Mirrors 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 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.
Internal Transmittance Filter manufacturers usually provide data for a glass of nominal thickness. Using Bouger s law, you can calculate the transmission at other thicknesses. Manufacturers usually specify Pd, so you can calculate the external transmittance from internal transmittance data. Prisms 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.