Lenses | Power of lenses#!#Lens formula

If two lenses of focal lengths f_(1) and f_(2) are kept in contact with each other, state the formula for the focal length of the combination . If P_(1) and P_(2) are the powers of these lenses, state the formula for the power of the combination.

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Two lenses of power 2.5 D and 1.5 D are joined together. The power of the new lens formed is

Two thin lenses, when in contact, produce a combination of power +10 dioptres. When they are 0.25m apart, the power is reduced to +6 dioptres. The power of the lenses in dioptres, are

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Power (P) of a lens is given by reciprocal of focal length (f) of the lens. i.e. P = 1//f . When f is in metre, P is in dioptre. For a convex lens, power is positive and for a concave lens, power is negative. When a number of thin lenses of powers p_(1), p_(2), p_(3).... are held in contact with one another, the power of the combination is given by algebraic sum of the powers of all the lenses i.e., P = p_(1) + p_(2) + p_(3) + ........ Answer the following questions : When a third lens of focal length - 20 cm is placed in contact with the two lenses, power of the three would be

a) What do you understand by the power of a lens? Name one factor on which the power of a lens depends. b) What is the unit of power of a lens? Define the unit of power of a lens. c) A combination of lenses for a camera two converging lenses of focal length 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.

Surabhi from Std. X uses spectacles. The power of the lenses in her spectacles is 0.5 D . Identify the type of lenses used in her spectacles.

Surabhi from Std. X uses spectacles. The power of the lenses in her spectacles is 0.5 D . Find the focal length of the lenses used in her spectacles.