When a light ray hits the surface of a transparent medium at an angle, the ray is bent at the point of incidence as it crosses into the different medium. This change in the path of the light is known as refraction. A common example of refraction can be observed when a pencil is placed in a glass of water. The part of  the pencil inside the water seems to be bent in relation to the part out of the water. The extent and direction of refraction of light depends on the angle of incidence. A ray of light passing through a pane of glass will bend once, towards a line perpendicular to the plane of the glass at the point of incidence, and again as it emerges from the glass, this time being deflected away from the perpendicular . If both sides of the pane of glass are parallel, the angle of refraction will be the same at either side of the pane of glass, so that the ray will emerge parallel to the original ray but at a certain distance.

The extent of refraction also depends on the optical density of the medium. Different mediums cause different changes in direction - the extent of deflection of light in air, water or glass is different. Optical density effects the speed at which the light travels through the medium. The higher the optical density, the slower the light speed. In a vacuum, and at a good approximation in air, light travels at about 300,000 km per second, whereas in glass or water it travels more slowly - 200,000 km per second in glass and 225,000 km per second in water.  The optical density of  transparent mediums is expressed by its Index of Refraction. The denser the medium is, the higher its refractive index will be. The lowest refractive index is 1, used for air or a vacuum, where light travels fastest. Glass has an approximate refractive index of 1.5 (depending on the type of glass) and water of 1.33 . These principles are applied in lenses, which can converge or refract the light at a particular angle, depending on the curvature of the lens.

Converging Light by Refraction

When the surface of incidence and the surface of emergence of a transparent medium such as glass are not parallel, a ray of light passing through the medium will emerge at an angle which is not identical to the angle of incidence .
The laws of refraction are employed when designing lenses for theatre luminaires. When a ray of light passes through a curved surface, it will bend away from the perpendicular at the point of exit . The curvature of the lens is calculated precisely in order to bend the rays of light in the desired direction, creating a focused and directional beam of light . The lenses of theatre luminaires use plane, convex, or concave surfaces in order to create different patterns of light beams.
The degree of deflection, and thus the "bending power" of a lens, depends on the following factors:

• The angle of incidence of the light.
• The curvature of the lens face. The more curved it is, the more it bends the light.
• The index of refraction of the material from which the lens is made.
• The wavelength of the light ray. Short waves (blue-violet) are deflected more than longer waves(red).    

 

As the light emerges from the prism it bands way from the perpendicular to the surface, those enlarging the angle.
		The prism refracts the different wavelength comprising white light at difrent angle, so that we see each color of the spectrum separately

Converging lenses have convex surfaces and are thick at the center and thin at the edges. They collect the rays of light to a real focal point. When a beam of light hits a converging lens, it concentrates the light rays.		Diverging lenses have concave surfaces and are thin at the center and thick at the edges, causing the light to spread out.
When a light beam passes through converging lenses, the lens concentrates the rays of light.		When a light beam passes through a diverging lens, the lens spreads the rays of light.

When a point light source is placed on the optical axis at the focal point of the lens , the emerging light beam will be parallel. When the source is moved towards the lens , the light will converge further. When the source is moved away from the lens , the light will spread out. Owing to the difference in convergence of different light waves,chromatic aberration is caused which results in a ring of rainbow colors round the edge of the beam of light, particularly when sharply focused. This can be rectified by combining two different types of glass in the same lens. This type of high quality color corrected lenses is used in sophisticated lighting equipment such as scenic projectors and intelligent luminaires.

the light at the desired angle. As it is difficult to manufacture thick and heavy lenses, plano-convex lenses rarely exceed 240mm in diameter. Originally the lenses of PC luminaires were produced by hand polishing. In recent years, moulded  lenses have become available. The plane side of these lenses, called pebble-convex or prism-convex lens, is often roughened with little bubbles in order to slightly blur the margins of the beam . PC lenses with a large diameter and a prominent curvature, which could theoretically have been very efficient both in collecting the light and in bending it, are not used in the theatre. A large, thick glass lens is extremely heavy and would absorb a lot of heat which causes the glass to crack. For these reasons, the lenses of theatrical luminaire lenses are usually quite thin. When a more

The Pebble Convex lens of CCT Minuette.