A thin convex lens has a focal length of $15, cm$ and an object is placed $50, cm$ in front of it. Find the image distance in centimeters and state whether the image is real or virtual.

easy 2 marks Class 12 · Physics · Ray Optics and Optical Instruments

Answer

30, cm

, real

The correct answer is $30, cm$ , real — Invert to get $v = \frac{1}{0.0467} \approx 21.4, cm$

Step-by-step solution

Use the thin‑lens formula $\frac{1}{v} = \frac{1}{f} - \frac{1}{u}$ .
Insert $f = + 15, cm$ and $u = + 50, cm$ : $\frac{1}{v} = \frac{1}{15} - \frac{1}{50} = 0.0667 - 0.0200 = 0.0467$
Invert to get $v = \frac{1}{0.0467} \approx 21.4, cm$

Common mistake: Mixing the signs of

f

and

u

; forgetting that for a real object

u

is positive

Concept tested: Thin‑lens formula