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Light. Click to move on. Sources of Light. Anything that produces light is called a light source. Light sources can be natural, eg. the sun, or artificial, such as light bulbs or television screens. Click to move on. Travelling Light.

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Presentation Transcript
slide1

Light

Click to move on

Mike Turner, May 2004

slide2

Sources of Light

Anything that produces light is called a light source.

Light sources can be natural, eg. the sun, or artificial, such as light bulbs or television screens.

Click to move on

Mike Turner, May 2004

slide3

Travelling Light

Light travels from a light source in straight lines. We call these light rays.

When we draw light rays, we must use a ruler, and we must always draw an arrow on the line to show the direction the light is travelling in, away from the source.

Click to move on

Mike Turner, May 2004

slide4

Travelling Light

Light passes easily through air. We say it is transparent.

If light hits an opaque object, it cannot pass through. This causes a shadow, as the light rays cannot bend around the object.

Some materials, eg. tracing paper, will let some light through, but we cannot see through them clearly as if we were looking through a transparent material such as glass. We say these materials are translucent.

Click to move on

Mike Turner, May 2004

slide5

Why We See Things

We see things when rays of light from them enter our eyes.

The rays of light can travel directly into our eyes if they are from a light source such as from a light hulb. However, we see most things because they reflect some light into our eyes.

Click to move on

Mike Turner, May 2004

slide6

The Eye and the Camera

The eye and the camera both work in a very similar way. They each have a hole which allows light rays in, and produce an inverted (upside down) image.

Rays of light travel in all directions from the head, but most of them cannot get into the box.

However, imagine a ray of light from the top of the head that is heading towards the pinhole. (Click).

Imagine another ray of light travelling from the bottom of the head. This ends up at the top of the image.

We end up with an image that is inverted.

The Eye

Click to move on

Mike Turner, May 2004

slide7

Reflection and Scattering

When light hits an object, some of it is reflected. How much is reflected depends on the colour (see later), and how rough or smooth the surface is.

A mirror has a very smooth surface and is an extremely good reflector.

Most surfaces have a rough texture, and they scatter the light in all directions.

Road surafces are rough, but when the surface is wet water fills in the holes and the smooth surface becomes very shiny as it reflects a lot more light.

Excellent interactive reflection demo

Click to move on

Mike Turner, May 2004

slide8

The Law of Reflection

If we shine a ray of light at a mirror we can predict how the ray will bounce off.

Click to move on

Mike Turner, May 2004

slide9

The Law of Reflection

Ray box

Angle of incidence

I

Normal Line

R

90o

Angle of reflection

The angle of incidence is always equal to the angle of reflection.

Reflected ray

Mike Turner, May 2004

slide10

periscope

How a Periscope Works

Mike Turner, May 2004

slide11

glass

Bending Light - Refraction

Air

A ray of light changes direction when it goes into a different substance. This is called refraction.

Mike Turner, May 2004

slide12

Air

glass

Bending Light - Refraction

Mike Turner, May 2004

slide13

Air

glass

Bending Light - Refraction

Mike Turner, May 2004

slide14

Apparently bent pencil

actual pencil

Fill a beaker with water, put your pencil in it and look at the pencil from above, draw the diagram below to show what you see and try to draw in the light rays.

Bent Pencil

refraction

Mike Turner, May 2004

slide15

Apparently bent pencil

actual pencil

The ray of light seems to come from here, this is where you see the pencil

refraction

Mike Turner, May 2004

slide16

Refraction At Work

A fish swimming underwater looks to an observer above the water to be in a different position to where it actually is. Remember the coin in the cup trick!

Looking into water

Click to move on

Mike Turner, May 2004

slide17

White Light?

  • What we think of as white light is actually a mixture of all colours of light.
  • Sir Isaac Newton decided that white light is made up of seven colours :
  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet
  • We now know that the human eye can distinguish millions of different colours and shades.

We can use a saying such as :

RichardofYorkgavebattleinvain

✩Try to think of your own saying to help you remember the colours of a spectrum.

Newton's prisms

Click to move on

Mike Turner, May 2004

slide18

Prisms

Red

Orange

Yellow

Green

Blue

Indigo

Violet

White light

Dispersion of white light to produce a spectrum occurs because some colours refract more than others.

Mike Turner, May 2004

slide19

Red Filter

BlueFilter

Colour Filters

When white light is shone towards a colour filter, not all of it is transmitted through the filter.

The red light passes through the filter.

All the other colours are absorbed.

This time blue light passes through the filter.

All the other colours are absorbed.

Click to move on

Mike Turner, May 2004

slide20

Mixing Colours

We can mix coloured light to form other colours. Mixing light is very different to mixing paints.

The three primary colours are red, green and blue. These are the only colours that a television screen produces. The whole picture is made up of red, green and blue dots!

When mixed in pairs these produce the three secondary colours, cyan, magenta and yellow (these are the three coloured inks you find in a colour printer).

Click to move on

Mike Turner, May 2004

slide21

The Primary Colours

Click to move on

Mike Turner, May 2004

red blue

Magenta

Red + Blue =

Click to move on

Mike Turner, May 2004

red green

Yellow

Red + Green =

Click to move on

Mike Turner, May 2004

blue green

Cyan

Blue + Green =

Click to move on

Mike Turner, May 2004

red green blue

White

Red + Green + Blue =

Mixing Primary Colours

Click to move on

Mike Turner, May 2004

slide26

Coloured Objects

A green object looks green because it reflects green light and absorbs the other colours.

Can you explain these examples?

Click to move on

Mike Turner, May 2004

slide27

Why do different objects look different colours?

  • We see non-luminous objects because they reflect light into our eyes.

✩RED objects look red becausethey reflect only RED light

✩BLUE objects look BLUE because they reflect only BLUE light

Click to move on

Mike Turner, May 2004

slide28

Remember!

Only RED light can get through a RED filter

Only BLUE light can getthrough a BLUE filter

Click to move on

Mike Turner, May 2004

blue object and blue filter
BLUE object and BLUE filter

BLUE objects reflect only blue light

So it looks BLUE

A BLUE filter lets blue light through

Click to move on

Mike Turner, May 2004

red object and red filter
RED object and RED filter.

So it looks RED

RED objects reflect only RED light

A RED filter lets RED light through

Click to move on

Mike Turner, May 2004

red object and blue filter
RED object and BLUE filter

BLUE filters only let through BLUE light. The RED light gets absorbed

RED objects reflect only RED light

No light gets here so the object looks BLACK

Click to move on

Mike Turner, May 2004

slide32

1. Draw a diagram to show how we see a non-luminous red object in white light, your diagram should show why the object appears red.

The bulb produces white light.

TheREDobject reflectsREDlight into our eye, all other colours of light are absorbed by the object.

Click to move on

Mike Turner, May 2004

slide33
2. What colour would a red object appear in red light?

RED

3. What colour would a blue object appear through a blue filter?

BLUE

4. Why do black objects appear black in white light?

Because they absorb all colours of light.

5. What colour would a blue object appear through a red filter?

BLACK

Click to move on

Mike Turner, May 2004

slide34

You have now reached the end of the revision presentation for the ‘Light’ topic.

I hope you have found it useful. If you have any ideas to improve this presentation please let Mr. Turner know.

Thank you for looking at this work, and good luck in your test.

Click to finish

Mike Turner, May 2004