Light reflecting off of a polished or mirrored surface obeys the law of reflection: the angle between the incident ray and the normal to the surface is equal to the angle between the reflected ray and the normal.
Precision
optical systems use first surface mirrors that are aluminized on
the outer surface to avoid refraction, absorption, and scatter
from light passing through the transparent substrate found in
second surface mirrors.
When light obeys the law of reflection, it is termed a specular reflection. Most hard polished (shiny) surfaces are primarily specular in nature. Even transparent glass specularly reflects a portion of incoming light.
Diffuse reflection is typical of particulate substances like powders. If you shine a light on baking flour, for example, you will not see a directionally shiny component. The powder will appear uniformly bright from every direction.
Many
reflections are a combination of both diffuse and specular
components. One manifestation of this is a spread
reflection, which has a dominant directional component that is
partially diffused by surface irregularities. 
When light passes between dissimilar materials, the rays bend and change velocity slightly, an effect called refraction. Refraction is dependent on two factors: the incident angle, q, and the refractive index, n of the material, as given by Snell’s law of refraction: n sin(q) = n’ sin(q’)
For a typical air-glass boundary, (air n = 1, glass n’ = 1.5), a light ray entering the glass at 30° from normal travels though the glass at 10.5° and straightens out to 30° when it exits out the parallel side.
Note that since sin(0°) = 0, light entering or exiting normal to a boundary does not bend. Also, at the internal glass-air boundary, total internal reflection will occur when n’sin(q’) = 1. This occurs at q’ = 41.8° for n’ = 1.5 glass.
The index
of refraction itself is also dependent on wavelength. This
angular dispersion causes blue light to refract more than red,
causing rainbows and allowing prisms to separate the spectrum.
Diffraction is another wave phenomenon that is dependent on wavelength. Light waves bend as they pass by the edge of a narrow aperture or slit. This effect is approximated by: q = l / D
where q is the
diffraction angle, l the wavelength of radiant energy, and D the
aperture diameter. This effect is negligible in most
optical systems. A diffraction grating uses the interference of
waves caused by diffraction to separate light angularly by
wavelength. Narrow slits then select the portion of the
spectrum to be measured. The narrower the slit, the
narrower the bandwidth that can be measured. However,
diffraction in the slit itself limits the resolution that can
ultimately be achieved.
