making microscope, telescope and slide projectors . They are used to correct the defect of vision called hypermetropia . REFRACTION THROUGH A CONCAVE LENS Let us discuss the formation of images by a concave lens when the object is placed at two possible positions. Object at Infinity When an object is placed at infinity, a virtual image is formed at the focus.
The size of the image is much smaller than that of the object (Figure . ). C F O Figure . Concave lens-Object at infinity Object anywhere on the principal axis at a finite distance When an object is placed at a finite distance from the lens, a virtual image is formed between optical center and focus of the concave lens.
The size of the image is smaller than that of the object (Figure . ). C F O B A B A N M Figure . Concave lens-Object at a finite distance But, as the distance between the object and the lens is decreased, the distance between the image and the lens also keeps decreasing.
Further, the size of the image formed increases as the distance between the object and the lens is decreased. This is shown in (figure . ). C C F F Figure .
Concave lens-Variation in position and size of image with object distance . APPLICATIONS OF CONCAVE LENSES . Concave lenses are used as eye lens of ‘Galilean Telescope’ . They are used in wide angle spy hole in doors.
. They are used to correct the defect of vision called ‘myopia’ . LENS FORMULA Like spherical mirrors, we have lens formula for spherical lenses. The lens formula gives the relationship among distance of the object (u), distance of the image (v) and the focal length (f) of the lens.
It is expressed as f = v − u . . . .
. . It is applicable to both convex and concave lenses. We