© Boardworks Ltd 2003 Light : What is Light? Light carries energy and travels as a wave. Light travels at 300 000 000 m/s or 300 000 km/s (much faster

© Boardworks Ltd 2003

Light : What is Light?

Light carries energy and travels as a wave.

Light travels at 300 000 000 m/s or 300 000 km/s (much faster than sound).

Light waves travel in straight lines.


Light : What materials will light travel through?

Different materials are placed between the lamp and the screen. Use the results table and shading chart on the next slide to estimate the opacity of different materials.


What materials let light through?

Material Opacity [%]

Tracing paper

1 ply tissue

2 ply tissue

Smoked glass

Glass

Perspex

Wood

Opaque objects do not allow light to pass through them - transparent ones do. Translucent materials only let through part of the light.

Shading chart.

75 %

100%

0%

15 %

2.5 %

50 %

30 %10 %

25 %20 %

5 %


How does light travel?

A clear glass trough or empty fish tank is filled with smoke.

Rays of light are shone through the tank

This demonstrates that:

LIGHT WAVES TRAVEL IN STRAIGHT LINES.


Seeing objects

How do we see an object?

The light from the object enters our eye.

Do we see all objects in the same way?

There are two ways we see objects…..

• We see some objects by reflected light.

• We see other objects because they are light sources.


Light sources

How do light rays from a light bulb and other light sources reach our eye?

They travel in a straight line directly into our eye.


Reflected light

How do light rays from a book and other such (non-luminous) objects reach our eye?

Light from the lamp or another light source strikes the book and some of the light is reflected into our eye.


Reflection : Good or Poor?

Objects which reflect light well have smooth, shiny surfaces and are usuallypale colours.

They give clear images because they reflect the light regularly.

Mirrors are excellent reflectors.

Objects which do not reflect light well have rough, matt surfaces and are usually darkcolours.

They give no or diffuse images because they reflect the light irregularly.


Reflection : Good or Poor?

Arrange these items along the arrow:

White Paper

Red

Roses

Polished Black Shoes

Al foil

Yellow

Banana

Blue Car

Tree Bark

Tangerine

Green Leaf

Tarmac Road

Best Reflectors

Worst Reflectors


What happens to the light which isn’t reflected?

• Some of this light may be absorbed (e.g. as heat).

• Some of this may also be transmitted (e.g glass reflects a small amount of light, absorbs some of the rest and allows most of it to pass through.


Reflection : 1. Lateral inversion

The way plane mirrors reflect light regularly gives a clear image which is the same size as the object

- but what is different about the image ?

This is called lateral inversion.


Reflection : 2. How far away is the image?

1. Fix a plane mirror along the centre of a

piece of A4 paper and draw around it.

Place a pin as the object in front of the

mirror.

2. Line up a ruler with the image of the pin and draw along the

edge of the ruler on the paper. Repeat for 3 more positions of

the ruler.

3. Remove the mirror and ruler. Where the lines cross is the

image position. What are the distances between the mirror and

object and its image?

I


Reflection : 3. The Maths of Reflection

Fix a plane mirror to a piece of A5 paper and draw around it.

Angle ofI ncidence [i]

Angle ofReflection [r]

Angle i

Angle r

Draw a normal line (at 90º) through the middle of the mirror outline.

Use a ray box to shine an incident ray at the mirror - plot the incident and reflected rays.

Measure the angles of incidence [i] and reflection [r] and write the answers in the table on the right.

Repeat for another 5 angles of incidence.


Reflection : 4. Reflecting without mirrors

Mirrors are good reflectors but not perfect - they give 2 reflections.

We use glass prisms instead of mirrors in good quality binoculars and other instruments.


Reflection : 4. Reflecting without mirrors


Using plane mirrors

By positioning two plane mirrors at 45° to each other at either end of a tube we can make a _________.periscope

Periscopes are used in _________.submarines


Reflection : Summary

You should now know that :

Pale and shiny surfaces are good reflectors, dark and rough surfaces are not.

The image in a plane mirror is laterally inverted.

The image is the same distance behind the mirror as the object is in front.

The image in a plane mirror is the same size as the object.

angle of incidence = angle of reflection

¡ = r


Refraction


Refraction : Bending light

The speed of light waves depends on the material they are travelling through.

