Question

The Focus distance of a convex lens is 20 cm. How far away is an object to place from this lens so that its reflection is 2 times larger than the object, while the reflection is real?​

Answer 1

Answer:

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Answer 2

Answer:

Explanation:

Given:

The focal length of convex lens (f) = 20cm

Magnification (m) = 2

Position of object (u) =?

Now, using the magnification formula

m = v/u

⇒ -2 = v/u

⇒ v = -2u....................(i)

Now, using lens formula

1/f = 1/v - 1/u...................(ii)

Substituting (i) in (ii)

1/20 = 1/-2u - 1/u

⇒ 1/20 = -1 - 2/2u

⇒ -3/2u = 1/20

⇒ -2u/3 = 20

⇒ u = - 20 × 3/2

⇒ u = -10 × 3

⇒ u = -30cm