what is meant by power of a lens? write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. write the nature and power of lens.
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Answer:
Convex lenses are also known as converging lenses since the rays converge after falling on the convex lens while the concave lenses are known as diverging lenses as the rays diverge after falling on the concave lens. Images formed by these lenses can be real or virtual depending on their position from the lens and can have a different size too. The image distance can be calculated with the knowledge of object distance and focal length with the help of lens formula. In optics, the relationship between the distance of an image (i), the distance of an object (o), and the focal length (f) of the lens are given by the formula known as Lens formula. Lens formula is applicable for convex as well as concave lenses. These lenses have negligible thickness. It is an equation that relates the focal length, image distance, and object distance for a spherical mirror. It is given as,
1i + 1o = 1f
i= distance of the image from the lens
o= distance of the object from the lens
f= focal length of the lens.
power of a lens is its ability to bend a ray of light falling on it and is reciprocal of its focal length. Its unit is dioptre. Power of a lens is defined as the reciprocal of its focal length i.e, P=1/F