Colour subtraction, absorption and reflection guide for KS3 physics students

Key points

Colours can be subtracted from light using filters, or by .
Filters transmit certain colour and absorb others.
Coloured objects reflect certain colours and absorb others.

Filters

Filters transmit some colours of light and absorb other colours of light.

Colours which are transmitted pass through the filter and come out the other side.
Colours which are absorbed do not pass through the filter.

The colours which are absorbed by the filter are subtracted from the light.

A photo of different colours of stage lights beaming down onto a smoky stage. — Image caption,
Filters transmit some colours of light and absorb other colours of light

This effect can be demonstrated using red/green 3-D glasses or coloured cellophane – like the type used to wrap some sweets.

Viewing coloured objects, or images through different coloured pieces of cellophane can cause objects to appear a different colour. They may even seem to disappear.

This is because the cellophane acts as a filter, partially or completely absorbing some colours of light, while other colours are transmitted.

A woman stood in the wilderness holding up a large piece of red and a large piece of green cellophane in front of her - It makes the images behind her appear different — Image caption,
Different colours of light transmit or are absorbed by different coloured filters

It can be useful to think about the primary colours of light when working out which colour of light will be transmitted by a filter. Remember that white light is made up of red, green and blue light.

Three circles overlapping: one red, one green, one blue. Where red and green overlap, there is yellow, where green and blue overlap, there is cyan, where red and blue overlap, there is magenta. In the middle, where all of the colours overlap, there is white. — Figure caption,
Colour wheel showing additive mixing of primary colours of light

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John Chase explore how different colours reflect different colours of light

Filters: primary colours

Red, green and blue filters each transmit a single primary colour. Any other colours present in the light are absorbed.

A red filter only transmits red light. It absorbs blue light and green light.
A blue filter only transmits blue light. It absorbs red light and green light.
A green filter only transmits green light. It absorbs red light and blue light.

Example 1

If white light shines at a red filter, red light passes through the filter. This is because:

the red light is transmitted
the blue light and green light are absorbed

A diagram of a red filter: red, green and blue light rays travelling towards filter. The red light ray is transmitted and passes through the filter, but the green and blue light rays are absorbed in the filter. — Image caption,
A red filter only transmits red light. It absorbs blue light and green light.

Example 2

If yellow light shines at a red filter, red light passes through the filter. This is because:

yellow light is a combination of red light and green light
the red light is transmitted
the green light is absorbed

Image of a red filter: red and green light rays travelling towards filter. The red light ray is transmitted and passes through the filter, but the green ray is absorbed in the filter. — Image caption,
Yellow light is a combination of red and green light. The red filter absorbs the green light, but allows the red light to pass through.

Example 3

If cyan light shines at a red filter, no light passes through the filter. This is because:

cyan light is a combination of blue light and green light
the blue light and green light are absorbed
there is no red light to be transmitted

An image of a red filter: blue and green light rays travelling towards filter. Both rays are absorbed in the filter and no light is transmitted. — Image caption,
Cyan light is a combination of blue and green light. The red filter absorbs blue light and green light, so no light passes through the filter.

Filters: secondary colours

Cyan, magenta and yellow filters transmit two primary colours. Any other colour present in the light is absorbed.

A cyan filter transmits blue light and green light. It absorbs red light.
A magenta filter transmits red light and blue light. It absorbs green light.
A yellow filter transmits blue light and green light. It absorbs red light.

Example 1

If white light shines at a cyan filter, cyan light passes through the filter. This is because:

the blue light and green light have been transmitted
the red light has been absorbed

An image of a cyan filter – red, green and blue light rays travelling towards filter. The blue and green rays are transmitted and pass through the filter, but the red light ray is absorbed in the filter. — Image caption,
A cyan filter transmits blue light and green light. It absorbs red light.

Example 2

If yellow light shines at a cyan filter, green light passes through the filter. This is because:

yellow light is a combination of red light and green light
the red light is absorbed
the green light is transmitted

A image of a cyan filter red and green light rays travelling towards filter. The green light ray is transmitted and passes through the filter, but the red ray is absorbed in the filter. — Image caption,
Yellow light is a combination of red and green light. The cyan filter absorbs the red light, but allows the green light to pass through.

Example 3

If green light shines at a magenta filter, no light passes through the filter. This is because:

the green light is absorbed
there is no red or blue light to be transmitted

An image of a magenta filter – green light rays travelling towards filter. The green light ray is absorbed and doesn't pass through the filter. — Image caption,
Green is a primary colour. The magenta filter absorbs the green light and no light passes through.

Subtracting colours – diffuse reflection

Diffuse reflection from surfaces can also subtract colours from light.

White surfaces reflect all colours of light, which is why they appear white.
Black surfaces absorb all colours of light, which is why they appear black.
Coloured surfaces reflect some colours and absorb others.

Image showing red, green and blue rays hitting a white surface (all rays reflected) and a black surface (all rays absorbed). — Image caption,
Black surfaces absorb all colours of light. White surfaces reflect all colours of light.

When looking at a coloured surface, the colours which are reflected enter the eye, which determines the colour the surface appears.

Image showing red, green and blue rays hitting red, green and blue surfaces, with the corresponding colour of light being reflected. — Image caption,
Red surfaces only reflect red light, blue surfaces only reflect blue light, and green surfaces only reflect green light.

Image showing red, green and blue rays hitting cyan, magenta and yellow surfaces, with the two primary colours that make that secondary colour of light being reflected — Image caption,
The two primary colours that make that secondary colour of light are the ones reflected by a secondary coloured surface.

Just like filters, it can be useful to think about the primary colours of light when working out which colour of light will be reflected by a surface.

Example 1

If white light shines at a blue surface, it appears blue. This is because:

the blue light is reflected and enters the eye
the red light and green light are absorbed

Red, green and blue light rays travelling towards red surface. The blue light ray is reflected, but the red and green light rays are absorbed. — Image caption,
A blue surface only reflects blue light, which is why it appears blue.

Example 2

If cyan light shines at a red surface, no light is reflected and the object appears black. This is because:

cyan light is a combination of blue light and green light
the blue light and green light are both absorbed
there is no red light to be reflected

Blue and green light rays travelling towards red surface. Both rays are absorbed in the filter. — Image caption,
Cyan light is a combination of blue and green light. Both these colours are absorbed by the red surface, so no light is reflected – and the surface appears black.

Example 3

If magenta light shines at a yellow surface, it appears red. This is because:

magenta light is a combination of red light and blue light
the red light is reflected and enters the eye
the blue light is absorbed

Red and blue light rays travelling towards a yellow surface.The red ray is reflected by the surface and the blue ray is reflected. — Image caption,
Magenta light is a combination of red and blue light. The yellow surface absorbs the blue light, and reflects the red light – so the object appears red.

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