# Light Energy ICSE Class-8th Concise Selina Physics Ch-5

Light Energy ICSE Class-8th Concise Selina Physics Solutions Chapter-5. We Provide Step by Step Answer of Objective, True False , Fill in the blanks,Match the following , Short/Long Answer Type of Exercise-5 Light Energy . Visit official Website  CISCE  for detail information about ICSE Board Class-8.

## Light Energy ICSE Class-8th Concise Selina Physics

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Objective Questions

### A. Objective Questions Light Energy ICSE Class-8th Concise Selina

1. Write true or false for each statement

(a) Water is optically denser than glass.
Water is optically denser than air.

(b) A ray of light when passes from glass to air, bends towards the normal.

(c) The speed of light is more in glass than in water.

(d) The depth of a pond when seen from above appears to be less.

(e) Light travels at a lower speed in water than in air.

(f) Light travels in the same straight line path while passing through different media.

(g) The angle formed between the normal and the refracted ray is known as the angle of incidence.

(h) At the point of incidence, a line drawn at right angles to the surface, separating the two media, is called the normal.

(i) Image is formed by a mirror due to refraction of light.

(j) Rays of light incident parallel to the principal axis pass through the focus after reflection from a concave mirror.

(k) A convex mirror is used as a shaving mirror.

(l) The focal length of a convex mirror is equal to its radius of curvature.

(m) A concave mirror converges the light-rays, but a convex mirror diverges them.

(n) A virtual image formed by a spherical mirror is always erect and situated behind the mirror.

### 2. Fill in the blanks

(a) Water is opitcally denser than air.
(b) Air is optically rarer than glass.
(c) When a ray of light travels from water to air, it bends away from the normal.
(d) When a ray of light travels from air to glass, it bends towards the normal.
(e) When white light passes through a prism, it disperses
(f) The splitting of white light into its constituent colours is called dispersion.
(g) A concave mirror is obtained on silvering the outer surface of a part of a hollow glass sphere.
(h) Radius of curvature of a spherical mirror is two times its focal length.
(i) The angle of incidence for a ray of light passing through the centre of curvature of a spherical mirror is
(j) A convex mirror always forms a virtual image.
(k) A concave mirror forms a virtual image for an object placed between pole and focus.

### 4. Select the correct alternative

(a) The speed of light in air or vacuum is

1. 3 × 10M s-1
2. 2.25 × 108 m s-1
3. 332 ms-1
4. 2.0 × 108 ms-1

3 × 10M s-1

(b) A ray of light moving from an optically rarer to a denser medium

1.  bends away from the normal
2.  bends towards the normal
3.  remains undeviated
4.  none of the above

bends towards the normal

(c) The angle between the normal and refracted ray is called

1.  angle of deviation
2.  The angle of incidence
3.  angle of refraction
4.  angle of emergence.

angle of refraction

(d) The property of splitting of white light into its seven constituent colours is known as

1.  rectilinear propagation
2.  refraction
3.  reflection
4.  dispersion

dispersion

(e) The seven colours in the spectrum of sunlight in order, are represented as :

1.  VIBGYOR
2.  VIGYBOR
3.  BIVGYOR
4.  RYOBIVG

VIBGYOR

(f) A ray of light passing through centre of curvature of a spherical mirror, after reflection

1. passes through the focus
2.  passes through the pole
3.  becomes parallel to the principal axis
4.  retraces its own path.

retraces its own path.

(g) If the radius of curvature of a concave mirror is 20 cm, its focal length is:

1.  10 cm
2.  20 cm
3.  40 cm
4.  80 cm

10 cm

(h) The image formed by a convex mirror is

1.  erect and diminished
2.  erect and enlarged
3.  inverted and diminished
4.  inverted and enlarged.

erect and diminished

(i) The image formed by a concave mirror is of the same size as the object, if the object is placed

1. at the focus
2. between the pole and focus
3.  between the focus and centre of curvature
4.  at the centre of curvature.

at the centre of curvature.

