Q.  A student places a candle flame at distance of about 60 cm from a convex lens of focal length 10 cm and focuses the image of the flame on a screen. After that he gradually moves The flame towards the lens and each time focuses the image on the screen. 
(a) In which direction towards or away from the lens, does he move the screen to focus the image? 
(b) How does the size of the image change? 
(c) How does the intensity of the Image change as the moves towards the Lens? 
(d) Approximately for what distance between the flame And the lens. the image formed or, the screen is inverted and of the same size? 
 

Dear Student,
No worries my explanation will clear your doubt easily.
See, as the objects moves away from the convex lens the field of view of the lens increases due to which the more number of rays from the object passes through the lens.We know that the more the number of rays from the object the better the intensity of the image formed by it.
Now in the question it is said that flame moves towards the lens so it means distance between these two decreases.It means less ray will fall on lens hence low intensity.

Hope it clears your doubt.
Regards

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