Lenses can be divided into two types, depending on their shape:
Converging lenses are fatter in the middle than at the edges;
Diverging lenses are thinner in the middle than at the edges.
Features of lenses:
1. The optical centre, C – the point midway between the lens surfaces on its principal axis. 2. The principal axis – the line passing symmetrically through the optical centre of the lens. 3. The principle focus, F – all rays close to and parallel to the principle axis will converge after refraction by the lens at a point known as the principle focus. Also known as the focal point. 4. The focal length, f – distance between the optical centre and the principle focus. 5. The focal plane – the plane which passes through F and P. The focal plane is perpendicular to the principle axis.
Converging Lenses
An incident ray through the optical centre C passes without bending.
An incident ray parallel to the principal axis is refracted by the lens to pass through F.
An incident ray passing through F is refracted parallel to the principal axis.
Diverging Lens
A diverging lens causes parallel rays to spread out. A diverging lens only produces virtual images.
Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point).
Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.
An incident ray that passes through the centre of the lens will in affect continue in the same direction that it had when it entered the lens.
An image produced by a lens can be larger or smaller than the object, i.e. it can be magnified or reduced.
The linear magnification, m is defined as:
Linear magnification, m = height of image hi ÷ height of object ho
m = image distance ÷ object distance, m = v ÷ u
Uses of a single lens:
Magnifying glass: a virtual image is produced which is upright and magnified
Camera/ eye: a real image is produced which is inverted and diminished
Projector/ photographic enlarger: a real image is produced which is inverted and magnified
Short Sight
A short sighted person is only able to focus on close objects and is not able to view objects far away. This may be caused to the eyeball being too long or the lens too rigid and unable to change its shape. The rays converge in front of the retina causing a blurred image. This defect is corrected using a diverging lens which makes the parallel rays from the object diverge. Then the strong action of the eye causes them to converge on the retina.
Long-sight
A long sighted person is only able to focus on distant objects and is not able to view objects close to the eye. The eyeball may be too short or the lens cannot be made fat enough, so the rays from one point of an object close to the eye come together behind the retina. This defect can be corrected using a converging lens. The converging lens causes rays from a close object to be less diverging and then the weekend action of the eye allows the rays to be brought to a focus on the back of the retina and the person is able to focus on close objects.