1. Discover PHYSICS for GCE ‘O’ Level Science
Unit 10 Light
Refraction

2. 10.4 Refraction at Plane Surfaces
Learning Outcomes
In this section, you’ll be able to:
Understand the terms used for refraction:
Normal
Angle of incidence
Angle of refraction
Recall that , and solve related problems
Define refractive index of a medium in terms of ratio of the speed of light in vacuum to that in the medium

3. 10.4 Refraction at Plane Surfaces
In Figure 10.28, we can observe that the pencil seems to be bent. What causes this effect?
Figure 10.28

4. 10.4 Refraction at Plane Surfaces
Light can travel through transparent materials such as glass, water or plastic.
Figure shows a light ray traveling from air into glass, and then into air again.
We can see the bending effect on the light ray when it passes from one medium into another. We call this refraction.
Figure 10.29

5. 10.4 Refraction at Plane Surfaces
Refraction of light is the bending of light ray as it passes from one medium into another.

6. 10.4 Refraction at Plane Surfaces
What causes refraction?
Light travels at different speed in different media.
For example, in air its speed is 3.0 × 108 m s-1, while in glass it is 2.0 × 108 m s-1.
At the boundary of the two media such as air and glass, there is a sudden change in speed of the light. This change in speed causes the path of the light to bend, resulting in refraction.

7. 10.4 Refraction at Plane Surfaces
Laws of Refraction
The incident ray, the normal and the refracted ray all lie in the same plane.
For two particular media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, i.e.

8. 10.4 Refraction at Plane Surfaces
Snell’s Law
The equation
where n = constant, is also known as Snell’s Law.

9. 10.4 Refraction at Plane Surfaces
Refractive Index
For the case of a light ray traveling from air
into a medium such as glass, then
where n is the refractive index of the medium
incident ray i
air
Medium (glass) r
refracted ray

10. 10.4 Refraction at Plane Surfaces
Refractive Index, n
For light passing from air or vacuum into a medium, then

11. 10.4 Refraction at Plane Surfaces
Table 10.3 Refractive indices of some transparent
materials

12. 10.4 Refraction at Plane Surfaces
Refractive index
Glass (nglass = 1.50) a higher refractive index than water
(nwater= 1.33). When a light ray enters glass, it will bend
towards the normal more than compared to when it enters
water. i i
air
air rw rg
Glass, nglass = 1.50
water, nwater = 1.33

13. 10.4 Refraction at Plane Surfaces
Can reflection and refraction occur
simultaneously?
Light is both reflected and refracted at air-glass interface. However the amount of light reflected is usually smaller than the amount of light refracted.
Figure 10.36

14. 10.4 Refraction at Plane Surfaces
One-way Mirrors
A one-way mirror is made of a sheet of glass coated with a thin reflecting layer of metal. This mirror reflects half the light and allows the other half to pass through.

15. 10.4 Refraction at Plane Surfaces
Phenomena of Refraction
‘Bent’ Objects – when a rod is placed in water, it appears ‘bent’.
In Figure 10.39, the light rays traveling from water to air bend away from the normal.
However, our brain tends to tell us that the light rays travel in straight line.
Thus, we tend to visualize the rod as ‘bent’.
Figure Ray diagram of the ‘bent’ image of a rod in a glass of water

16. 10.4 Refraction at Plane Surfaces
Phenomena of Refraction
Misperception of Depth – The effect of refraction can make a swimming pool seem shallower than it really is.
Figure Ray diagram of the image I of a point O at the bottom of a swimming pool.

17. 10.4 Refraction at Plane Surfaces
The Secret of the Archer Fish
How is the Archer fish able to overcome the visual distortion caused by refraction?
Answer: The Archer fish position itself directly under the prey. This way, the prey appears the least distorted, as the light rays entering the water surface perpendicularly are not refracted.

18. 10.4 Refraction at Plane Surfaces
News Reader Prompter
Have you wondered how a news reader is able to read the script and yet maintain eye contact with the camera lens?
A news reader prompter
Answer: The news reader reads the script off a partially reflected image, which is formed on a one-way mirror. The camera is positioned behind the mirror.

19. 10.4 Refraction at Plane Surfaces
Key Ideas
Refraction occurs because the speed of light changes when travelling through different optical media.
The two Laws of Refraction are:
The incident ray, the normal and the refracted ray all lie in the same plane.
For two particular media,
where i is the angle of incidence in air.

20. 10.4 Refraction at Plane Surfaces
Key Ideas
The refractive index n of a transparent medium is
where c is the speed of light in vacuum or air,
v is the speed of light in the medium 4.
where i is the angle of incidence of light in air or vacuum
5. Light is both reflected and refracted at the boundary of two optical media.

21. 10.4 Refraction at Plane Surfaces
Test Yourself 10.4
1. Draw a diagram to show how the direction of light changes when it travels from air into water.
Answer:
water
air i r
normal
incident ray
refracted ray
i = angle of incidence,
r = angle of refraction

22. 10.4 Refraction at Plane Surfaces
Test Yourself 10.4
2. How is the speed of light in glass related to the angle of incidence and angle of refraction?
Answer: i
air
glass
where i = angle of incidence in vacuum or air,
r = angle of refraction in glass
c = speed of light in vacuum or air
v = speed of light in glass r

23. 10.4 Refraction at Plane Surfaces
Test Yourself 10.4
3. At what angle of incidence in air will light pass through another transparent medium without being refracted?
Answer:
When the angle of incidence, i = 0° from the normal, the light ray passes straight through with refraction or bending.
air
medium

24. 10.4 Refraction at Plane Surfaces
Test Yourself 10.4
4. Draw a diagram to show how the eye sees a coin at the bottom of a bucket of water.
water
air
coin
image of coin