(j) A convex mirror is used

1.  as a shaving mirror
2. and as a head mirror by a dentist
3.  or as a rear view mirror by a driver
4.  as a reflector in torch

as a rear view mirror by a driver

Short/ Long Answer Question Chapter-5 Light Energy Selina Physics for class-8

Page-99

#### Question-1 :-

State the speed of light in (a) air, (b) water, and (c) glass

Speed of light in
Air – 3 × 108 ms-1  or 300000 km/h
Water – 2.25 × 108 ms-1
Glass – 2 × 108  ms-1

#### Question-2 :-

How does the speed of light determine the optical density of a medium?

If the speed of light in a medium is less than the speed of light in air, means the MEDIUM is DENSER than air.
If the speed of light is more than the speed of light in AIR, means the MEDIUM is LESS DENSER than air.
Speed of light is 1/density of medium

#### Question-3 :-

Which is optically denser: water or air? Give reason

Denser medium are those in which speed of light gets reduced when light travels through that medium.
when light travels through water or glass it’s speed get reduced hence they are optically denser medium.

WATER is optically DENSER as the speed of light is less in water than in air.
2.25 × 108 ms-1   <   3 × 108 ms-1
speed of light in Water < speed of light in Air.

#### Question-4 :-

Out of air and glass, which is optically rarer? Give reason.

Air is RARER as the speed of light in air is more than the speed of light in glass.
3 × 108 ms-1  >  2 × 108 ms-1
speed of light in air > speed of light in glass.

#### Question-5 :-

What do you understand by refraction of light?

Refraction is the bending of light as it passes from one substance to another. Here the light ray passes from air to glass and back to air. The bending is caused by the differences in density between the two substance.

#### Question-6 :-

Describe an experiment to show that a light ray bends when it passes from one transparent medium into another transparent medium.

EXPERIMENT: Spread and fix a sheet of white paper on the drawing board.
At the centre of the paper, place a glass slab XYX ‘ Y’ and draw its boundary.
A ray of light AB travelling from air (rarer medium) to glass slab (denser medium). Part of path BC in denser medium bends towards the normal.
∠r  ∠i. This shows that when light travels from RARER to DENSER medium bends towards the normal. Ray BC travels from DENSER medium to RARER medium in air (RAY CD) bend away from normal. ∠e >∠r.
This shows that when a ray of light travels from DENSER to RARER medium bends AWAY from normal.

#### Question-7 :-

Draw a ray diagram to show that the depth of a vessel containing water when seen from above, appears to be less than its real depth.

When there is no water in the vessel (a) when water is added in the vessel coin appear to be at B than actually, the coin is at A. Hence, the coin appears to be raised up in water.
i.e. its height appears to be less than actual.

#### Question-8 :-

Define the following terms :
Incident ray, Refracted ray, Angle of incidence, Angle of refraction.

INCIDENT RAY AB: The ray light AB which is in air strikes the glass slab at B.
Or
“A ray of light falling on the surface separating the two media.”
REFRACTED RAY BK: A ray of light which after passing the first medium is in second medium i.e. ray BK.
“A ray of light travelling in other medium in the changed direction.”
ANGLE OF INCIDENCE: “The angle which the incident ray makes with the normal is called angle of incidence.”
i.e. ∠i
ANGLE OF REFRACTION: “The angle which the refracted ray makes with the normal is called angle of refraction.”
i.e. ∠r

#### Question-9 :-

A ray of light falls normally on a glass slab. What is the angle of incidence?

When a ray of light falls on a glass slab normally, the angle between normal and incident ray is zero.
∴ Angle of incidence is zero.

#### Question-10 :-

A ray of light travels from a rarer medium to a denser medium. How will it bend?

When a ray of light travels from a Rarer medium to a denser medium, it bends towards the normal.

#### Question-11 :-

A ray of light travels from a denser medium to a rarer medium. How will it bend?

When a ray of light travels from a DENSER medium to a RARER medium it will bend AWAY from normal

#### Question-12 :-

The diagram given below in fig shows a ray of light AO falling on a surface separating two media. Draw the refracted ray in each, case.

