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Explain the different types of concave and convex lens.
Lenses are one of the most important devices in optics. Lenses are used is many applications requiring the manipulation of light waves.

Lenses work by refractinglight waves in a particular manner denoted by the construction of the lens itself. Different type of lenses will either refract light to a point, called a Focus Point, or they may cause the light waves to diverge outwards.

One of the most common lens is the Plano - Convex lens, this lens has one side which is flat, and the other side which is curved like a sphere. This type of lens is thicker at the centre than the edges - a positive lens.

 Positive Lenses

Positive lenses are lenses which are thicker at the centre than at the edge. The combined effect of refraction at the front and back surfaces of the lens  will bend light to a point or Focus - The light from this type of lens is focused to a point IN FRONT of the lens.

Positive lenses produce a REAL IMAGE, -  the image produced by this type of lens CAN be projected onto a screen.

The three basic positive lenses are:
Double Convex lens
Plano - Convex lens
Positive Meniscus lens


 Negative Lenses

Negative  lenses are lenses which are thinner at the centre than the edge. The same optic laws apply to negative lenses as they do with positive lenses, but a negative lens works in a somewhat different way than a positive lens.

The focal lengths of this type of lens are negative, this can be worked out mathematically by assigning a negative radii of curvature  to a concave lens surface. If you pass parallel light rays through a negative lens, they seem to spread out  from a point behind the lens, the distance from this point to the centre line of the lens is the focal length; this is given a negative value, as it is on the opposite side of the lens from the focal point of a positive lens.

Because negative lenses do not bring parallel light rays together, they DO NOT produce a real image, but they do produce a VIRTUAL IMAGE.

The three basic negative lenses are:
Double Concave lens
Plano - Concave lens
Negative Meniscus lens


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