Prisms are sometimes used separate light into different wavelengths. The physical principle is called refraction. Although the result is the same, the mechanism is quite different from what happens in a diffraction grating. Light changes direction as it enters and leaves the prism, and the amount of change in direction depends on the wavelength. This effect is not very big, except when the light beam that leaves is travelling almost parallel to the final surface. That is why we used diffraction gratings instead of prisms to study a spectrum.

Using refraction to separate light into different wavelengths gives only one copy of the spectrum, whereas diffraction gratings give many (three on each side, for the diffraction gratings in the kit).
In these pictures sunlight is being refracted by a bevelled pattern on the glass window. By moving just a few inches up or down the color changes.

A diamond gets its multicolored sparkle from refraction, like a prism, except that now the light makes many reflections inside the diamond before it emerges, which magnifies the color effect. A rainbow also involves both reflection and refraction.

Prisms can also serve as a kind of mirror, and are sometimes used this way -- in particular, in binoculars.

The first use of a Fresnel lens was in lighthouses The goal was to make a really large lens to direct the light from a light source into a beam. A solid lens would be too heavy and difficult to make; building it from prisms is much simpler.

Prisms and lenses around us

Any glass object is a crude prism or lens. The light beams that enter the cut glass jewel make several reflections before it escapes, and this helps accentuate the small differences in direction taken by the different wavelengths of light in the beam.

The bright spots within the shadow of the glass are the beams that have been redirected and perhaps somewhat focussed; looking through the glass, we see a distortion of the background, because light has changed direction as it enters the glass. In the same way, we can tell that the glass is partly filled with water.

Here is another example. The glass casts a shadow, because it is deflecting light by reflection and refraction. Every bottle or bowl made of clear glass gives rise to an optical effect for us to explore.

The ripples of light on the bottom of a swimming pool are caused by the lens effect of the waves on the surface. (This pool is being disturbed by the man cleaning it -- you can see his shadow).

Air is usually uniform, and then light travels in straight lines in it; but when there is a large temperature difference in air, there can be a tiny amount of change of direction. This is the source of mirages; it is also the shimmer seen over a fire or other really hot object. As shown in the picture at left, a flame has a plume of hot air rising from it, and the deflections give rise to a shadow pattern (the picture shows both the bunsen burner and its shadow on a screen).

Drops of water on your glasses become little lenses, that can turn distant street lights into complex and wonderful objects. The random lens has turned the single spot of light into a complicated star shape in the picture at right.

Whenever light moves from one medium to another with differing optical properties, it refracts. If the surface is curved, we get a kind of lens. Accidental lenses are very common, but we tend to ignore them because the images don't make any sense, like the view through this rain-streaked glass door.