Unit 3 Part 2

Question 1

Luminous objects:

Answer 1

objects that emit their own lights, e.g., the sun, stars, fire and light bulbs.

Question 2

Non-luminous objects

Answer 2

objects that do not emit light. We see them because they reflect light, e.g., Moon, desk, chair and human(s), etc.

Question 3

The light bends ….. the normal when it speeds up.

Answer 3

The light bends away from the normal when it speeds up.

Question 4

the light bends, …. normal when it slows down

Answer 4

the light bends away normal when it slows down

Question 5

The light bends towards normal when it …….

Answer 5

The light bends towards normal when it slows down

Question 6

The light bends away from the normal when it ……

Answer 6

The light bends away from the normal when it speeds up.

Question 7

What is TOTAL INTERNAL REFLECTION?

Answer 7

Only occurs when light rays travel towards a boundary of a less optically dense medium ( smaller n).

Reflection inside a material; [1 mark]
Reflection is inside the more optically dense medium/ the medium with higher refractive index; [1 mark]
All the light is reflected; [1 mark]
Occurs when the incident angle is larger than the critical angle; [1 mark]

Question 8

What happens when the angle of incidence increases?

Answer 8

The angle refraction also increases, until it reaches a special angle called the critical angle (c).

Question 9

What is the critical angle?

Answer 9

The smallest possible angle of incidence at which light rays are totally internally reflected.

Question 10

What happens to the refracted and reflected rays when the angle of incidence is greater than the critical angle?

Answer 10

There is no light refracted, however, the light is reflected at the boundary.

Question 11

What is the critical angle for light passing from water to air?

Answer 11

49 degrees

Question 12

What is the critical angle for light passing from glass to air?

Answer 12

42 degrees

Question 13

What is the refractive index of glass?

Answer 13

About 1.5

Question 14

What is the refractive index for water?

Answer 14

1.3

Question 15

Define refractive index

Answer 15

A measure of the change in speed a wave experiences when it travels across the boundary between two media, e.g. air and glass, the index also describes how much the direction of the wave changes

Question 16

How can you investigate the refractive index of glass?

Answer 16

Equipment: a ray box and a rectangular glass block.

First: shine a ray of light onto one of the sides of the glass block so that the ray emerges on the opposite side of the block. Mark the directions of both of these rays with crosses.
Second: draw around the glass block before removing it.
Using the crosses, draw in the direction of both rays.
Fourth: draw in the direction of the ray that travelled inside the glass block.
Five: Draw a normal (a line at 90 degrees to the glass surface) where the ray enters the block.
Six: Measure the angles of incidence (i) and refraction (r)
Lastly, use the equations n= sin i/ sin r to find the refractive index of the glass block.

Question 17

What are the uses of total internal reflection?

Answer 17

Any two from:

Optical fibres
for communication; [1 mark]
endoscopes/medical uses; [1 mark]
decorative lamps; [1 mark]
Safety reflectors; [1 mark]
Prisms in
binoculars/telescope; [1 mark]
camera; [1 mark]
periscope; [1 mark]
range-finder; [1 mark]