Rainbows and spectra

You may be able to see a fainter, secondary spectrum which is produced because some light is reflected inside the prism and comes out in a different place.

Isaac Newton identified seven colours: red; orange; yellow, green, blue, indigo and violet. Whether it is possible to distinguish accurately between blue and indigo is open to question so you can remember the colours in their correct order by reciting either:

Richard Of York Gave Battle In Vain

Or

Richard Of York Gave Battle Vainly

A spectrum can also be produced using a diffraction grating which is a transparent surface on which very fine lines have been engraved.

This in effect acts as a collection of very small prisms set very close together and produces a spectrum when white light is shone through it.

A similar effect can be observed when white light is reflected back from the surface of a CD. Indentations in the disc form a diffracting surface which being solid reflects a spectrum rather than transmitting the light.