It is often necessary to diffuse light, either through transmission or reflection. Diffuse transmission can be accomplished by transmitting light through roughened quartz, flashed opal, or polytetrafluoroethylene (PTFE, Teflon). Diffusion can vary with wavelength. Teflon is a poor IR diffuser, but makes an excellent visible / UV diffuser. Quartz is required for UV diffusion.
Lenses are often employed to redirect light or concentrate optical power. The lens equation defines the image distance q, projected from a point that is a distance p from the lens, based on the focal distance, f, of the lens. The focal distance is dependent on the curvature and refractive index of the lens. Simply put, all rays parallel to the optical axis pass through the focal point. Since index of refraction is dependent on wavelength, chromatic aberrations can occur in simple lenses.
When light reflects off of a rear surface mirror, the light first passes through the glass substrate, resulting in reflection losses, secondary reflections, and a change in apparent distance. First surface mirrors avoid this by aluminizing the front, and coating it with a thin protective SiO coating to prevent oxidation and scratching.
Concave mirrors are often used to focus light in place of a lens. Just as with a lens, a concave mirror has a principal focus, f, through which all rays parallel to the optical axis pass through. The focal length of a spherical concave mirror is one half the radius of the spherical surface. Reflective systems avoid the chromatic aberrations that can result from the use of lenses.
Prisms use glass with a high index of refraction to exploit the variation of refraction with wavelength. Blue light refracts more than red, providing a spectrum that can be isolated using a narrow slit.
Internal prisms can be used to simply reflect light. Since total internal reflection is dependent on a difference in refractive index between materials, any dirt on the outer surface will reduce the reflective properties, a property that is exploited in finger print readers.
