Course: Separate Physics only | Tier: Foundation + Higher | See also: Lenses and Visible Light
Notes
White light and the visible spectrum
White light is a mixture of all the colours of the visible spectrum: red, orange, yellow, green, blue, indigo, violet. Each colour has its own narrow band of wavelengths and frequencies. Red has the longest wavelength (lowest frequency) and violet has the shortest wavelength (highest frequency).
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- The colour of an opaque object depends on which wavelengths of light it reflects most strongly. The other wavelengths are absorbed.
- A white object reflects all wavelengths equally.
- A black object absorbs all wavelengths.
- Transparent objects transmit light so you can see clearly through them.
- Translucent objects transmit light but scatter it, so you can’t see clearly through them.
- A colour filter transmits only its own colour (wavelengths) and absorbs all the others.
Working out what colour something looks
Ask two questions: What colour light is reaching the object? and Can the object reflect that colour? If it can, it looks that colour. If it can’t, it absorbs the light and looks black.
| Object | Light shining on it | Looks |
|---|---|---|
| Red | White | Red |
| Red | Red | Red |
| Red | Blue | Black |
| White | Green | Green |
| Black | Any colour | Black |
Filters
White light through a red filter comes out red, because the filter absorbs all the other colours. If that red light then hits a blue filter, the blue filter absorbs it, so no light gets through.
Questions
Q1 (F) Which colour of visible light has the longest wavelength?
Show answer
Red.
Q2 (F) Why does a white object look white in sunlight?
Show answer
It reflects all wavelengths (colours) of light equally.
Q3 (F) What is the difference between a transparent and a translucent material?
Show answer
Both transmit light. A transparent material lets you see clearly through it; a translucent material scatters the light, so you can’t see clearly through it.
Q4 (F) What does a green filter do to white light?
Show answer
It transmits green light and absorbs all the other colours.
Q5 (F/H) A blue shirt is viewed under red light. What colour does it look, and why?
Show answer
Black. The shirt only reflects blue light. It absorbs the red light and there is no blue light for it to reflect.
Q6 (F/H) A sheet of white paper is lit with red light. What colour does it look?
Show answer
Red. White paper reflects all colours, but only red light is reaching it.
Q7 (H) White light passes through a red filter and then a blue filter. Explain what you would see.
Show answer
No light (darkness). The red filter absorbs every colour except red. The blue filter then absorbs the red light, so nothing is transmitted.
Q8 (H) A green car is seen through a green filter in sunlight. A red car is seen through the same filter. Compare how they look and explain.
Show answer
The green car looks green: it reflects green light, which passes through the filter. The red car looks black: it reflects red light, which the green filter absorbs, so no light from it reaches your eye.
