A prism turns white light into a rainbow, and the visible rainbow is only a sliver of the whole electromagnetic spectrum.
Dispersion is the splitting of white light into its colours by a prism. The seven colours, in order from least to most bent, are red, orange, yellow, green, blue, indigo and violet (VIBGYOR read from the other end). Different colours travel at different speeds in glass, so glass has a different refractive index for each: red is bent the least and violet the most. A second prism placed the other way round recombines the colours into white light.
Which colour is bent the most by a prism, and which the least?
Violet is bent the most and red the least
Why does a prism split white light into colours?
The colours travel at different speeds in glass, so each is refracted by a different amount
An opaque object has the colour of the light it reflects and absorbs the rest. A red rose reflects red and absorbs the other colours, so in green light it appears black. The primary colours of light are red, green and blue, which together make white. Mixing two primaries gives a secondary colour: red + green = yellow, green + blue = cyan, and red + blue = magenta.
What colour does a red rose appear in green light?
Black, because it absorbs green light and has no red light to reflect
What colour is produced by mixing red and green light?
Yellow
In order of decreasing wavelength, the electromagnetic spectrum is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All travel at 3 × 10⁸ m/s in vacuum, and speed = frequency × wavelength (c = fλ). Infrared is used in remote controls, microwaves in cooking and radar, ultraviolet in sterilising, X-rays in medical imaging, and gamma rays in treating cancer.
Light has a wavelength of 500 nm (5 × 10⁻⁷ m). Find its frequency.
f = c ÷ λ = 3 × 10⁸ ÷ 5 × 10⁻⁷ = 6 × 10¹⁴ Hz
A radio wave has a wavelength of 3 m. Find its frequency.
f = 3 × 10⁸ ÷ 3 = 1 × 10⁸ Hz
Which electromagnetic waves have the shortest wavelength?
Gamma rays
1. What is dispersion of light?
2. Name the colours of the spectrum in order.
3. Which colour is deviated the most by a prism?
4. Which colour is deviated the least by a prism?
5. Why does dispersion occur in glass?
6. Name the primary colours of light.
7. What colour results from mixing red and green light?
8. Which electromagnetic waves are used in television remote controls?
9. Light has a wavelength of 500 nm. What is its frequency? (c = 3 × 10⁸ m/s)
10. What is the speed of all electromagnetic waves in vacuum?