Download 10.4 Refraction at Plane Surfaces

Discover PHYSICS
for GCE ‘O’ Level Science
Unit 10 Light
Refraction
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
sin i
= constant , and solve related
• Recall that
problems sin r
• Define refractive index of a medium in terms of
ratio of the speed of light in vacuum to that in
the medium
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
10.4 Refraction at Plane Surfaces
•
•
Light can travel through transparent materials such as glass,
water or plastic.
Figure 10.29 shows a light ray traveling from air into glass,
and then into air again.
Figure 10.29
•
We can see the bending effect on the light ray when it passes
from one medium into another. We call this refraction.
10.4 Refraction at Plane Surfaces
Refraction
Refraction of light is the bending of light ray
as it passes from one medium into another.
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.
10.4 Refraction at Plane Surfaces
Laws of Refraction
1. The incident ray, the normal and the
refracted ray all lie in the same
plane.
2. 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.
sin i
= constant
sin r
10.4 Refraction at Plane Surfaces
Snell’s Law
The equation
sin i
=n
sin r
where n = constant, is also known as
Snell’s Law.
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
sin i
=n
sin r
where n is the refractive index of the medium
incident ray
air
i
Medium (glass)
r
refracted ray
10.4 Refraction at Plane Surfaces
Refractive Index, n
Refractive index of medium =
n
=
speed of light in vacuum or air
speed of light in medium
c
v
For light passing from air or vacuum into a medium, then
sin i
c
=n=
sin r
v
10.4 Refraction at Plane Surfaces
Table 10.3 Refractive indices of some transparent
materials
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.
air
i
air
rg
Glass, nglass = 1.50
i
rw
water, nwater = 1.33
10.4 Refraction at Plane Surfaces
Can reflection and refraction occur
simultaneously?
Light is both reflected and refracted at airglass interface. However the amount of light
reflected is usually smaller than the amount of
light refracted.
Figure 10.36
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.
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
Figure 10.39 Ray diagram of the ‘bent’
‘bent’.
image of a rod in a glass of water
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 10.40 Ray diagram
of the image I of a point O
at the bottom of a
swimming pool.
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.
http://www.naturia.per.sg/buloh/verts/archer_fish.htm
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?
Answer: The news reader
reads the script off a
partially reflected image,
which is formed on a oneway mirror. The camera is
positioned behind the
mirror.
A news reader prompter
10.4 Refraction at Plane Surfaces
Key Ideas
1. Refraction occurs because the speed of light
changes when travelling through different optical
media.
2. The two Laws of Refraction are:
a. The incident ray, the normal and the refracted
ray all lie in the same plane.
b. For two particular media,
sin i
= constant
sin r
where i is the angle of incidence in air.
10.4 Refraction at Plane Surfaces
Key Ideas
3. The refractive index n of a transparent
c
medium is
n=
v
where c is the speed of light in vacuum or air,
v is the speed of light in the medium
sin i
=n
4.
sin r
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.
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:
incident ray
normal
i
air
water
r
refracted ray
i = angle of incidence,
r = angle of refraction
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:
sin i
c
=n=
sin r
v
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
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
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.
air
water
image of coin
coin