CBSE Class 10 Science 2019 · Paper 31/2/1 · Question 13(a)
A student holding a mirror in his hand, directed the reflecting surface of the mirror towards the Sun. He then directed the reflected light on to a sheet of paper held close to the mirror.
- (a)What should he do to burn the paper ?
- (b)Which type of mirror does he have ?
- (c)Will he be able to determine the approximate value of focal length of this mirror from this activity ? Give reason and draw ray diagram to justify your answer in this case.
Answer
He focuses sunlight into a sharp spot and holds it steady; the mirror is concave; the distance gives its approximate focal length.
(a) He should move the sheet of paper (or the mirror) back and forth until the reflected sunlight is focused into the smallest, sharpest bright spot on the paper, and then hold the mirror and the paper steady in that position for a few minutes. The sunlight collected by the whole mirror is concentrated on that spot, and the paper begins to smoke and catches fire.
(b) A concave (converging) mirror.
(c) Yes. The Sun is very far away, so its rays reach the mirror parallel to the principal axis, and after reflection from a concave mirror they meet at the principal focus F. The sharp bright spot on the paper, the image of the Sun, is therefore formed at F, and the distance between the mirror and the paper (PF) gives the approximate focal length of the mirror.
Ray diagram: rays from the Sun parallel to the principal axis are reflected by the concave mirror and all pass through F on the axis, where the paper is held; PF .
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