Q
A convex lens forms a real image 30 cm from its optical centre. The object is placed 20 cm away from the lens. Using the sign convention, what is the correct sign and value for the focal length of the lens?
RRB Group D
09 Aug, 2026 (9:00 AM - 10:30 AM)
2026
✓ Answer:
C
+12 cm
Using 1/f = 1/v − 1/u with u = −20 cm and v = +30 cm gives f = +60 cm.