Refracted ray 0 C is shown in each case.

Towards the normal or ∠r < ∠i

Away from the normal or ∠r > ∠i

Refracted ray goes undeviating

or

∠i = 0

∠r = 0

#### Question-13 :-

Draw a diagram showing the refraction of a light ray from water to glass. Label on it the incident ray, the angle of incidence (/), and the angle of refraction (r)

Water is rarer than glass.
Light travels from water to glass mean light travels from a rarer to a denser medium.
The incident ray is AO.
Angle of incidence is ∠i
Angle of refraction is ∠r

#### Question-14 :-

The diagram in figure shows a ray of light AO falling on a rectangular glass slab PQRS. Complete the diagram till the ray of light emerges out of the slab. Label on the diagram the incident ray, the refracted ray and the emergent ray.

Incident ray AO
Refracted ray OB
Emergent ray BE are shown.

#### Question-15 :-

Explain the following:

(a) A coin placed at the bottom of a vessel appears to be raised when water is poured in the vessel.

(b) A straight stick partly dipped in water obliquely, appears to be bent at the surface of water.

(c) The sun is seen before the sunrise and after the sunset.

(a) A coin at A appears to be at B i.e. depth of coin is observed is less than the actual depth at A.

The ray of light starting from A (denser) medium bends away from the normal. Due to the refraction of light, the coin appears at B at a lower depth. Hence, in the same way, the depth of water appears to be less. `

(b) When light passes from denser medium (water) to rarer medium (air) appears to come from Q’ virtual image in spite of Q. This is due to refraction of light, that stick appears to be raised or bent.

(c)

At sunrise, when the sun is just below the horizon, the light from the sun suffers refraction from rarer to denser medium and bends towards the normal at each refraction. As a result the sun is seen in advance before it rises above the horizon in the morning and similarly after the sunset above the horizon after the sunset.

#### Question-16 :-

What is mirage? Give a reason for its formation?

A mirage is a naturally occurring optical phenomenon in which light rays bend via refraction near the surface due to heat, to produce an image of distant objects or the sky.

Sand becomes very hot during noon and the air in contact becomes rarer while upper layers of air are still a denser medium. When light travels from denser to rarer medium (starting from tree top), the light rays are bent away from normal. When refracted from rarer to denser medium (going towards the eye), the light rays bend towards the normal and the tree appears to be inverted.

#### Question-17 :-

What is a prism? Draw a ray diagram to show the refraction of a light ray through prism.

Prism is a transparent refracting medium bounded by two plane surfaces inclined at the same angle.

#### Question-18 :-

What do you mean by the term dispersion?

The breaking up of the white light into its constituent colours as it passes through a refracting medium such as prism is known as dispersion.

#### Question-19 :-

A ray of light falls on a prism. Draw a ray diagram to show that the prism disperses the white light.

#### Question-20 :-

In figure, AO is the ray of white light falling on a prism PQR. Complete the diagram till the light emerges out from the prism and falls on the screen.

#### Question-21 :-

What do you understand by the term spectrum? Name the various colours present in the spectrum of sunlight.

Spectrum is the band of colours obtained on a screen when white light passes through a prism and splits into its constituent colours. The colours of the spectrum are violet, indigo, blue, green, yellow, orange and red {VIBGYOR}

#### Question-22 :-

You are given a disc divided into seven sectors with colours violet, indigo, blue, green, yellow, orange and red in them. What would be its colour when it is rotated rapidly?

Newton’s colour disc— It is a circular disc taken by Newton to demonstrate that the recombination of seven colours produces white light. A circular cardboard disc which is divided into seven sectors and they were painted with the seven colours of VIBGYOR. The disc when rotated at a very high speed appears WHITE.

#### Question-24 :-

What is a spherical mirror?

Spherical mirror. “A mirror which is made from a part of a hollow sphere is called SPHERICAL MIRROR.

