Transcript Understanding Waves: Seismic Waves and Ultrasound

Understanding Waves, Reflection
and Refraction
Physics Revision
In this lesson you will revisit:
•
•
•
•
The basic principles of waves.
The wave formula.
Reflection.
Refraction and Total Internal Reflection.
You need to be able to:
• Describe the parts of a wave.
• Know the difference between longitudinal and
transverse waves.
• Use the wave equation.
• Explain the processes of reflection, refraction and
total internal reflection.
• Know how sound waves behave and the uses of
ultrasound.
• Describe how seismic waves behave and what
they tell us about Earth’s structure.
Waves : Introduction
a = amplitude

a

Time
a


Displacement
+
Crest / Peak
 = wavelength
Frequency is the number
of waves that pass a
point each second.
Trough
Transverse Waves
• Most waves are transverse.
• Transverse waves include light and all other
electromagnetic waves, ripples on water,
waves on strings and a rope being flicked
up and down.
• The vibrations are at 90º to the direction of
travel of the wave.
Longitudinal Waves
• The only ones that you need to know are
sound waves and earthquake p-waves.
• The vibrations are along the same direction
as the wave is travelling.
The Wave Formula
V = x 

v

f
x

Reflection of Light
• It allows us to see objects.
• From a rough surface, like paper, light
reflects in all directions – DIFFUSE
REFLECTION.
• When light reflects from an even surface it’s
all reflected at the same angle – CLEAR
REFLECTION.
• Angle of incidence = Angle of reflection
Refraction – a property of all waves.
• This is when waves change direction as they
enter a different substance.
• It is caused by the change in speed of the
waves.
• In water waves slow down in shallower
regions.
• Sound waves can also be refracted.
Air to Perspex
angle of incidence > angle of refraction
i
>
r
 As the light ray
moved from air into
perspex it moved
towards the normal.
 If light rays move
from a less dense
medium (air) to a
more dense medium
(perspex) they
‘bend’ towards the
normal.
i > r
Perspex to Air
angle of incidence < angle of refraction
i
<
r
 As the light ray moved
from perspex into air it
moved away from the
normal.
 If light rays move from
a more dense medium
(perspex) to a less
dense medium (air)
they ‘bend’ away from
the normal.
i < r
Total Internal Reflection
This angle is called the critical angle [ c]
i<c
i=c
Refraction
Critical case
i>c
Total Internal
Different materials have different critical angles. Reflection
Diamond has the lowest at 24º which is why it
[TIR]
reflects so much light.
Uses of Total Internal Reflection
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•
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Binoculars.
Periscopes
Reflectors
Optical Fibres
• You will need to know the position of the
prisms in binoculars and periscopes.
Which wave below is a longitudinal wave?
A. Ultraviolet
B. Water
Which wave below is a longitudinal wave?
C. Light
D. Sound
What are the units of frequency?
A. Metres
B. Hertz metres
C. Hertz/metres
D. Hertz
A wave has a wavelength of 6m and a
frequency of 5Hz.
What is the speed of the wave?
A. 1.2 m/s
B. 0.88 m/s
C. 3.0 m/s
D. 30 m/s
Which of the following is not a light source?
A. The Sun
B. A star
C. Traffic lights
D. A book
Which of the following is not seen by reflected
light?
A. Your hand
B. Jupiter
C. Light bulb
D. The Moon
Which of the following is the most dense?
A. Air
B. Water
C. Glass
D. Lead
When light changes direction as it moves from
one medium to another we call this effect what?
A. Reflection
B. Refraction
C. Diffraction
D. Total internal reflection
What happens to the speed of light as it
moves from air into glass?
A. Decreases
B. Increases
C. No effect
D. Decreases and increases
If a ray of light moves from air to glass parallel
to the normal what happens?
A. No change in direction
B. It bends away from the normal
C. It bends towards the normal
D. It stops