CBSE Class 10 Science 2019 · Paper 31/2/2 · Question 5

Light - Reflection and Refraction 2 marks Medium

The power of a lens is

+ 5

diopters. What is the nature and focal length of this lens ? At what distance from this lens should an object be placed so as to get its inverted image of the same size ?

Answer

The lens is convex with $f = + 20$ cm; the object must be 40 cm in front of it, at $2 F_{1}$ .

Focal length:

f = \frac{1}{P} = \frac{1}{+ 5}

= + 0.2

= + 20

cm. Nature: the focal length is positive, so the lens is a convex (converging) lens. Object distance: an inverted image of the same size is formed when the object is at

2 F_{1}

, that is,

2 f = 40

cm in front of the lens (

u = - 40

cm). The image is then formed at

2 F_{2}

40

cm behind the lens (

v = + 40

cm), real, inverted and of the same size.

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CBSE Class 10 Science 2019 · Paper 31/2/2 · Question 5

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