The cholesteric liquid crystal molecules, so named because of their discovery in cholesterol derivatives, have a parallel alignment in the same plane, but in the next layer, the alignment angle of the molecules is slightly out of phase with the previous plane . This phase shift in a regular sequence gives a helical structure to the molecular stack. We then observe a selective reflection whose wavelength, and therefore colour, will depend on the magnitude of the pitch of the helix and the angle of sight. The reflected light will be polarized circularly. As the temperature increases, the pitch of the helix shortens, allowing reflection of shorter wavelengths. Here, cholesteric liquid crystals are on a black disk to block light coming from behind. A finger touched the disc when taking the photograph. The blue-green zone shows a higher temperature at the point of contact.