How do we know that light is a wave, and what are colours?
Around 1801 the English physician Thomas Young let sunlight pass through two narrow slits close together, and looked at the light falling on a screen behind them. Instead of two bright lines he saw a whole row of bright and dark stripes, called fringes.
Particles could not do that: two openings should give two bright patches. Waves do it naturally. The waves from the two slits overlap, and where crest meets crest the light is brighter, while where crest meets trough it cancels into darkness. Young had shown that light is a wave.
You move the two slits closer together. What happens to the fringes on the screen? The spacing of the fringes is λ L / d: the smaller the distance d between the slits, the wider the fringes. Try it in the figure.
The colours we see are simply different wavelengths. Violet light has a wavelength of about 400 nanometres, red light about 700. In 1865 James Clerk Maxwell showed that light is an electromagnetic wave: a ripple of electric and magnetic fields that travels at 299,792 kilometres per second.
Visible light is only a thin slice of a much wider family. Longer wavelengths give infrared, microwaves and radio waves; shorter ones give ultraviolet, X-rays and gamma rays. They are all the same thing as light, with different wavelengths.
Bright fringes appear where the paths from the two slits differ by a whole number of wavelengths: d sin θ = m λ, with m = 0, 1, 2…
On a screen at distance L, bright fringes are spaced λ L / d apart. Red light (650 nm), slits 0.25 mm apart, screen 1 m away: 2.6 mm between fringes.
Frequency of light: f = c / λ. Green light (535 nm) oscillates about 560 thousand billion times a second (5.6 × 10^14 Hz).
Light makes interference fringes, so it is a wave: an electromagnetic wave, and each colour is a different wavelength.