CONCAVE MIRROR:

Silvered surface is away from the centre of curvature and focal length is negative, i.e. reflecting surface is towards the centre of curvature.
CONVEX MIRROR:

Silvered surface is towards the centre of curvature and focal length is positive i.e. reflecting surface is away from the centre of curvature. It always forms diminished (small) image which is VIRTUAL.

#### Question-26 :-

Explain the following terms:

Pole, Centre of curvature, Radius of curvature, Principal axis.

Show them on separate diagrams for each of the concave and convex mirrors.

Pole – The geometric centre of the mirror is called pole.

The mid point of aperture AB is P, P is the pole of the mirror.

Centre of curvature – It is the centre of the hollow sphere which the mirror forms a part. It is represented by C.

Radius of curvature – It is the radius of the hollow sphere of which the mirror is a part. It is represented by R.

Principal axis – It is a straight line joining the pole of the mirror to its centre of curvature and extended further.

#### Question-27 :-

What do you understand by the focus and focal length of a spherical mirror? Show them on the separate diagrams for each of a concave mirror and convex mirror.

Focus – The point on the principal axis at which the incident rays which are parallel to the principal axis, meet after reflection from the mirror is called focus.

Focal length – The distance between focus and pole is focal length.

#### Question-28 :-

Draw a suitable diagram to illustrate how a beam of light incident parallel to the principal axis is reflected by:

(a) a concave mirror

(b) a convex mirror

#### Question-29 :-

How is a spherical mirror used to converge a beam of light at a point? Name the type of mirror used.

If a parallel beam of light is incident on a concave mirror, it converges the beam to a point called focus.

#### Question-30 :-

How is a spherical mirror used to diverge a beam of light from a point? Name the type of mirror used.

In the case of convex mirrors, the reflected rays diverge and do not meet at a point after reflection. They appear to come from a point’ F on the principal axis.

The type of mirror used is convex mirror.

#### Question-31 :-

State the direction of incident ray which after reflection from a spherical mirror gets reflected along its own path. Give a reason.

A ray passing through the centre of curvature is incident normally on the spherical mirror, gets reflected back along its own path.

#### Question-32 :-

How is the focal length of a spherical mirror related to its radius?

Radius is twice as focal length.

Focal length = Radius of curvature/2

Or 2F = R

#### Question-33 :-

The diagram (figure) given below shows two parallel rays 1 and 2 incident on (a) a concave mirror, (b) a convex mirror. Draw the reflected rays and mark the focus by the symbol F

#### Question-34 :-

Complete the following diagrams in figure by drawing the reflected rays for the incident rays 1 and 2 if F is the focus and C is the centre of curvature.

For concave mirror:

Ray passing through F after reflection becomes parallel to the principal axis.

Ray (2) passing through the centre of curvature travels back along its own path.

For convex mirror:

#### Question-35 :-

Which are the two convenient rays that are chosen to construct the image by a spherical mirror for a given object? Explain with the help of suitable ray diagrams.

Two convenient rays that are chosen to construct the image by a spherical mirror for a given object:

• A ray passing through the centre of curvature of a concave mirror, or a ray directed in the direction of centre of curvature of a convex mirror is reflected back along its own path.

• A ray of light parallel to the principal axis, passes through the principal focus after reflection in a concave mirror, or appears to diverge from its path in a convex mirror.

#### Question-36 :-

Draw a ray diagram to show the formation of an image of an object placed beyond the centre of curvature of a concave mirror.State the position, size and nature of the image.

A real, inverted and smaller image is formed between the centre of curvature and focus.

Position of image – Between C and F

Size of image – smaller than object.

#### Question-37 :-

Draw a ray diagram to show the formation of image of an object placed at the centre of curvature of a concave mirror. State the position, size and nature of the image.

Object placed at C:

Nature – A real, inverted image of the same is formed at the centre of curvature.

Position of image – Image formed is at C.

Size of image – equal to the object AB.

#### Question-38 :-

Draw a ray diagram to show the formation of image of an object placed between the focus and centre of curvature of a concave mirror. State the position, size and nature of the image.

