What is meant by power of a lens ? Name and define its S.I. unit. One student uses a lens of focal length +50 cm and another of –50 cm. State the nature and find the power of each lens. Which of the two lenses will always give a virtual, erect and diminished image irrespective of the position of the object ?
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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 = 50cm = 0.5m
P1 = 1/f1
P1 = 1/0.5
P1 = 2D
Since Power is +ve therefore the lens with focal lenght 50cm is convex lens.
f2 = -50cm = -0.5m
P2 = 1/f1
P2 = 1/-0.5
P2 = -2D
Since Power is -ve therefore the lens with focal lenght -50cm is concave lens.
The image formed by the concave lens is virtual, erect and diminished irrespective of the position of the object.
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 = 50cm = 0.5m
P1 = 1/f1
P1 = 1/0.5
P1 = 2D
Since Power is +ve therefore the lens with focal lenght 50cm is convex lens.
f2 = -50cm = -0.5m
P2 = 1/f1
P2 = 1/-0.5
P2 = -2D
Since Power is -ve therefore the lens with focal lenght -50cm 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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