Bending Of Light
Topic
The path taken by light passing through a transparent solid
Introduction
Have you ever wondered why a drinking straw in a glass of water appears to be
bent where it meets the surface of the water, when you know it isn’t? Scientists
have shown that light travels at different speeds in different transparent
substances such as air, water, glass and plastic. Light travels faster in air than in
liquids or solids. If a light ray is traveling at an angle into a more optically dense
medium (e.g., from air to water or air to glass) part of the ray reaches the
junction before the rest, causing it to slow down earlier, and hence making the
ray bend. This bending is called refraction. In this experiment, you will
investigate how light passes through a transparent solid.
Time required
45 minutes
Materials
light box with single slit plate (see Experiment 4.01)
rectangular clear plastic or glass block (6.5 × 11.5 × 2.0 cm)
four sheets of white, unlined 81/2 × 11 paper
pencil
protractor
scientific calculator
Safety note
Be careful if using a glass block, as the edges may be sharp. If a plastic block is
used, handle it carefully as the surface scratches easily. Do not stare directly
into bright light sources. Do not touch the light box when switched on.
Procedure
Perform this experiment in a room where you can restrict external light, i.e., the
windows can be covered. Begin the experiment with windows/blinds closed and
the room lights on. It can be performed using the light box constructed in Part A
of Experiment 4.01.
1. Set up the light source and rectangular block on a sheet of paper as shown in
diagram 1 on the next page. Mark the position of the block on the paper using
the pencil and then turn on the light. Turn off the room lights.
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light box with single slit
1
incident
ray
rectangular
block
Relative positions of light box and block
2. With the pencil, make a few dots to mark the path of the ray of light from the
light box to the glass block (referred to as the “incident ray”) and the path
taken by the ray emerging from the block (the “emergent” ray).
3. Turn on the room lights and turn off the light. Remove the block and light
box from the paper.
4. Remove the sheet of paper on which you have marked the rays. Use the pencil
and ruler to connect the dots you made to mark the incident ray. Label this
line “incident ray.” Do the same with the dots you made to mark the emergent
ray and label this line “emergent ray.”
5. Using the protractor and the ruler, draw a line at right angles (90°) to the
block at the point where the incident ray strikes the block (see diagram 2
below). This line is called the “normal.”
normal
2
incident
ray
i
A
r
glass or rectangular
plastic block
refracted ray
B
emergent
ray
Light ray passing through a rectangular glass block
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6. Use the protractor to measure the angle between the incident ray and the
normal. This angle is known as the “angle of incidence” (i) and enter the value
in the data table below. Use your scientific calculator to find the sine of the
angle and enter this in the data table on the next page.
7. Connect the points where the light enters (A) and emerges (B) from the block
(see diagram 2 above). This is the path taken by the light refracted by the
block (the “refracted” ray).
8. Measure the angle between the refracted ray and the normal. This is the
“angle of refraction” (r) and enter the value in the data table. Find the sine of
this angle and enter this in the data table.
9. Repeat steps 1 to 8 three more times using a clean sheet of paper each time.
Number each sheet of paper.
DATA
Angle of incidence (i)
Sin i
TABLE
Angle of refraction (r)
Sin r
sin i
sin r
1
2
3
4
Analysis
1. How is the angle of incidence related to the angle of refraction? Use the values
in the data table to help you work out the relationship.
2. Were you always able to see the path of the incident ray, the refracted ray,
and the emergent ray on the paper you were using, i.e., were they in the
same “plane”?
3. What can you say about the incident and emergent rays?
Want to know more?
Click here to view our findings.
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PHYSICS EXPERIMENTS ON FILETM
OUR FINDINGS • 10.18
mirror. Because light rays are not focused on reflection by a convex mirror, an
image seen in this type of mirror is a “virtual” image – it cannot be projected
onto a screen.
concave mirror
C
convex
mirror
F
P
Key
C = center of curvature
r = radius of curvature
F = focal point of the mirror
f = focal length of the mirror
F
P
f
C
f
r
r
Reflection of light by a convex mirror
Reflection of light by a concave mirror
3. Curved mirrors have many uses. Concave mirrors focus light from distant
objects to a point close to the mirror so, for example, they are useful when
shaving. Dentists also use concave mirrors when examining teeth. The rearview mirror in an automobile is a convex mirror – it magnifies distant objects.
4.03 Bending Of Light
1. The first law of refraction states that the sine of the angle of incidence divided
by the sine of the angle of refraction is a constant for a particular pair of
media, such as air to glass or glass to air. This is known as Snell’s Law. The
constant for a particular boundary between two media is called the refractive
index. The refractive index depends on the direction the light travels in – i is
always the angle of incidence at the boundary and r the angle of refraction
from that boundary (see the diagram below).
normal
incident
ray
i
A
r
glass or rectangular
plastic block
refracted ray
i1
B
r1
emergent
ray
Angles formed by a light ray passing through a rectangular glass block
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10.19 • OUR FINDINGS
PHYSICS EXPERIMENTS ON FILETM
2. As you were able to see the path of the incident and reflected rays on the
paper, they were always in the same plane. If your light box is not very
powerful, the path of the refracted beam will not be visible within the block,
but the ray will still be visible when it emerges.
3. The incident ray and the emergent ray are parallel to each other. This is
because you were using a block with parallel sides and the angles i and r1 are
equal - as are the angles r and i1 (see the diagram on the previous page).
Light rays traveling through a glass or plastic block are bent because light travels
more slowly in a dense medium such as plastic or glass than it does in air. A ray
of light striking a transparent block at a smaller angle than 90º is refracted. Part
of the ray strikes the block first and is slowed down to travel through the block
while the remainder of the ray is traveling in air. This differential slowing bends
the ray. If a beam of light strikes a transparent block at right angles (i.e., along
the normal), it is not refracted (see the diagram below).
incident ray
A
glass or rectangular
plastic block
B
emergent ray
Light ray strikes block at right angles
4.04 Passage Of Light Through Lenses
Part A: Convex and concave lenses
1. Parallel rays of light passing through a convex lens are brought together
(“focused”) at a point (F), the principal focus of the lens. The principal focus
of a convex lens is said to be “real” because the rays are focused.
2. The distance of this point from the lens is called the focal length of the lens (f)
(see the diagram on the next page left). Because a convex lens focuses the rays,
f is regarded as positive.
3. Parallel rays of light passing through a concave lens spread apart (“diverge”).
If the diverging lines are traced back, they meet at a point behind the lens.
This is called the apparent principal point (F) of the lens (see the diagram
above right). The principal focus of a concave lens is said to be “virtual”
because the rays cannot come to a focus.
4. The distance of this point from the lens is called the focal length of the lens (f)
(see the diagram on the next page right). Because a concave lens does not
focus the rays, f is regarded as negative.
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Published by Facts On File, Inc. All electronic storage, reproduction,
or transmittal is copyright protected by the publisher.