SNC 2D
Unit 3: Light and Optics
Refraction of Light
Name: ______________________
Date: ______________________
Important Points from Previous Work with Refraction
You have already defined the following terms:

refraction

medium

speed of light

index of refraction
1.
a)
b)
Choose the word(s) within the brackets that best complete each statement.
The index of refraction is the (value, ratio, measure) of the speed of light in a vacuum to the
speed of light in any other medium.
Light will travel fastest in (a vacuum; any medium other than a vacuum; in water). Because the
c
speed of light, c, is in the numerator of n  this means that that the denominator, v, will always
v
be (larger, smaller) than the value of c. If the light is traveling in a medium other than a vacuum,
c
the fraction will always be (larger; smaller) than 1.000. So, the index of refraction, n, in a
v
vacuum is the (highest; lowest) value possible, at 1.000. All other media will have an index of
refraction (less than; greater than) that for a vacuum. (See Table 11.1 page 454).
Working with Index of Refraction, n
n
c
v
where n = index of refraction (no units)
c = speed of light in a vacuum [3.0 x 108
m
s
]
(Light travels fastest in a vacuum.)
v = speed of light in another medium in
m
s
.
Calculating the index of refraction, n, always involves comparing the speed of light in
a vacuum, c, to its speed in another medium because light travels fastest in a vacuum.
The index of refraction for light in a vacuum is 1.00 (exactly). All other media have
larger values. The larger the value of its index of refraction, n, the slower light
travels in the medium and the more dense the medium is. For example, n = 1.0003 in
air and n = 1.33 in water so light will travel more slowly in water than in air and water
is more dense (1.0
2.
g
cm 3
) than air (0.0013
g
cm 3
).
Read through Sample Problem: Calculating the Speed of Light in Different Media on page 455.
Use the GRASP method as you solve Practice Problems #1, 2 and 3 please. Use Table 11.15, page
454, as required.
Understanding the Mechanics of Refraction

The bending of light rays as they pass between two different transparent media is called
refraction. Refraction of light occurs at the boundary between the two media.

The amount by which a transparent medium decreases the speed of light is called the index
of refraction – the larger this value, the more the medium decreases the speed of light.

The larger the index of refraction, the more dense the medium – so more dense media
slow light more than less dense media.

The angle of incident, reflected and refracted rays are measured from the normal, drawn at
90o to the surface where the light rays cross between two media.

The following mnemonics will help you to remember how light bends when it moves from
one medium to another:
FST = Fast to Slow, Towards Normal
[You can remember this as “Freaky Science Teachers”.]
If light moves from a medium in which it travels faster to a medium in which it travels slower, it will
bend toward the normal. Remember that light will move faster in a medium which is less dense and
which has a smaller index of refraction. E.g. Glass is denser and has a larger refractive index than air
so light travels slower in glass than in air.
normal
angle i
> angle R
i
air
nair = 1.0003
boundary
glass
R
nglass = 1.47
SFA = Slow to Fast, Away from Normal
[You can remember this as “Sam Fights Alot.”]
If light moves from a medium in which it travels slower, to a medium in which it travels faster, it will
bend away from the normal. E.g. Glass is denser and has a larger refractive index than air so light
travels slower in glass than in air.
Normal
angle i
< angle R
i
nglass = 1.47
glass
boundary
air
R
nair = 1.0003
Refraction Practice
1
a) In which medium is light always travelling at its fastest? ________________________
b) What is the value of the index of refraction, n, for a vacuum? _________
c) Why is it not possible to have an index of refraction less than 1? (Think about how n is
calculated…)
2.
Under what conditions could you slow light down and then speed it up again?
3.
Sometimes incident light can be both reflected and refracted. Label the relevant angles in the
diagram as one of these:
i = angle of incidence
r = angle of reflection
R = angle of refraction
normal
nair = 1.0003
interface
nglass = 1.47
4.
Choose the correct phrase within the bracket to complete each statement.
a) Incident and refracted rays are measured from (the interface of two different media; a
normal drawn at 90o to the point of incidence).
b) In all cases, the angle of reflection (is larger than; equals; is smaller than) the angle of
incidence.
c)
d)
e)
f)
g)
h)
5.
When light passes from a more dense medium to a less dense medium, it will bend (towards;
away from) the normal.
When light passes from a less dense medium to a more dense medium, it will bend (towards;
away from) the normal.
When light passes from a medium with a higher index of refraction, n, to one with a lower
index of refraction, it will bend (towards; away from) the normal.
When light passes from a medium with a lower index of refraction, n, to one with a higher
index of refraction, it will bend (towards; away from) the normal.
When light passes from a medium in which it is travelling faster to a medium in which it will
travel slower, it will bend (towards; away from) the normal.
When light passes from a medium in which it is travelling slower to a medium in which it will
travel faster, it will bend (towards; away from) the normal.
In each diagram below, draw in the missing refracted ray. Do not worry about what the exact
angles are – just show whether the refracted ray is closer to or further from the normal
compared to the incident ray. [Use the FST and SFA rules from the last page…]
a)
Medium 1 is more dense than
medium 2.
normal
b) Light travels fastest in Medium 1.
normal
medium 1
interface
medium 2
c)
medium 1
interface
medium 2
normal
n = 1.0003
interface
n = 1.5
6.
a) A light ray passes from a vacuum into a substance where its speed is 2.25 x 108
m
s
. Calculate
the index of refraction for this substance. [Recall that the speed of light in a
vacuum = c = 3.00 x 108
b)
m
s
and
n
c
.]
v
7.
Use Table 11.1 page 454
to identify the substance based on its index of refraction, n. _______________________
Will light move faster or slower in this substance than in a vacuum? _________
Is this substance denser or is it less dense than air? _________________
If light is passing from air to this substance, will it refract toward the normal or away from the
normal? __________________________
Answer #1 page 456.
8.
Read page 453 in the textbook. Define the term dispersion. Answer #5 page 456.
9.
a)
Read pages 458 and fill in the blanks in the note below.
Partial reflection and refraction is a phenomenon that occurs when _____________________
c)
d)
e)
________________________________________________________________________
___________________________________________________________________ . An
example of this is __________________________________________________________
________________________________________________________________________
________________________________________________________________________
10.
a)
Read page 462 and fill in the blanks below.
The critical angle is ___________________________________________________
__________________________________________________________________
__________________________________________________________________
b)
The following diagram shows an incident ray with (as always) angle of incidence = angle of
reflection. Some of the light is also refracted, though. This time, the refracted ray is lying at
90o to the normal (right at the boundary of the two media). Based on this information, label
each of the following in the diagram below. If necessary, use Figure 11.16 pg. 462 to help.
incident ray
reflected ray
critical angle, c ,
refracted ray
angle of reflection,  r,
angle of refraction,  R
normal
air
water
c)
boundary
Total internal reflection is __________________________________________________
_______________________________________________________________________
_______________________________________________________________________
d)
Total internal reflection is what makes it possible for fibre optic cables to carry hundreds of
telephone conversations or streams of data without any of the information getting lost through
the cable walls along the way.
i) Read pages 465 – 466 and answer question #7 page 467.
ii) List two other applications for total internal reflection as described on pages 464 – 465.