Radiations considered waves emitted by a heated object ,span from zero to infinitely long wavelength. These waves carry electric anf magnetic fields while traveling from one place to another and hence the name. The distance between the adjacent peaks in a wave is called Wavelength. For most experiments with light, it seemed to behave as a wave. However, there are a class of experiments in which light behaves as particles.
Light when treated as particles, is named Photon.
Ordinary lenses suffer from defects in that the light passing through these lenses are not focussed at one point for all colors. This defect is eliminated by designing the lens with additional elements, such lens is called Achromatic lens. Ability to rotate the telescope to compensate for the westward motion of the stars so that a star is always in focus. These two seemingly complicated movements of the telscope are efficiently made by Computers, guiding the telescope. Thin mirrors used to reduce the weight of telescope.