Dispersion of light. Spectral analysis
In 1666 Newton used a prism to show that white sunlight splits into seven colours (red, orange, yellow, green, light blue, blue, violet) and with a second prism recombined them into white light; this is the dispersion of light. The cause: the refractive index n of a substance depends on wavelength (frequency); since n = c/v, rays of different colours travel at different speeds in a medium: red fastest and violet slowest, which is why red is refracted least and violet most. Visible light has wavelengths from about 380 nm (violet) to 760 nm (red). In vacuum all rays travel at the same speed, so there is no dispersion. Spectra: continuous (hot solids and liquids, an incandescent lamp, the Sun), band (molecules) and line (individual atoms in rarefied gases); an absorption spectrum has dark lines on a continuous background because a gas absorbs the frequencies it emits when heated. Each chemical element has its own line spectrum, a kind of «fingerprint», so determining the composition of a substance from its spectrum, spectral analysis, is very sensitive and also gives the composition of the Sun and stars. Helium was first found in the solar spectrum.
At home: fill a clear glass with water, place it in the light from a window and look for rainbow colours on the wall; or make a spectrum from a lamp on a CD surface. Do not look at the Sun, and never look at the Sun through a prism or binoculars. Observing a salt flame spectrum must be done only under adult supervision in suitable conditions.