Power of a lens of focal length 25 cm is
- (a)+2·5 Dioptre
- (b)+3 Dioptre
- (c)+4 Dioptre
- (d)+5 Dioptre
Correct — C, +4 Dioptre. The power of a lens is the reciprocal of its focal length in metres: P = 1/f. A focal length of 25 cm is 0·25 m, so P = 1/0·25 = +4 dioptres. The positive sign shows it is a converging (convex) lens. Hence the power is +4 D.
- (a)+2·5 Dioptre — A power of +2·5 D would correspond to a focal length of 1/2·5 = 0·40 m (40 cm), not 25 cm.
- (b)+3 Dioptre — A power of +3 D would mean a focal length of 1/3 = about 0·33 m (roughly 33 cm), not 25 cm.
- (d)+5 Dioptre — A power of +5 D would mean a focal length of 1/5 = 0·20 m (20 cm), not 25 cm.
The power of a lens measures how strongly it converges or diverges light and is defined as P = 1/f, with f in metres. Its unit is the dioptre (D): one dioptre is the power of a lens of focal length one metre. A convex (converging) lens has positive power; a concave (diverging) lens has negative power. The key step is converting centimetres to metres before taking the reciprocal.
The only calculation needed is 25 cm = 0·25 m, then 1 divided by 0·25 = 4. Each distractor value matches a different focal length, so a unit slip (forgetting to convert cm to m) or a wrong reciprocal is what the trap punishes.
- Power of a lens P = 1/f, where f is the focal length in metres; the unit is the dioptre (D).
- One dioptre is the power of a lens of focal length 1 metre.
- Convex (converging) lenses have positive power; concave (diverging) lenses have negative power.
- For f = 25 cm = 0·25 m, P = 1/0·25 = +4 D.
- Forgetting to convert 25 cm to 0·25 m before taking the reciprocal.
- Reading the sign wrong — a positive power means a convex (converging) lens.
Asked as a direct calculation — convert the focal length to metres and use P = 1/f.
No directly related past PYQ was found.
- practice — not a real PYQ
The power of a concave lens of focal length 50 cm is
- (a)+2 D
- (b)-2 D
- (c)+0·5 D
- (d)-0·5 D
Answer(b) -2 D — for a concave lens f = -0·5 m, so P = 1/(-0·5) = -2 D.
- practice — not a real PYQ
Two thin lenses of powers +3 D and +2 D are placed in contact. The power of the combination is
- (a)+1 D
- (b)+5 D
- (c)+6 D
- (d)+2·5 D
Answer(b) +5 D — powers add for lenses in contact.