What is meant by power of a lens ? Write its SI unit. A student uses a

1.	What is meant by power of a lens ? Write its SI unit. A student uses a lens of foclength 40 cm and another of -20 cm. Write the nature and power of each lens.
Answer» The ability of a lens to converge/diverge a beam of light rays is expressed in terms of its Power(P). P=1/f The SI unit of power of lens is Dioptre.It is denoted by the letter "D" SI unit of length is metre(m) therefore 1D = 1/m or 1D = 1m-1 f1 = 40cm = 0.4m P1 = 1/f1 P1 = 1/0.4 P1 = 2.5 DSince Power is +ve therefore the lens with focal lenght 40cm is convex lens. f2 = -20cm = -0.2m P2 = 1/f1 P2 = 1/-0.2P2 = -5D Since Power is -ve therefore the lens with focal lenght -20cm is concave lens. The image formed by the concave lens is virtual, erect and diminished irrespective of the position of the object.

What is meant by power of a lens ? Write its SI unit. A student uses a lens of foclength 40 cm and another of -20 cm. Write the nature and power of each lens.

Answer»

The ability of a lens to converge/diverge a beam of light rays is expressed in terms of its Power(P). P=1/f The SI unit of power of lens is Dioptre.It is denoted by the letter "D" SI unit of length is metre(m) therefore 1D = 1/m or 1D = 1m-1

f1 = 40cm = 0.4m P1 = 1/f1 P1 = 1/0.4 P1 = 2.5 DSince Power is +ve therefore the lens with focal lenght 40cm is convex lens.

f2 = -20cm = -0.2m P2 = 1/f1 P2 = 1/-0.2P2 = -5D Since Power is -ve therefore the lens with focal lenght -20cm is concave lens.

The image formed by the concave lens is virtual, erect and diminished irrespective of the position of the object.

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