Object placed between C and F

Nature – A real, inverted and magnified image is formed beyond the centre of curvature.

Position of image – Image formed is beyond C

Size of image – The image is enlarged

#### Question-39 :-

Draw a ray diagram to show the formation of image of an object placed between the pole and focus of a concave mirror. State the position, size and nature of the image.

Object lies between focus and pole:

Nature – Virtual and erect

Position of image – On the other side of mirror

Size of image – Enlarged/ magnified

#### Question-40 :-

Draw a ray diagram to show the formation of image of an object placed on the principal axis of a convex mirror. State the position, size and nature of the image. What happens to the image as the object is moved away from the mirror?

Object placed in front of convex mirror:

Position – Between pole and focus

Size – diminished

Nature – Virtual and erect

As the object is moved away from the mirror, the image shift towards the focus and the size decreases.

#### Question-41 :-

Draw separate diagrams for the formation of virtual image of an object by convex mirror. State the difference between concave and convex image.

## Image Formation By Convex Mirror

The image formed in a convex mirror is always virtual and erect, whatever be the position of the object. In this section, let us look at the types of images formed by a convex mirror.

• When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is much smaller as compared to that of the object.

• When an object is placed at a finite distance from the mirror, a virtual image is formed between pole and focus of the convex mirror. The size of the image is smaller as compared to that of the object.

Difference between convex and concave mirrors:

 Concave mirror Convex mirror What are convex and concave mirrors? If the inner side of the spherical mirror is reflecting, it is called a concave mirror. If the outer side of the spherical mirror is reflecting, it is called a convex mirror. Image Concave mirrors can form inverted and real images and also virtual and erect images. Convex mirrors form virtual and erect images Size Size can be smaller, larger or of the same size depending on the position of the object. Smaller than the size of the object, always. Position Depends on the position of the object. Always within the focus, irrespective of the position of the object.

#### Question-42 :-

Name the mirror which always forms an erect and virtual image.What is the size of the image as compared to that of the object?

It is CONVEX MIRROR. The size of the image is smaller than the size of the object, but when the object is at infinity the size of image is POINT size.

#### Question-43 :-

Name the mirror which forms an erect, virtual and enlarged image of an object. What is the position of object relative to the mirror?

Concave mirror forms an erect, virtual and enlarged image when the object is between focus and pole of mirror.

#### Question-44 :-

What is a real image ? Name the mirror which can be used to obtain the real image of an object. What should be the position of the object relative to the mirror?

REAL IMAGE. “When rays of light after reflection or refraction actually meet at some other point” the image is real. Concave mirror.
Except between pole and focus, the image formed is REAL

#### Question-45 :-

How can a concave mirror be used to obtain a virtual image of an object? Draw a diagram to illustrate your answer.

The concave mirror forms a virtual image of an object, when the object is placed between the focus and pole of the mirror.

#### Question-46 :-

State two uses of a concave mirror.

Two uses of concave mirror are:

(i) They are used as a reflector in the headlights of cars and in search of light.

(ii) Doctors examine the throat, ear, nose and eyes with light focus with the help of a concave mirror.

#### Question-47 :-

State two uses of convex mirror.

Two uses of convex mirror are:

(i) It is used as a rear view mirror in vehicles.

(ii) It is used as a vigilance mirror.

(iii) it is used as a reflector in street lamps.

#### Question-48 :-

A driver uses a convex mirror as a rear view mirror.Explain the reason with the help of a ray diagram.

A convex mirror always forms a small and upright image between pole and focus which helps the driver to see all the traffic coming behind in a small area of the mirror.

#### Question-49 :-

State the kind of mirror used

(a) by a dentist, and

(b) as a street light reflector

(a) concave mirror

(b) convex mirror

#### Question-50 :-

Name the kind of mirror used to obtain

(a) a real and enlarged image

(b) A virtual and enlarged image

(c) a real and diminished image, and

(d) a virtual and diminished image

(a) concave mirror

(b) concave mirror

(c) concave mirror

(d) convex mirror

Thanks

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