JEE Main · 2023 · 25 Jan 2023 · Shift 2 · Geometrical Optics

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An object is placed on the principal axis of convex lens of focal length 10 cm as shown. A plane mirror is placed on the other side of lens at a distance of 20 cm . The image produced by the plane mirror is 5 cm inside the mirror. The distance of the object from the lens is

____

cm.

Answer: 30

Solution

The flat mirror forms its image

5 cm

behind itself, which means the rays reaching it were converging toward a point

5 cm

in front, that is

20 - 5 = 15 cm

from the lens. That point is the image the lens makes, so

v = 15 cm

. From

\frac{1}{v} - \frac{1}{u} = \frac{1}{f}

with

f = 10 cm

\frac{1}{u} = \frac{1}{15} - \frac{1}{10} = - \frac{1}{30}

. The object stands

30 cm

from the lens.

JEE Main · 2023 · 25 Jan 2023 · Shift 2 · Integer answer

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