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  • 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 direct

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 ?

 

 

Answers (1)

 

(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 F and 2F on other side of the lens. Hence student will move the lens towards candle.

( F means a location at a distance from lens that equals the focal length of lens. 2F means distance that equals twice the focal length)

(b) Image is keep on magnified,  when object is moving towards lens, from a distance beyond 2F, then 2F, then less than 2F

(c) as the object (candle) is moved towards lens more light  intensity is collected by lens, hence 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.

He will get diffused light on the screen.

 

Posted by

Satyajeet Kumar

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