PROPERTIES OF WAVES
Refraction of Light and Sound Waves
REFRACTION
When light bends in going obliquely from one
medium to another, we call this process refraction.
REFRACTION
 Refraction
occurs to
minimize the time taken
by light to travel from A
to B.
 Just
as if you wanted to
save someone from
drowning, the quickest
path would not be a
straight line – it would be
the dashed path shown.
REFRACTION
Light follows a less inclined path in the glass.
•
Light travels slower in glass than in air, so it minimizes the
time it spends in the glass.
REFRACTION
Light rays pass from air into water and water into air.
•
Pathways are reversible for both reflection and refraction.
INDEX OF REFRACTION
Index of refraction, n, of a material
 indicates how much the speed of light
differs from its speed in a vacuum.
 indicates the extent of bending of rays.
 ratio of speed of light in a vacuum to the
speed in a material.
INDEX OF REFRACTION
The generally accepted speed of light in a vacuum
is 3.00 x 108m/s; however, the speed of light varies
according to the material that is travelling
through. The speed of light in liquids and solids
is significantly less than in a vacuum.
 The ratio of the speed of light in a vacuum (c),
to the speed of light in a given material (v), is
called the absolute index of refraction (n) of
the material.
 n= c
also known as c = nv also known as v = c
v
c = vn
n

.
.
Just by looking at this formula, we can predict:
 The unit(s) of the index of refraction: there are
none! It’s a ratio
 The greater the index of refraction for a given
material, the slower the speed of light in that
substance.
 The minimum possible index of refraction of a
substance would be 1. An index of refraction less
than 1 would mean that…
Example 1:
The speed of light in water is m/s. What is the
index of refraction for water?
Example 2:
The index of refraction for diamond is 2.42. What
is the speed of light in diamond?
REFRACTION OF LIGHT


The change in the speed of light as it passes from
one medium into another causes it to bend.
Refraction is the bending of light that takes place
at a boundary between two materials having
difference indices of refraction.
REFRACTION OF LIGHT
CAREFUL….with reflection, the angle of incidence equaled the
angle of reflection. With refraction the same is not true.
How can we determine the angle of refraction given the angle of
incidence? Let’s do the refraction lab to figure this out!
Refraction
CHECK YOUR NEIGHBOR
REFRACTED LIGHT THAT BENDS TOWARD THE
NORMAL IS LIGHT THAT HAS
A. slowed down.
B. sped up.
C. nearly been absorbed.
D. diffracted.
Refraction
CHECK YOUR ANSWER
REFRACTED LIGHT THAT BENDS TOWARD THE
NORMAL IS LIGHT THAT HAS
A. slowed down.
B. sped up.
C. nearly been absorbed.
D. diffracted.
Refraction
CHECK YOUR NEIGHBOR
REFRACTED LIGHT THAT BENDS AWAY FROM THE
NORMAL IS LIGHT THAT HAS
A. slowed down.
B. sped up.
C. nearly been absorbed.
D. diffracted.
Refraction
CHECK YOUR ANSWER
REFRACTED LIGHT THAT BENDS AWAY FROM THE NORMAL
IS LIGHT THAT HAS
A.
B.
C.
D.
slowed down.
sped up.
nearly been absorbed.
diffracted.
Explanation:
This question is a consistency check with the
question that asks about light bending toward the
normal when slowing.
REFRACTION
Illusions caused by refraction
 Objects
submerged in water appear closer to
the surface.
REFRACTION
Illusions caused by refraction (continued)
 Objects
such as the Sun seen through air
are displaced because of atmospheric
refraction.
REFRACTION
Illusions caused by refraction (continued)
 Atmospheric
refraction is the cause of mirages.
Refraction
CHECK YOUR NEIGHBOR
WHEN LIGHT TRAVELS FROM ONE MEDIUM TO ANOTHER
AND CHANGES SPEED IN DOING SO, WE CALL THE PROCESS
A. reflection.
B. interference.
C. dispersion.
D. refraction.
Refraction
CHECK YOUR ANSWER
WHEN LIGHT TRAVELS FROM ONE MEDIUM TO ANOTHER
AND CHANGES SPEED IN DOING SO, WE CALL THE PROCESS
A. reflection.
B. interference.
C. dispersion.
D. refraction.
DISPERSION
 Newton performed experiments that illustrated
the dispersion of sunlight into a spectrum and
subsequent recombination into white light.

Components of whit light are dispersed in a
prism (and in a diffraction grating.)

A dispersive medium is one in which different
wavelengths of light have slightly different
indices of refraction. For example, crown glass is
a dispersive medium since the index of refraction
for violet light in crown glass is higher than for
red light. This is responsible for chromatic
aberration.
RAINBOWS
Rainbows are a result of dispersion by many
drops.
 Dispersion of light by a single drop
RAINBOWS
Sunlight incident on two sample raindrops, as shown,
emerges from them as dispersed light.
 The observer sees the red light from the upper drop and
the violet light from the lower drop.
 Millions of drops produce the whole spectrum of visible
light.

