1. Refraction - at the air-glass boundary
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Refraction - at the air-glass boundary

2. Refraction through a glass block:
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Wave slows down and bends towards the normal due to entering a more dense medium
Wave slows down but is not bent, due to entering along the normal
Wave speeds up and bends away from the normal due to entering a less dense medium

3. Light travels at 300 000 km/s in a vacuum.
The speed of light
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Light travels at km/s in a vacuum.
As it enters denser media the speed of light decreases.
Looking at the chart, which do you think is denser, Perspex or water?
Perspex must be denser because light travels more slowly through Perspex than water.

4. Words – speed up, water, air, bent, medium
Refraction
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Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through.
In this case the light rays are slowed down by the water and are _____, causing the ruler to look odd. The two mediums in this example are ______ and _______.
Words – speed up, water, air, bent, medium

5. Why does light change direction as it enters a material?
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In the muddy field the car slows down as there is more friction.
If it enters the field at an angle then the front tyres hit the mud at different times.
Tyre one hits the mud first and will move more slowly than tyre two.This causes the car to turn towards the normal.
Tyre Tyre2
Road
Mud

6. Refraction : Effects of Refraction
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This ruler appears bent because the light from one end of the ruler has been refracted,
but light from the other end has travelled in a straight line.
Would the ruler appear more or less bent if the water was replaced with glass?

7. The Archer fish
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The Archer fish is a predator that shoots jets of water at insects near the surface of the water, say on a leaf.
The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.
image of prey
prey location

8. Finding the Critical Angle…
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1) Ray gets refracted
2) Ray still gets refracted
4) Ray gets internally reflected
3) Ray still gets refracted (just!)
THE CRITICAL ANGLE

9. TOTAL INTERNAL REFLECTION
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weak reflected
ray
As the angle of incidence
increases towards the critical
angle ( glass = 420 ) the
refracted ray gets weaker
and the reflected ray gets
stronger.

10. Uses of Total Internal Reflection
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Optical fibres:
An optical fibre is a long, thin, _______ rod made of glass or plastic. Light is _______ reflected from one end to the other, making it possible to send ____ chunks of information
Optical fibres can be used for _________ by sending electrical signals through the cable. The main advantage of this is a reduced ______ loss.
Words – communications, internally, large, transparent, signal

11. Other uses of total internal reflection
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1) Endoscopes (a medical device used to see inside the body):
2) Binoculars and periscopes (using “reflecting prisms”)

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16. Wave diagrams 1) Reflection 2) Refraction 3) Refraction 4) Diffraction
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1) Reflection
2) Refraction
3) Refraction
4) Diffraction

17. Diffraction
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More diffraction if the size of the gap is similar to the wavelength
More diffraction if wavelength is increased (or frequency decreased)

18. Sound can also be diffracted…
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The explosion can’t be seen over the hill, but it can be heard. We know sound travels as waves because sound can be refracted, reflected (echo) and diffracted.

19. Diffraction depends on frequency…
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A high frequency (short wavelength) wave doesn’t get diffracted much – the house won’t be able to receive it…

20. Diffraction depends on frequency…
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A low frequency (long wavelength) wave will get diffracted more, so the house can receive it…