AQA GCSE Viewing objects through filters

AQA GCSE Viewing objects through filters(Physics)

Viewing objects

We see objects because light from a source (e.g. sun, lamp) will strike an object, then that light is reflected into our eyes.

In the case below white light strikes the white ball, the white will will absorb none of the light and all of the white light is reflected into our eyes. So, we see the ball as white.

Coloured Filters

Coloured filters absorb some of the wavelengths of light, but allow other wavelengths of light to be transmitted.

In the example below, white light strikes the red filter. The red filter absorbs all of the wavelengths of light other than red. It will allow red light to pass through. As a result we see red light transmitted from the filter.

Below is a summary table, showing which wavelengths of light are absorbed and transmitted for different filters.

Colour of filter	Colour of light entering filter	Colour of light absorbed by filter	Colour of light transmitted from filter
Red	White	Blue & Green	Red
Green	White	Red & Blue	Green
Blue	White	Red & Green	Blue
Magenta	White	Green	Red & Blue (Magenta)
Yellow	White	Blue	Red & Green (Yellow)
Cyan	White	Red	Blue & Green (Cyan)
Black	White	Red, Green & Blue	None

Viewing Objects through filters.

In the example below, white light shines onto the white ball. The white ball will absorb no wavelengths of light, it will reflect all the light as white light.

This white light then passes through a red filter, which absorbs all the wavelengths of light apart from red. It will transmit wavelengths due to red, so the person will see the ball as red through the filter.

You can use the table above to predict the colour of the object that is seen when looking at the object assuming that it is white light that travels from the object to the filter.

If white light travels from the object to the filter, the colour of the object will always be the colour of the filter!

What happens if a different colour of light shines onto the object?

This makes it slightly more tricky to deal with.

In the example below, red light strikes the white ball, which absorbs none of the wavelengths of light. The white ball will reflect he wavelengths due to red light.

When the red wavelenghts of light strike the green filter, they are absorbed and no wavelenghts of light are transmitted. So, the ball is seen as black by the person’s eye when they look at the filter.

The easy way to work it out, is to remember that the colour of the filter is the wavelengths of light that it will allow to pass through.

The summary table below might help you

If no light passes through, the object appears as black!

Colour of filter	Wavelengths of light that can pass through
Red	Red
Green	Green
Blue	Blue
Magenta(red and blue)	Magenta, red or blue
Cyan(blue and green)	Cyan, blue or green
Yellow(red and green)	Yellow, red or green

Practice Question

1.White light travels from an object to a magenta filter. When a person looks at the magenta filter what colour do they see the object to be?

2. Blue light travels from an object to a Magenta filter. When a person looks at the magenta filter what colour does the object appear to be?

3. White light passes through a blue filter before striking a white ball. The reflected light from the ball then strikes a red filter. A person then looks at the red filter. What colour does the ball appear to be in the red filter?