SNELL’S LAW

Dutch Mathematician Willebrod Snell (1591 –
1626) determined the exact relationship between
the angle of incidence and the angle of refraction.
LAW OF REFRACTION
Summarized:
The ratio of the sine of the angle of incidence to
the sine of the angle of refraction is a constant
(also known as Snell’s Law).
 The incident ray and the refracted ray are on
opposite sides of the normal at the point of
incidence, and all three are coplanar.

EXAMPLE 1:
Light passes from air into water at an angle of
incidence of 50 degrees. What is the angle of
refraction?
EXAMPLE 2:
If the angle of refraction of light in crown glass is
30 degrees, what is the angle of incidence?
Index of Refraction for Crown glass = 1.52
GENERAL FORM OF SNELL’S LAW

PROBLEM! This version of Snell’s Law is only
valid of the first medium is air. For any two
media we must use the General Form of
Snell’s Law:
n1sinθ1 = n2sinθ2
* This equation works for all cases of refraction!
EXAMPLE 3:
Light passes from water to into quartz at an angle
of incidence of 30 degrees. What is the angle of
refraction? Include a diagram.
EXAMPLE 4:
Light passing from flint glass into ethanol refracts
at an angle of degrees. What is the angle of
incidence? Include a diagram.
ASSIGNEMENT:
Snell’s Law Assignment (p.18)
 Refraction Problems (p.19)

CRITICAL ANGLE AND TOTAL INTERNAL
REFLECTION

At a boundary, an incident ray can undergo
partial reflection, or, in certain situations total
internal reflection.
The critical angle is the angle of incidence for
which the angle of refraction is 90. At this
angle, the refracted ray glances parallel to the
boundary
 At any angle of incidence greater than the critical
angle, total internal reflection occurs. This
means that no light passes through the
boundary.
 Total internal reflection is only possible if light is
travelling from a more refractive medium to a
less refractive medium. (i.e. n2 < n1 )


Without total internal reflection, there would be
no such thing as fibre optics! YIKES! A world
without fibre optics technology would be a world
without:



The Internet
Cable TV
medical cameras
EXAMPLE:
Determine the critical angle for a crown glass and
water boundary.
LENSES
Types of Lenses:
Lenses are of two basic types – Converging and
Diverging

Converging – thicker in the middle than the ends

Diverging – thinner in the middle than the ends
Note:
 Incident light rays are refracted twice by a lens;
once at each boundary. To simplify matters on
ray diagrams, incident rays can be shown to
refract at the construction line passing through
the optical centre of the lens. For a thin lens this
leads to a reasonably close approximation
because the lateral displacement is quite small.
ABERRATIONS

Lens defects are called aberrations. They hinder
the quality of the image formed. They can be
corrected by using aspheric lenses or by using
thin lens combinations that cancel out
aberrations.
RAY DIAGRAMS FOR LENSES

How lens diagrams are different from curved
mirror diagrams:
 Instead of a Vertex (V), a lens has an optical centre
(O) which is located at its geometric centre
 A lens has two foci, equidistant on either side of
the lens, since light behaves the same way when
travelling in either direction (Principle of
Reversibity). The two foci, F and F' are called the
primary principal focus and the secondary
principal focus, respectively. F, sometimes also
referred to as the primary focal point, is shown on
the right side of a converging lens, and on the left
side of a diverging lens, while F', the secondary
focal point is shown on the opposite side of each
respective lens.
RULES FOR DRAWING RAY DIAGRAMS FOR
CONVERGING AND DIVERGING LENSES
(Parenthetical remarks refer specifically to
diverging lenses)
 An incident ray that is parallel to the principal
axis is refracted such that it passes through (or
appears to have originated from) the principal
focus (F).
 An incident ray passing through (or heading
toward) the secondary principal focus (F') is
refracted such that it travels parallel to the
principal axis.
 An incident ray passing through the optical
centre of the lens continues to travel in a straight
line.

EXAMPLES FOR CONVERGING &
DIVERGING LENS:
THIN LENS EQUATION
Lens problems are very similar to those done
with curved mirrors.
Thin Lens Equation
1
1
1
= +
𝑓
𝑑𝑜
𝑑𝑖
Magnification Equation

M=
ℎ𝑖
ℎ𝑜
=-
𝑑𝑖
𝑑𝑜
SIGN CONVENTION:
di
is positive for real images and
negative for virtual images
f is positive for converging
lenses, and negative for
diverging lenses.

EXAMPLE 1:

An object is 32.0 cm to the left of a converging
lens with a focal length of 8.0 cm. Where is the
image located?
EXAMPLE 2:

An object 3.0 cm tall is placed 2.0 cm to the left of
a diverging lens with a focal length of 3.5 cm.
Where is the image located, and what is its size?