If the light waves enter a different material [e.g. from glass into air] the speed changes.

This causes the light to bend [or refract].

Air = Fastest Diamond = slowestGlass = slower

Glass


Refraction - at the air-glass boundary


Refraction :Investigating Refraction

Angle i

Angle r

What can you remember about i and r?


Air to Glass:angle of incidence > angle of refraction

As the light ray moved from air into glass it moved towards the normal.

If light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal.

i > r


Glass to Airangle of incidence < angle of refraction

i < r

As the light ray moved from glass into air it moved away from the normal.

If light rays move from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal.

i < r


Angle of incidence = 0°

When the angle of incidence is 0 the light ray is not deviated from its path.

Un-deviated light ray


Refraction in a rectangular block


Revision tip

Remember the word:TAGAGA

Towards (normal) Air Glass Away (from normal) Glass Air


Fast and slow

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 beach?

In a similar way as light moves from one medium to another of different density the speed of light changes.

Do you think light moves faster or slower in a more dense medium?In the water.

Light moves slower through a more dense medium.


The 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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Wat

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Per

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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.


Why does light change direction as it enters a material?

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 one hits the mud first and will move more slowly than tyre two.This causes the car to turn towards the normal.

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

Tyre 1 Tyre2

Mud

Road


No change in direction

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.


Same for light

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______.

If the light enters a new medium along the normal (i = 0) then it does not ‘bend’ because all of the light ray slows down at the s___ t___.

ormal

lows own

irection

ame ime


Refraction : Effects of Refraction

Many visual effects are caused by refraction.

This ruler appears bent because the light from one end of the ruler has been diffracted, 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?


Refraction : Apparent Depth

The rays of light from the stone get bent [refracted] as they leave the water.

Your brain assumes they 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


The 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.

The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey.

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.


Refraction : Magic coins

Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin.

Gradually add water to the bowl and watch the coin through the tube float up - can you explain this?


Refraction : Summary

When light bends this is called refraction.

Refraction happens because the light changes speed [or velocity].

When light enters a more dense medium [e.g. glass], it bends towards the normal.

When light enters a less dense medium [e.g. air], it bends away from the normal.

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

Remember that the angle of reflection [r] and angle of refraction [r] have the same symbol.

In reflection, I = r

In refraction, I r


Colour


1. Shine a ray of bright white light at a prism, as shown above, and move the prism until colours appear.

Colour : splitting white light up


Explanation

What happens?

The white light ray is split into a spectrum of colours. This is known as DISPERSION.

Why?

The different colours of light have different wavelengths. Different wavelengths are refracted different amounts.

Red light is refracted least.

Violet light is refracted the most.

Which colour is refracted the most?How do you remember the order of the colours?Richard Of York Gave Battle In Vain




Until now we have used the title ‘splitting white light up’. However this process has a scientific name.

The splitting of white light into a spectrum of colours is called DISPERSION.

But why does white light get dispersed?



Dispersion

• Each of the colours [ROYGBIV] has a slightly different waveform - what is different?

They each have a different wavelength [].


Because the different colours of light have different wavelengths they are bent (refracted) by different amounts.

But which colour do you think is bent the most?

Red light is refracted least because it has a long wavelength.

Violet light is refracted the most because it has the shortest wavelength.

Dispersion



Colour : joining the colours back together

Remember how you dispersed white light to give a spectrum of colours?

Now do the opposite to it - you’ll need 2 prisms!

A similar effect can also be seen using a colour wheel [or Newton’s disc].


Newton’s disc


Seeing colours

We see objects by light striking an object and then reflecting into our eye.

How do we see colour?

Why does a red dress look red?

Why does a green apple look green?

How do we see objects like a book?


Primary Colours Animation


Primary Colours

You can make any colour by mixing three colours. Do you know what they are?

Red Green

Blue

The colours labelled in white text are the PRIMARY COLOURS.

The colours labelled in blue text are called SECONDARY COLOURS. They can be

made by mixing the

primary colours

Cyan

Yellow

Magenta



Red objects

Why does a red snooker ball look red in white light?

Remember white light is made of a spectrum of colours.

The snooker ball absorbs all the colours of the spectrum EXCEPT red, so red light is reflected into our eye. The snooker ball appears red.


