04/19/23Refraction - at the air-glass boundary

04/19/23Refraction through a glass Refraction through a glass

block:block:

Wave slows down and bends towards the normal due to

entering a more dense medium

Wave speeds up and bends away from the normal due to entering a less dense

medium

Wave slows down but is not bent, due to

entering along the normal

04/19/23The speed of lightThe speed of light

Light travels at 300 000 km/s in a vacuum.As it enters denser media the speed of light decreases.

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Vac

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m

Wat

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Per

spex

Speed oflight(thousandskm/s)

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.

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RefractionRefraction

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

04/19/23Why does light change direction Why does light change direction as it enters a material?as it enters a material?

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 1 Tyre2

Mud

Road

04/19/23Refraction : Effects of Refraction

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?

04/19/23The Archer fishThe Archer fish

The Archer fish is a predator that shoots jets of water at insects near the surface of the water, say on a leaf.

image of prey

prey location

The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

04/19/23Finding the Critical Angle…Finding the Critical Angle…

1) Ray gets refracted

4) Ray gets internally reflected3) Ray still gets refracted (just!)

2) Ray still gets refracted

THE CRITICAL ANGLE

04/19/23TOTAL INTERNAL REFLECTION

As the angle of incidenceincreases towards the criticalangle ( glass = 420 ) therefracted ray gets weaker and the reflected ray getsstronger.

weak reflectedray

04/19/23Uses of Total Internal Uses of Total Internal ReflectionReflection

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

04/19/23Other uses of total internal Other uses of total internal reflectionreflection1) Endoscopes (a medical device used to see inside the

body):

2) Binoculars and periscopes (using “reflecting prisms”)

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Wave diagramsWave diagrams1) Reflection

4) Diffraction3) Refraction

2) Refraction

04/19/23DiffractionDiffraction

More diffraction if the size of the gap is similar to the wavelength

More diffraction if wavelength is increased (or frequency decreased)

04/19/23Sound can also be diffracted…Sound can also be diffracted…

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.

04/19/23Diffraction depends on Diffraction depends on frequency…frequency…

A high frequency (short wavelength) wave doesn’t get diffracted much – the house won’t be able to receive

it…

04/19/23Diffraction depends on Diffraction depends on frequency…frequency…

A low frequency (long wavelength) wave will get diffracted more, so the

house can receive it…