Colour n lenses

CCOOLLOOUURR ANDAND LENSESLENSES

ColourColour

White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.

We can demonstrate this by splitting white light with a prism:

This is how rainbows are formed: sunlight is “split up” by raindrops.

The colours of the rainbow:The colours of the rainbow:

RedOrangeYellowGreenBlue

IndigoViolet

Adding coloursAdding coloursWhite light can be split up to make separate

colours. These colours can be added together again.

The primary colours of light are red, blue and green:Adding blue and

red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white again

Adding red and green makes yellow

Seeing colourSeeing colourThe colour an object appears depends on the

colours of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is

reflected

A white hat would reflect all seven colours:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and

blue):

Purple light

White

light

Using coloured lightUsing coloured light

If we look at a coloured object in coloured light we see something different. For example, consider a football kit:

White

light

Shorts look blue

Shirt looks red

In different colours of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Some further examples:

Object Colour of lightColour object seems to be

Red socks

Red Red

Blue Black

Green Black

Blue teddy

Red Black

Blue

Green

Green camel

Red

Blue

Green

Magenta book

Red

Blue

Green

Using filtersUsing filtersFilters can be used to “block” out different colours of

light:

Red Filte

r

Magenta

Filter

Investigating filtersInvestigating filters

Colour of filter Colours that could be “seen”

Red

Green

Blue

Cyan

Magenta

Yellow

Red

Magenta

White

Yellow

Blue Green

Cyan

Concave and Convex Concave and Convex LensesLenses

Look at the convex and concave lenses below:

The curved surface/surfaces of a lens bends the light. 

Notice the paths of light of the convex and concave lenses shown below.

Convex LensesConvex Lenses

A convex lens is thicker in the middle.A convex lens bends light rays so that

theymeet at a focal point. This means that theLight rays are converging.

Concave LensConcave Lens

A concave lens is thinner in the middle.A concave lens bends light rays to make them spread out. This means that the light rays are diverging.

Corrective LensesCorrective Lenses

The Eye (pg. 229)

Vis

ion

Near-sighted VisionNear-sighted Vision

•See objects up close but not at a distance.

•Concave lenses are used to correct this vision.

Near-

sig

hte

d

Vis

ion

Far-sighted VisionFar-sighted Vision

•See objects at a distance but not up close.

•Convex lenses are used to correct this vision.

Far-

sig

hte

d V

isio

n