Green objects

Why does a green snooker ball look green in white light?

The snooker ball absorbs all the colours of the spectrum EXCEPT green, green light is reflected into our eye. The snooker ball appears green.


Black objects

Why does a black snooker ball look black in white light?

The snooker ball absorbs all the colours of the spectrum. NO light is reflected into our eye. The snooker ball appears black.


White objects

Why does a white snooker ball look white in white light?

The snooker ball doesn’t absorb any of the colours of the spectrum. The whole spectrum of light is reflected into our eye. The snooker ball appears white.


Magenta objects

Why does a magenta ball look magenta in white light?

The ball absorbs all the colours of the spectrum EXCEPT red and blue, red and blue light is reflected into our eye. The ball appears to be magenta, a mixture of red and blue light.


Look at the clothes below. What colour light is reflected by these clothes?


What colours are absorbed by this frog’s skin? What colours are reflected into your eyes?

This part of the skin absorbs all the colours of the spectrum but reflects red light.

This part of the skin absorbs all the colours of the spectrum and none are reflected.


What colours are absorbed by this flower? What colours are reflected into your eyes?

This part of the flower absorbs all the colours of the spectrum but reflects yellow (a mixture of red and green) light.

This part of the flower absorbs no colours, it reflects them all.


Filters

Filters let certain colours of light pass through, but absorb all other colours.

Using different coloured filters placed in front of your eye, look around the classroom and see what effect they have on your vision.

Objectfilter


Red, blue and green filters

Red filters absorb all colours….

Blue filters absorb all colours….

Green filters absorb all colours….

… apart from red light

… apart from blue light

… apart from green light


Magenta, cyan and yellow filters

Magenta filters absorb all colours….

Cyan filters absorb all colours….

Yellow filters absorb all colours….

… apart from red and blue light

… apart from blue and green light

… apart from red and green light



But why do colours look different in different coloured light?

Lets start with the example of a red ball in red light.

The red light shines on the ball. The red ball

reflects red light and so appears red.


What about the red ball in green light?

The green light shines on the ball.

The red ball only reflects red light so it absorbs the green light and reflects nothing. Because it doesn’t reflect any type of light it appears black.So what colour does a green

ball appear in blue light?

The green ball only reflects green light so it absorbs the red light and reflects nothing. Therefore it appears black.


But what if the filter you are using lets through more than one type of light. For example what will our red ball look like in magenta light.

The magenta light shines on

the ball. Remember that

magenta is a mixture of blue and red light The ball reflects only red light.

Therefore it absorbs the blue light and reflects the red light. It will appear to be red.


Over the next two slides you will be shown the same girl as in the previous example. However, she will be standing in a different coloured light each time. The colour of this light is written at the top of the slide.

Your task is to drag the correct shirt and trousers onto the girl to represent what those clothes would look like in this coloured light.




Object (Colour) Colour Filter Appearance

red ball red

red ball blue

blue book green

blue book magenta

green apple cyan

green apple magenta

red and blue tie red

red

black

black

blue

green

black

red and black

Complete the table below by adding in the colour that each object would appear to be in the conditions listed.


Flag colours

For the flag shown, what it will look like in:

a) Red light b) Green light c) Blue light

Actvity


Multiple Choice Questions


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


If a ray of light moves from a more dense medium to a less dense medium at an angle to

the normal what happens?

A. It continues with no change of

direction

B. It bends towards the normal

C. It bends away from the normal

D. It stops


Which colour has the longest wavelength?

A. Blue

B. Indigo

C. Yellow

D. Red


Which colour below is refracted (bent) the least by a glass prism?

A. Red

B. Orange

C. Yellow

D. Green


Which of the following is not a primary colour?

A. Red

B. Blue

C. Cyan

D. Green


Which two primary colours make magenta?

A. Red and cyan

B. Red and yellow

C. Red and blue

D. Blue and violet


If white light passed through a magenta filter and then a blue filter, what colour would emerge?

A. Red

B. Red and blue

C. Blue

D. Black


What colour would a red dress look in cyan light?

A. Red

B. Green

C. Blue

D. Black

Documents

© Boardworks Ltd 2003 Light : What is Light? Light carries energy and travels as a wave. Light travels at 300 000 000 m/s or 300 000 km/s (much faster