Critical Angle and TIR

Question 1

Give three cases in relation to i and C

Answer 1

1. When i ∠ i
2. When i = C: ∠ r becomes 90°. The refracted ray is along the glass-air interface and is very weak
3. When i>C: No refracted ray is obtained and the incident ray is totally reflected. The boundary surface behaves as a reflecting surface

Question 2

What is critical angle?

Answer 2

Critical angle in the angle of incidence in the denser medium corresponding to which the angle of refraction in the rarer medium is 90°

Question 3

Give the formula for critical angle with respect to refractive index

Answer 3

aμg = 1/sin C = cosec c

sin C = 1/ aμg

Question 4

Name the factors that affect the critical angle

Answer 4

1. The color of light

2. The temperature

Question 5

How does the critical angle depend on the color of light?

Answer 5

The refractive index of a transparent medium decreases with the increase in wavelength of light. Therefore, the critical angle increases with increase in wavelength of light - least for violet light and most for red light

Question 6

For which light is C most and least?

Answer 6

It is the least for violet light and most for red light

Question 7

How does the critical angle depend on temperature?

Answer 7

On increasing the temperature of a medium, its refractive index decreases, so the critical angle for a pair of media increases. Thus, C increases with the increase in temperature

Question 8

Define total internal reflection

Answer 8

When a ray of light travelling in a denser medium, is incident at the surface of a rarer medium at an angle of incidence greater than the critical angle for the pair of media, the ray is totally reflected back into the denser medium. This phenomenon is called TIR

Question 9

Name the essential conditions for total internal reflection

Answer 9

1. The light must travel from a denser to rarer medium

2. The angle of incidence must be greater than the critical angle for the pair of media

Question 10

Using which device can the phenomenon of refraction and total internal reflection be demonstrated?

Answer 10

A laser pen source

Question 11

Name the three purposes of the total reflecting prism

Answer 11

1. to deviate a ray of light through 90
2. to deviate a ray of light through 180
3. to erect an inverted image without producing deviation in its path

Question 12

Where is the ‘deviating a light ray through 90’ action of a right-angles isosceles prism used?

Answer 12

In a periscope where a total reflecting prism is preferred over a plane mirror

Question 13

Where is the ‘deviating a light ray through 180’ action of a right-angles isosceles prism used?

Answer 13

In a binocular and camera to invert the image without any loss of intensity

Question 14

When is the ‘erecting an inverted image’ action of an erecting prism used?

Answer 14

In a slide projector

Question 15

What is C for warer?

Answer 15

49°

Question 16

What is C for glass?

Answer 16

42°

Question 17

What is C for diamond?

Answer 17

25°

Question 18

Why is the phenomenon of TIR used in the construction of periscope. binocular and certain types of camera?

Answer 18

In the process of TIR, 100 % energy or intensity of light is reflected back. No other device, such as plane mirror, etc. produces 100 % reflection (due to absorption of some part of light)

Question 19

Name the three different prisms in which the phenomenon of TIR is considered

Answer 19

1. Right angled isosceles prism / total reflecting prism
2. Equilateral prism
3. Right angled prism

Question 20

What are the angles in right angled isosceles prism?

Answer 20

45°, 90°, 45°

Question 21

What are the angles in an equilateral prism?

Answer 21

60°, 60°, 60°

Question 22

What are the angles in a right angled prism?

Answer 22

30°, 90°, 60°

Question 23

What is a total reflecting prism?

Answer 23

A prism having an angle of 90° between its two reflecting surfaces and the other two angles equal to 45°

Question 24

Why is a total reflecting prism called so?

Answer 24

The light incident normally at any of its faces, suffers TIR inside the prism

Question 25

What is a prism called when it is used to erect an inverted image without deviation?

Answer 25

An erecting prism

Question 26

What happens to a parallel beam of light incident normally on face AB of an equilateral prism?

Answer 26

After refraction, the beam splits into two equal beams travelling in different directions making an angle of 120° with each other

Question 27

What is the function of an equilateral prism?

Answer 27

It can be used to deviate a light ray through 60° by TIR

Question 28

What is the use of a right angled prism?

Answer 28

It can be used to deviate a light ray through an angle less than 60° by TIR

Question 29

When does non occurrence of TIR through a right angled prism take place?

Answer 29

1. When a light ray is incident normally on the face AB of ◺ ABC
2. When a light ray is incident normally on the upper part of the face AC of ◺ABC

Question 30

When does a ray suffer TIR and gets deviated by an angle greater than 60° in a right angled prism?

Answer 30

When the ray of light is incident normally on the hypotenuse AC of the prism ABC ◺ below the foot of the perpendicular on it from its opposite corner

Question 31

State the difference between TIR and reflection from a plane mirror w.r.t conditions for occurrence

Answer 31

TIR takes place only when light passes from a denser to a rarer medium at an angle of incidence greater than the critical angle for that pair of media, whereas, reflection from a plane mirror takes place when light is incident on a plane mirror from any medium at any angle of incidence

Question 32

State the difference between TIR and reflection from a plane mirror w.r.t light reflected

Answer 32

In TIR, the entire light is reflected, whereas in reflection from a plane mirror, only a part of light is reflected while rest is refracted and absorbed

Question 33

State the difference between TIR and reflection from a plane mirror w.r.t loss of energy

Answer 33

In TIR, there is no loss of energy. The energy of the reflected ray is the same as that of the incident ray. Whereas in reflection from a plane mirror, there is a loss of energy. The energy of the reflected ray is always less than that of the incident ray.

Question 34

State the difference between TIR and reflection from a plane mirror w.r.t brightness

Answer 34

In TIR, the image is much brighter and the brightness remains unchanged even after long use of the total reflecting device, whereas in reflection from a plane mirror, the image is less bright and the brightness gradually decreases as the silvering on the mirror becomes old and rough

Question 35

What is a mirage?

Answer 35

What appears to be a pool of water or a wet road on a hot sunny day at some distance. It is commonly observed in a desert

Question 36

What is used to transmit a light signal over a long distance without any loss of energy?

Answer 36

An optical fibre

Question 37

Give some consequences of TIR

Answer 37

1. A mirage seen commonly in a desert
2. An empty test tube placed in water in a beaker with its mouth outside the surface, shines like a mirror when seen from certain angles
3. A crack in a glass vessel often shines like a mirror
4. A piece of diamond sparkles when viewed from certain directions
5. An optical fibre is used to transmit a light signal over a long distance without any loss of energy