Question

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Find the power of a convex lens which forms a real and inverted image of magnification -1 of an object placed at a distance of 20cm from its optical centre.

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Answer 1

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How do you calculate the power of a convex lens?

• The power of a lens is defined as the reciprocal of its focal length in meters, or D = 1/f, where D is the power in diopters and f is the focal length in meters

At what distance should an object be placed from a convex lens?

• Using lens formula to find the position of object u. Therefore the object must be placed at a distance of 72 cm from the focal length of the lens. And the magnification of the image formed will be inverted this could be seen by the negative sign obtained

Where should an object be placed in front of a convex lens to get a real image of the same size of the?

• Where should an object be placed in front of a convex lens to get a real image of the size of the object? Answer: At twice the focal length, the image formed by the convex lens is real and of the same size as the object.

What is the minimum distance between an object and its real image in case of a convex lens?

• Ray Optics and Optical Instruments. Show that the least possible distance between an object and its real image in a convex lens is 4f where f is the focal length of the lens

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