Lenses

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A lens is a transparent medium bound by two surfaces. It bends light because a ray of light gets refracted twice while passing through it.

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A convex lens has two spherical surfaces puffed up outwards. It is thicker at the centre than at the edges and acts as a converging lens.

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A concave lens has both surfaces spherical on the inside. It is thinner at the centre than at the edges and acts as a diverging lens.

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The optical centre is the point inside a lens on the principal axis through which light rays pass without changing their path.

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The focal length is the distance between the optical centre and the principal focus of a lens.

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Lens formula: 1/v - 1/u = 1/f, where v is image distance, u is object distance, and f is focal length.

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Use P = 1/f(m). Here f = 25 cm = 0.25 m. So P = 1/0.25 = 4 D. The power is +4 dioptre for a convex lens and -4 dioptre for a concave lens of the same focal length magnitude.

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Use P = P1 + P2. So total power = +2 D + +3 D = +5 D.