Lenses

LENSES

Parallel rays of light that pass through a converging lens are refracted so that they converge to a point, which is known as the principal focus or the focal point.  The distance from the lens to the focal point is called the focal length.  Light can pass through the lens in both directions, so there is a focal point on both sides.  If the object is further away from the lens than the focal point, an inverted, real image is formed.  When the image is smaller than the object, it is said to be diminished.  The size of the image depends on the position of the object.  The nearer the object is to the lens, the larger the image is.  If the object is nearer to the lens than the focal point, an upright, virtual image is formed behind the object.  The image is also magnified.  The magnification can be calculated by using the equation:

magnification = image height / object height

Parallel rays of light that pass through a diverging lens are refracted so that they diverge away from the focal point.  The image produced by a diverging lens is always virtual.  When drawing ray diagrams, a converging lens is represented by a line with arrows pointing away from each other at each end.  A diverging lens is represented by a line with arrows pointing towards each other at each end.

USING LENSES

We can draw ray diagrams to find the image that different lenses produce with objects in different positions.  The line through the centre of the lens and perpendicular to it is called the principal axis (the horizontal line).  Ray diagrams use three construction rays from a single point on the object to locate the corresponding point on the image.  The…