Colour of lightIB MYP Physics: Revision notes
Section 1
White light and dispersion
White light is a mixture of all the colours of the spectrum: red, orange, yellow, green, blue, indigo and violet. When white light passes through a glass prism, each colour is refracted by a different amount. Violet bends the most and red the least, so the colours spread out. This is called dispersion.
A rainbow is formed when raindrops disperse sunlight.
Section 2
Primary and secondary colours of light
The primary colours of light are red, green and blue. They cannot be made by mixing other colours. Mixing them gives the secondary colours:
- red + green = yellow
- red + blue = magenta
- green + blue = cyan
- red + green + blue = white
This is different from mixing paints. Mixing lights adds colours together, so the result is brighter.
Do not use the paint colours red, yellow and blue for light. Light mixing uses red, green and blue.
Section 3
Colour filters
A colour filter is a piece of transparent coloured material. It lets through (transmits) light of its own colour and absorbs the other colours.
- A red filter transmits red and absorbs green and blue.
- A magenta filter transmits red and blue and absorbs green.
- A yellow filter transmits red and green and absorbs blue.
- A cyan filter transmits green and blue and absorbs red.
If white light goes through a red filter then a green filter, almost no light gets through.
Section 4
The colour of objects
We see an object because it reflects light into our eyes. The colour of an object depends on which colours it reflects and which it absorbs.
- A white object reflects all colours.
- A black object absorbs all colours.
- A red object reflects red and absorbs the others.
- A yellow object reflects red and green (which mix to look yellow) and absorbs blue.
A coloured object looks its normal colour only in white light, which contains every colour.
Section 5
Objects under coloured light
An object can only reflect colours that are in the light falling on it.
- A white object takes on the colour of the light: it looks red in red light and green in green light.
- A red object in red light looks red, but in green or blue light it looks black, because it absorbs those colours and there is no red to reflect.
- A yellow shirt in red light looks red, in green light looks green, and in blue light looks black.
To work out an answer: list the colours the object reflects, then list the colours in the light. The colour seen is where they overlap. If there is no overlap, the object looks black.
Check every colour-of-object question with two lists: what the object reflects, and what the light contains. The overlap is the colour you see.
Must know
- White light is a mixture of colours; a prism disperses it into a spectrum.
- Primary colours of light: red, green, blue. Secondary: yellow, magenta, cyan.
- Filters transmit their own colour and absorb the others.
- Objects reflect their own colour and absorb the rest; white reflects all, black absorbs all.
- Under coloured light an object can only reflect colours that are present.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Colour of light
- A student shines a narrow beam of white light through a glass prism in a darkened room. A band of different colours appears on a white screen on the other side of the prism, with red at one end and violet at the other.Explain why the prism splits the white light into different colours.2 marks
- A theatre lighting technician is setting up spotlights on a white screen. She has three spotlights that shine pure red, pure green and pure blue light. She also has a magenta filter, which she can place in front of a spotlight that gives white light.Describe what happens to red, green and blue light when white light passes through the magenta filter.2 marks
- A student investigates how objects look under different coloured lights in a dark room. She uses lamps that give pure red, pure green and pure blue light, and three objects: a white sheet of paper, a red apple and a blue book. Under green light, the white sheet looked green and the red apple looked black. Under red light, the white sheet looked red and the blue book looked black.Identify the independent variable, the dependent variable and one control variable in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).