Two lens of power 15D and 5D are placed is contact co axially the focal lenght of equivalent lens is
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Power of a lens is an ability to converge or diverge light rays depends on its focal length. There are two types of lens concave and convex lens. The degree of convergence or divergence of light rays achieved by concave or convex lens in terms of power. It can be defined as reciprocal of focal length. Unit of power is dioptre.
P=1/f
There are number of lenses in many of optical instruments. They are combined to increase its magnification and sharpness of image. So net power is total sum of power of all lenses in that instrument.
a combination of two lens of power 15D and 5D is equivalent to a single lens of power, P = 25D.
the focal length of lens f= 1/P
therefore f=1/25 = 0.04m
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