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Home » A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
CBSE | Class 10 | Light Reflection and Refraction | NCERT Exemplar | Science
A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Answer-

Because the image is actual and the same size, it should be positioned at 2F.

The image of the needle is assumed to be formed at a distance of 50 cm from the convex lens. As a result, the needle is positioned 50 cm in front of the lens.

Object distance (u) is – 50 cm and Image distance (v) is 50 cm

Let the focal length be f

Using the lens formula, we get –

1/f = 1/v – 1/u

Upon substituting values, we get –

1/f = 1/50 – (1/-50) = 2/50

f = 25 cm

Expression for the power of lens is given by –

Power = 1/f(in meters) = 1/0.25

Power = + 4 Dioptre

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