Shortcuts form the basis for many other menu structures in Windows 9x. The desktop approach to displaying a hierarchy of windows onscreen was originally developed for the ALTO computer. It is usually placed on top of other windows and more distinctly colored. Mouse pointers may also change appearance when over a window with focus. The study of optics is divided into geometrical and physical optics.
Geometrical optics applies the laws of reflection and refraction of light to the design of lenses .
Physical optics studies polarization, interference , diffraction , and the emission, composition, and absorption of light. The concept of photons is used to explain the interactions of light and matter that produce a change in the form of energy. The concept of transverse waves is used to explain the passage of light through substances and some facts of image formation. Transverse waves may be described as a set of points moving back and forth perpendicularly to the direction the wave is advancing. A wave's frequency is the number of complete motions back and forth per second of any point on the wave.