Colour (cont.d)

Lesson aim To understanding that white light is made from different colours To understand how coloured filters work and why we see coloured objects as we do.

Learning Outcomes By the end of the lesson pupils should know:

• how coloured filters change white light

• to combine knowledge from different sources to explain how coloured filters work

•how coloured objects appear in white light and in different colours of light

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?

Remember: splitting white light up

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

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 black 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.

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

True or false?

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 a girl. 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 choose the correct shirt and trousers to represent what those clothes would look like in each 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.

• HomeworkSuggest what clothes you would wear on a

night out, where the lighting on the dance floor was mainly red and green.

Draw an outfit you might wear, explaining why you chose those colours.

Revise for a test next lesson

Flag colours

For the flag shown, draw 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