A-level Physics (Advancing Physics)/Lenses/Worked Solutions
1. A lens has a focal length of 10 cm. What is its power, in dioptres?
Always use SI units, so 10 cm = 0.1m.
2. Light reflected off a cactus 1.5m from a 20D lens forms an image. How many metres is it from the other side of the lens?
u must be negative, as per. the Cartesian covention, so u = -1.5.
3. A lens in an RGB projector causes an image to focus on a large screen. What sort of lens is it? Is its power positive or negative?
The wavefronts are being caused to spread out (diverge) more by the lens. Hence, it is a diverging lens. The wavefronts are losing curvature, so the lens has a negative power.
4. What is the focal length of a 100D lens?
5. The film in a camera is 5mm from a lens when automatically focussed on someone's face, 10m from the camera. What is the power of the lens?
5mm = 0.005m
Since lenses are generally made to have nice round powers, this was probably a 200D lens. However, this is in real life, and this an hypothetical question with numbers picked out of thin air by me.
6. The light from a candle is enlarged by a factor of 0.5 by a lens, and produces an image of a candle, 0.05m high, on a wall. What is the height of the candle?