Waves – Topic 4
Chapters 26
Reflection & Refraction
Reflection

Law of Reflection
– The angle of
incidence equals
the angle of
reflection

The incident and
reflected rays lie in
the same plane
with the normal.
Image position
 You
think in
straight
lines!
 The
image is
located
behind the
mirror.
Diffuse vs Specular
Reflection

Diffuse Reflection
– Light incident upon
a rough surface
– Law of reflection
still holds; Normals
are not parallel.

Specular Reflection
– Mirror like reflection
– All Normals are
parallel
Refraction

Sudden change in direction of a wave as
it changes speed.
– It must enter obliquely to change direction!

Which direction does it bend as it slows?
Refraction

In both cases the speed of the wave has
decreased. This is indicated by the
decrease in wavelength!
Refraction
 In
which medium does light travel
faster? (glass rod appears bent)
Speed of light

n is the absolute index
of refraction. This is a
measure of optical
density. n is defined
as the ratio of the
speed of light in a
vacuum to the speed
of light in a new
medium.



v is the speed of light
in the new medium.
c= 3.0 x 108 m/s
As the index increases
the speed decrease.
Draw a graph for
index vs. speed.
Relative Index of Refraction



n is the relative index of
refraction. If air is not
used, then remember
nrel = n2/n1
What is the relative index
when going from
diamond into lucite?


If nrel < 1 ;
speeds up
If nrel > 1 ;
slows down
Refraction
n(water)=1.33; n(glass)=1.50; n(air)=1.00
Calculate the
speed of light in
water and glass.
Vw = 2.26 x 108m/s
Vg = 2.00 x 108m/s
Refraction
n1- from
n2 - into


When a wave slows down it bends closer to the
normal. {less to more – toward} n2>n1
When a wave speed up it bends away from the
normal. {BLA – Big ―› Little – Away} n2<n1
Refraction

If light rays bend closer to the normal
when slowing down, why does the glass
rod seem to bend away form the normal?
Apparent Depth



R – Real Depth
A – Apparent Depth
Diverging rays
enter your eyes.
You “think” in
Straight Lines.
A virtual image
appears to come
from point y
Apparent Depth
 If
the chest is 20 m below the
surface at what depth will the image
appear? Assume nsea water = 1.34
Snell’s Law

n1sinθ1 = n2sin θ2

v1/v2 = λ1 / λ2
Example
A monochromatic light ray f= 5.09 x 1014 Hz
is incident on medium X at 55˚. The
absolute index of refraction for material X
is 1.66
1.
2.
3.
What is material X?
Determine the angle
of refraction.
Determine the
speed of light in
medium X.
Ex: Solution
The index of 1.66 is Flint Glass
To find the angle of
refraction use
Snell’s Law.
θ2= 30˚
To find the speed use
n=c/v.
v = 1.8 x 108 m/s
Dispersion
 The
breaking up of white light into its
component frequencies.
Refractive & Frequency


The refractive index
depends on the
medium & the
frequency of light.
Each frequency
“color” propagates at
a different speed and
bends a different
amount.
Critical Angles
 Def:
The angle of incidence when the
angle of refraction is 90 degrees.
 Only
exist when you have the BLA’s
– Big to Little Away
 When
a light ray exceeds the critical
angle it undergoes total internal
reflection (TIR)
Snell’s Law Lab
 What
factors effect the index of
refraction of a liquid?
 Write up a planning A and B for this
question. (Last IB Assessment)
 Research any procedures you might
want to use when investigating.
– I may modify your methods based on
materials we have available.