Concave Lenses

A concave lens is thinner at its centre than at its edges. It is a diverging lens: it spreads light rays out.When rays parallel to the principal axis pass through a concave lens, they diverge as if they had come from a point on the same side of the lens as the light source. This point is the principal focus F. Because the rays only appear to come from it, it is a virtual focus. The distance from the centre of the lens to F is the focal length f.Edexcel uses the real is positive convention: distances to real objects, images and foci are positive, and distances to virtual ones are negative. So a concave lens has a negative focal length. A concave lens of focal length 25 cm has f = −0.25 m.Two rays are used to draw ray diagrams for a concave lens:
1. A ray parallel to the principal axis leaves the lens diverging, in line with the principal focus on the object side.
2. A ray through the optical centre passes straight through, undeviated.The emerging rays never meet, but trace back to a point on the object side. So the image formed by a concave lens is always virtual, upright and diminished (smaller than the object), and on the same side as the object, wherever the object is.In diagrams a concave lens is often drawn as a vertical line with inward-pointing arrowheads at each end.Concave lenses are used in spectacles to correct short sight, and in peepholes in doors to give a wide field of view.