MODULE 3
WAVES and
THERMODYNAMICS
Year 11 Physics
Sylvia Park
3.2 Wave Behaviour
Inquiry Question:
How do waves behave?
Dot Point
• explain the behaviour of waves in a variety of situations by investigating the
phenomena of:
̶ reflection
̶ refraction
̶ diffraction
̶ wave superposition (ACSPH071, ACSPH072)
Reflection of Waves
A change in the physical properties in the
same medium (such as density or
temperature) can act like a change in
medium.
When the medium ends, or changes, the
mechanical wave does not just stop, but
instead the energy that the wave is
carrying undergoes three processes:
reflection
absorption
transmission
Reflection of Waves
Reflection occur when a wave hits a
surface and “bounce off” the surface. It can
be observed when you see your reflection
in a mirror or when sound wave echoes.
The normal is an imaginary line that is
perpendicular to the surface. The angle of
incidence and angle of reflection are both
measured from the normal to the ray.
In reflection, the angle of incidence (θi) and
the angle of reflection (θr) is always equal. θi = θr
This is the law of reflection.
The same behaviour is observed with light and other waves.
Reflection of Flat Surfaces
When reflection occur on a flat surface, all of the
reflected rays are parallel to each other. This is
why you see a clear image when you look into a
mirror.
Mirrors reflects almost all of the light that hits a
surface and absorbs very little of the light, plus it
has a smooth flat surface giving you a non-
distorted image.
When you look into the surface of water, you will
still see a reflection because water will also
reflect some of the light, however, the surface of
water is not smooth. Therefore you will see a
distorted image.
Reflection on Rigid Surface
When a transverse wave pulse reaches a hard surface, such as the fixed end of a rope, the wave is
reflected. When the end of the rope is fixed, the reflected pulse is inverted (shown below).
A wave crest would be reflected as a trough.
This inversion can also be referred to as a 180° change of phase or, expressed in terms of the
𝝀𝝀
wavelength, λ, a shift in phase of .
𝟐𝟐
Reflection on Rigid Surface
When a wave pulse hits the end of the rope that is free to move (i.e. free boundary), the pulse
returns with no change of pulse. A crest is reflected as a crest and a trough is reflected as a
trough (i.e. no change of phase). The amplitude of the reflected wave is less than the original as
part of the energy is absorbed by the post.
Reflection on Rigid Surface
The more rigid and/or dense the second
medium is, the more the wave energy will be
reflected – but some energy is always
absorbed by, or transferred to the second
medium. This is why sound can travel
through the walls.
When a transverse wave pulse is sent down
from the light rope to the heavier rope, a
smaller proportion of the wave is transmitted
into it and a larger proportion of it is reflected
back.
There is no inversion for a wave travelling
from a more dense medium to less dense.
The first particle of the less dense medium
does not have sufficient mass to overpower
the last particle of the more dense medium.
Wave Fronts
If there are many waves travelling in one or more
directions, there needs to be a method of keeping
track of where they are going. The concept of wave
front is used for this purpose. If a series of waves
travel in the same direction, one can join together
all the crests of neighbouring waves to form
wavefronts.
At any instant of time, a wave front thus represents
all the particles of a medium that are vibrating in
phase.
It’s often convenient to represent a light wave by
rays rather then wave fronts.
A ray is a line along the direction of travel of the
wave.
When waves travel in a homogeneous isotropic
material, the rays are always straight lines that are
perpendicular to the wave fronts.
Wave Front Diagrams
A wave front diagram is a simple way to demonstrate
waves and see how the waves are moving.
Let's imagine the ocean waves as they rise up and fall
down, creating crests and troughs. If you were to watch a
single wave it would go up and down in consistent sizes.
If we were to track this up and down pattern we would
end up with a pattern such as this:
Reflected Wave Fronts
The law of reflection states that the angle of the incident wave must equal the angle of the reflected wave and the
incident wave and the reflected wave must lie in the same plane.
The construction line perpendicular to the reflective surface is called the ‘normal’. The normal is used as it can be
identified experimentally regardless of the nature of the reflective surface. All angle measurements are taken with
respect to this line, not to the reflective surface. The normal, incident ray and reflective rays all lie in the same plane.
Refraction
• Refraction is a change in the direction of a wave caused by a change in its speed.
• Changes in the speed of a wave occur when the wave passes from one medium (substance) to
another.
• The direction of the refraction depends on whether the waves speed up or slow down when
they move into the new medium.
Refraction
Refraction of Water Waves
(less dense) (more dense)
Refraction (more dense) (less dense)
Refraction
Total internal reflection is also a kind of refraction, where the angle of refraction is so
large that the light is stuck inside the medium. It is used for optical fibre.
As the wave travels from more The angle at which the At any angles bigger than the
dense to less dense, its direction angle of refraction is 90°, critical angle will result total
of travel will bend away from the is called the critical angle. internal reflection.
normal.
Diffraction
When a plane (straight) wave passes through a
narrow opening, it bends. Waves also bend as
they travel around obstacles. The ‘bending’
phenomenon is known as diffraction.
Diffraction and Slit Width
If the wavelength is much smaller than the gap or obstacle, the degree of diffraction is less.
Diffraction of Waves
