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A student obtained a sharp image of a candle flame placed at the distant end of the laboratory table on a screen using a concave mirror to determine its focal length. The teacher suggested that he focus on a distant building about 1 km away from the laboratory, for getting a more correct value of the focal length. In order to focus the distant building on the same screen the student should slightly move the:
(a) mirror away from the screen
(b) screen away from the mirror
(c) screen towards the mirror
(d) screen towards the building
Answer
In case of Concave mirror , here,
- Initially, the student is taking an image of object kept beyond C
- When an object is kept beyond C, image is formed between C & F.
- So, initially, screen is kept between C & F
- But when we focus the distant building 1km away, the object is kept at infinity
- So, image formed will be at F
- Thus, we move the screen from between C&F to F,
- i.e. We move screen towards the mirror
So, the correct answer is (c)
