow or orange stain. To convert such stains to white, blue is used.

Question 25.
How would you determine whether a mirror is plane, concave or convex ?
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.

Question 26.
What type of mirror would you use as shaving mirror?
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 27.
A concave mirror of small aperture forms a sharper image why ?
It is because such a mirror is free from the defect due to spherical aberration.

Question 28.
The refractive index of diamond is much greater than the ordinary glass. Is this fact of some use to a diamond-cutter ?
An critical angle [sin-1(1/μ)] of diamond is much smaller and total internal reflection takes place easily.

Question 29.
Why does a diamand sparkle with great brilliance?
Multiple total internal reflections of light occur within the diamond. Diamond cutter uses this fact.

Question 30.
Why does a convex lens of glass μ = 1.5 behave as a diverging lens when immersed in a liquid of μ = 1.65 ?
The refractive index of glass with respect to the liquid $$\left(\frac{1.5}{1.65}\right)$$ is less than I.

Question 31.
On what factors does the angular dispersion depend?
The angular dispersion depends on

• refracting angle of the prism
• nature of the material of the prism
• wavelength of the incident light.

Question 32.
How does the ray passes through the prism in the minimum deviation position?
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.

Question 33.
What is myopia?
Myopia: The defect where far point is less than infinity is known as myopia. The defect is also called short sightedness.

Question 1.
Draw a labelled diagram to show refraction of a light ray through a prism.
Path of a ray of light through a prism : let A B C be the principal section of a prism A B and A C are two refracting surfaces. B C is the base. PQRS is the path of a ray. It is to be noticed that in a prism, the refracted ray and the emergent 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 ?
Conditions of formation of pure spectrum :
(i) Slit should be very narrow to allow rays of light to pass as minimum as possible.
(ii) The slit should be placed at the focus so that the emergent light rays may be parallel.
(iii) Prism should be placed at the minimum deviation position of spectrum i.e. at the mean deviation of yellow spectrum.

(iv) 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 ?
The light coming from stars is refracted continuously by different layers of the atmosphere before coming to us. Due to these 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 apperent 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?
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, but it is only laterally displaced.

Numerical Problems :

Example 1:
How far from a concave mirror of radius 2 m} would you place the object to get an image magnified 3 times?
Given m = ± 3 = – $$\frac{v}{u}$$ i.e. v = mp 3 u
For real image v = -3 u, r = -2 m and u is negative.

Example 2:
A concave mirror forms an image of 20 cm high object on a screen placed 5 m away from the mirror. The height of the image is 50 cm. Find the focal length of the mirror and the distance between ma.c and the object.
Given size of object =20 cm
size of the image =50 cm
v = -5 m

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

Example 3:
Light falls from glass of refractive index 1.5 to air. Find the angle of incidence for which the angle of deviation is 90°.
Refractive index of glass is 1.5 . So, critical angle of glass to air medium θ is given by

So, reflection in the glass 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°.

Example 4:
Where should an object be placed from a converging lens of focal length 20 cm, so as to obtain a real image of magnification 2 ?
Given m = $$\frac{v}{u}$$ = 2, or, v = +2 u, f = +20 cm