Lesson 3 – Hot or Cold Reactionsepacpunyaphat.weebly.com/uploads/3/1/9/4/... · 2018. 8. 29. · Refraction Learning Objectives •Investigate the behaviour of light as it travels

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Refraction Date

Key words

Reflection

Reflected Ray

Incident Ray

Normal

Refraction

Convex

What do you think is happening in the above photo and why?

https://www.youtube.com/watch?v=o08jgkut7e8

https://www.youtube.com/watch?v=o08jgkut7e8

Refraction

Learning Objectives

• Investigate the behaviour of light as it travels from one medium to another.

• Explain the process of refraction using key scientific terms.

• Construct ray diagrams.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20053 of 84

1. Place a rectangular

glass block on a

sheet of paper and

draw around it.

2. Draw a normal line

(at 90º) along the top

surface of the block.

3. Shine rays of light with incident [i] angles of 0º, 20º,

40º, 60º, 80º and 90º into the block, making sure they

all hit where the normal line crosses the glass surface.

Measure angle ‘r’ each time and record the results.

angle i

Refraction investigation

angle r


Record the results of the refraction investigation in a table:

Angle of incidence [i]

Angle of refraction [r]

20º

40º

60º

80º

90º

Describe the change of direction of:

the ray as it enters the glass;

the ray as it re-enters the air.

Refraction investigation – results


Refraction in a rectangular block


In the water.

Light moves slower through a more dense medium.

Travelling through different materials

If you were running along a beach and then ran into the

water when would you be moving slower – in the water

or on the sand?

Do you think light moves faster or slower in a more dense

medium?

In a similar way, as light moves from one medium to

another of different density, the speed of light changes.


glass

air

The speed of light waves depends on the material they

are travelling through.

air = fastest diamond = slowestglass = slower

Bending light

If light waves enter a different material (e.g. travel from

glass into air) the speed changes.

This causes the light to bend or refract.


mud

road

Why does light change direction?

Imagine a car driving from the road into a muddy field.

In the muddy field it 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 1 hits the mud first and will move more slowly than tyre 2. This causes the car to turn towards the normal.

When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal.

tyre 1 tyre 2


airglass

When light is refracted as it travels from air to glass:

angle of incidence > angle of refraction

i > r

What happens in refraction: air to glass

In general, when light rays

move from a less dense

medium (air) to a more dense

medium (glass) they ‘bend’

towards the normal.

As the light ray travels from

air into glass it moves

towards the normal.

i > r


air

glass

If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end.

When light is refracted as it travels from air to glass:

angle of incidence < angle of refraction

i < r

What happens in refraction: glass to air

In general, when light rays travel

from a more dense medium (glass)

to a less dense medium (air) they

‘bend’ away from the normal.

As the light ray travels from

glass into air it moves away

from the normal.

i < r



If the car approached the muddy field at an angle of

incidence of 0° then both front tyres would hit the mud at

the same time.

The tyres would have the same speed relative to each other

so the direction of the car would not change, it would just

slow down.


Refraction in a rectangular block


If the light enters a new medium along the normal

(i.e. angle of incidence = 0) then it does not ‘bend’

because all of the light ray slows down at the s___ t___.

When light hits a medium at an angle to the n_____

the light ‘bends’ in a similar way to that described for

the car in a muddy field.

Part of the light ray s____ d____ before the rest and

this causes the change of d_______.

ormal

lows

irection

ime


ame

own


Many visual effects are caused by refraction.

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?


The rays of light from a stone get bent (refracted) as they

leave the water.

Your brain assumes

these rays of light

have travelled in

straight lines.

Your brain forms an image

at the place where it thinks

the rays have come from –

the stone appears to be

higher than it really is.

actual location

image

Apparent depth


The Archer fish is a predator that shoots jets of water at

insects near the surface of the water, e.g. 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.

https://www.youtube.com/

watch?v=_6LsXA_FJIE

The Archer fish

The Archer fish allows

for the refraction of light

at the surface of the

water when aiming at

the prey.

https://www.youtube.com/watch?v=_6LsXA_FJIE




1. When light bends this is called r_ _ _ _ _ _ _ _ _.

Refraction summary

Remember that the angle of reflection [r] and the

angle of refraction [r] use the same symbol.

In reflection: i = r

In refraction: i r

2. Refraction happens because the light changes s_ _ _ _.

3. When light enters a more dense medium (e.g. glass),

it bends t _ _ _ _ _ _ the normal.

4. When light enters a less dense medium [e.g. air],

it bends a_ _ _ _ f_ _ _ the normal.

5. If the incident ray hits a surface at 0º, no refraction occurs

True or false?

Lesson 3 – Hot or Cold Reactionsepacpunyaphat.weebly.com/uploads/3/1/9/4/... · 2018. 8. 29. · Refraction Learning Objectives •Investigate the behaviour of light as it travels

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