The optical system, comprised of reflectors and lenses, is an essential component of all theatre luminaires and its function is to condense as much light as possible into the light beam and to focus, direct and shape the beam of light. The smaller the light source, the more condensed its light will be and the more efficiently it is manipulated by the optical system  .
The optical systems of theatrical luminaires implement the principles of optics -  transmission, absorption, refraction and reflection.

Transmission of light occurs in transparent mediums such as air, glass and water. The rays of light emitted by a light source travel in a straight line through the air until they reach a different medium. See more.		Absorption - when light hits an opaque, non reflective surface, it is absorbed by the material, converted into another form of energy (usually heat but sometimes electric or chemical energy) and emitted in its new form.See more.
Reflection - reflective opaque surfaces such as mirrors or a lake of water cast the rays of light back, in a different direction from that at which they hit the surface, unless perpendicular to the surface. See more.		Refraction - as light passes from one transparent medium to another, (for instance from air to glass) it is refracted, i.e. it changes direction by a certain angle at the point of incidence. See more.

The drop of water transmits light, refracting a few rays. The ice reflect most of the light, while the dark branches absorb it.

Most materials combine these principles, and transmission, refraction, absorption and reflection take place simultaneously  in different proportions. Differences in medium and surface cause one of the principles to be more significant than the others.  For example, when the same amount of light hits two opaque surfaces - one light and the other dark, the light surface will reflect the vast majority of the light hitting it, and only a small portion of the light will be absorbed and converted to heat. However the dark surface will convert most of the light to heat and will reflect only a small portion of the light. This is why when you wear a black T-shirt on a hot summer day you get much hotter than when wearing a white T-shirt. When light hits a glass surface most of the light is refracted and transmitted, a small portion is reflected, and a small amount is  converted to heat. If the glass is colored, less of the light will be reflected and more converted to heat.Conventional glass or plastic color mediums (filters) reflect only very little of the blocked light while converting most of it into heat. Dichroic filters reflect most of the light which is not transmitted. Photoelectric cells convert the absorbed light into electric energy.