Understanding Waves, Reflection and Refraction

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Understanding Waves, Reflection and Refraction

• Physics Revision

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In this lesson you will revisit

• The basic principles of waves.
• The wave formula.
• Reflection.
• Refraction and Total Internal Reflection.

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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 Earths structure.

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Waves Introduction
? wavelength Frequency is the number of waves
that pass a point each second.
a amplitude
Crest / Peak
?
a
?
Time
? Displacement
a
?
Trough
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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.

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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.

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The Wave Formula

• V ? x ?

v
?
?
??
f
x
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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 its all
  reflected at the same angle CLEAR REFLECTION.
• Angle of incidence Angle of reflection

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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.

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Air to Perspexangle of incidence gt angle of
refraction ?i gt ?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 gt ?r
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Perspex to Airangle of incidence lt angle of
refraction ?i lt ?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 lt ?r
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Total Internal Reflection
This angle is called the critical angle ? c
? i lt ? c Refraction
? i ? c Critical case
? i gt ? c Total Internal Reflection TIR
Different materials have different critical
angles. Diamond has the lowest at 24º which is
why it reflects so much light.
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Uses of Total Internal Reflection

• Binoculars.
• Periscopes
• Reflectors
• Optical Fibres
• You will need to know the position of the prisms
  in binoculars and periscopes.

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Which wave below is a longitudinal wave?

• Ultraviolet
• Water
• Light
• Sound

Which wave below is a longitudinal wave?
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What are the units of frequency?

• Metres
• Hertz metres
• Hertz/metres
• Hertz

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A wave has a wavelength of 6m and a frequency of
5Hz. What is the speed of the wave?

• 1.2 m/s
• 0.88 m/s
• 3.0 m/s
• 30 m/s

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Which of the following is not a light source?

• The Sun
• A star
• Traffic lights
• A book

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Which of the following is not seen by reflected
light?

• Your hand
• Jupiter
• Light bulb
• The Moon

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Which of the following is the most dense?

• Air
• Water
• Glass
• Lead

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When light changes direction as it moves from one
medium to another we call this effect what?

• Reflection
• Refraction
• Diffraction
• Total internal reflection

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What happens to the speed of light as it moves
from air into glass?

• Decreases
• Increases
• No effect
• Decreases and increases

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If a ray of light moves from air to glass
parallel to the normal what happens?

• No change in direction
• It bends away from the normal
• It bends towards the normal
• It stops