Two convex lenses of focal length 20 cm each are coaxial and separated by 60 cm. A distant object is viewed through the combination. The final image i s ______ cm from the first lens.

Question

G1 Physics · Accepted Answer

For the first lens, the object is at infinity, so the image forms at its focus: v₁ = 20 cm to the right of L₁. The second lens is 60 cm to the right of L₁, so this image is 40 cm to the left of L₂: u₂ = -40 cm For L₂, f₂ = +20 cm. Using 1/f = 1/v - 1/u: 1/20 = 1/v₂ + 1/40 1/v₂ = 1/40 v₂ = 40 cm Thus final image position from L₁: 60 + 40 = 100 cm Final value: 100