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

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Short/ Long Answer Question


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.
Answer. False.
Water is optically denser than air.

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

 

3. Match the following

Match the following

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

Answer

3 × 10M s-1

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(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

Answer

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.

Answer

 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

Answer

dispersion

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

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

Answer

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.

Answer

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

Answer

 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.

Answer

 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.

Answer

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

Answer

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

Answer-1 :-

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?

Answer-2 :-

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

Answer-3 :-

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.

Answer-4 :-
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?

Answer-5 :- 

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.

Answer-6 :- 

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.

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