A student focuses the image of a candle flame, placed at about 2 m from a convex lens of focal length 10 cm, on a screen. After that he moves gradually the flame towards the lens and each time focuses its image on the screen.(A) In which direction does he move the lens to focus the flame on the screen?(B) What happens to the size of the image of the flame formed on the screen?(C) What difference is seen in the intensity (brightness) of the image of the flame on the screen?(D) What is seen on the screen when the flame is very close (at about 5 cm) to the lens?
(a) Let us assume the screen to lens distance is greater than 20 cm. Since it is required to get image beyond $2F$, the object should be placed between $F$ and $2F$ on another side of the lens. Hence student will move the lens towards the candle. ($F$ means a position at a distance from the lens that equals the focal length of the lens. $2F$ means the distance that equals twice the focal length).
(b) Image keep on magnified when the object is moving towards the lens, from a distance beyond $2F$, then $2F$, then less than $2F$.
(c) As the object (candle) is moved towards the lens more light intensity is collected by the lens, hence the brightness of the image increases.
(d) When the candle is very close about $5\ cm$, focussing the flame is not possible. Hence student will not get any image on the screen, rather he will get only diffused light on the screen.
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