Light - Lenses 2

The earliest lenses were convex, so therefore they are classed as being converging lenses - lenses that bring rays of light to a place called the focal point.

At the focal point, they create an image that can be seen on a screen. This made them useful as they acted as a magnifying glass, but they could also be used to focus the Sun's heat on a small area. The heat was concentrated and they could be used to start fires. A lens that is flat on one side has the prefix plano- so a convex lens that is flat on one side is called a plano-convex lens. Lenses with concave surfaces cause light rays to diverge (spread apart) and are therefore referred to as diverging lenses.

Some lenses are more powerful than others, more curvature means that they will bend light through a larger angle and the focal length will be shorter. The power of a lens is measured in dioptres. A more powerful lens will bring light to a focus closer than a less powerful lens so the dioptre is defined as the reciprocal of the focal length. Theoretically a dioptre has the SI units of m^-1 but it is not an officially recognized SI unit and can be written just as the number, without units. It is of particular use to opticians as it is a lot easier to specify a lens of a certain power to correct sight defects.

Lenses refract different wavelengths of light through different angles so white light that passes through a lens is split into its constituent colors. This is called chromatic aberration and was a big problem to makers of early optical instruments. Chromatic aberration makes an image less sharp and you can see a colored 'halo' around the edges. When you are looking at tiny objects or distant objects like stars and planets, it is unhelpful as they appear to be slightly fuzzy. It was eventually discovered that different types of glass had different chromatic aberration and that putting two types of glass together to form a compound lens solved the problem. A lens system that does not suffer from chromatic aberration is described as being achromatic.

The eye is an optical instrument and often crops up in questions about optics in physics senior high school papers. You need therefore to be familiar with the main parts of the eye and their functions. The part of the eye that focusses the light is the lens. The shape of the lens is controlled by the ciliary muscles. To view distant objects, the shape of the lens needs to be changed to be thinner and vice-versa. The light rays are focussed onto the retina. The changes in the lens that allow it to be focused on near and far objects is called accommodation.