Question

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is ______ cm.

Options

• 15

• 20

• 10

• 25

Answer 1

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is 15 cm.

Explanation:

m = `"f"/("f"+"u")`

As, m₁ = -m₂

`"f"/("f"-10)=(-"f")/("f"-20)`

⇒ f = 15 cm