A student holds a convex lens in sunlight and finds the smallest bright spot on a sheet of paper 21…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student holds a convex lens in sunlight and finds the smallest bright spot on a sheet of paper from the lens.
- What is the focal length? (b) What is the power of the lens? (c) Why can we use the Sun for this? (d) Why must the student not hold the lens still for long?
Need a reminder? The key ideaShow
Power with in metres, in dioptres (D). Convex lenses have a positive power, concave lenses a negative power. For thin lenses in contact, the powers add: .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The spot is at the focus: .
Step 2. (b) .
Step 3. (c) The Sun is so far away that its rays arrive parallel, and a parallel beam meets at the focus.
Step 4. (d) All the light on the lens is squeezed into a tiny spot. It gets hot enough to burn the paper.
Answer: (a) . (b) . (c) Its rays are parallel. (d) The spot is very hot.
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