Question

A student focused the image of a candle flame placed at about 2m from a convex lens of focal length 10 cm , on a screen. After 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 difference is seen in the brightness of the image

Answer 1

since the f =+10cm.so 2f=+20cm.
for object distance=-200cm.
we have by the lens formula, 1/v-1/-200=1/10.
or,v=+200/19cm.
A)in positive x direction he should move the candle (in his right hand side direction).
B)as the object is taken towards the lens the image formed will be illuminated.becuz the object distance is decreasing.

Answer 2

(a) The student moves the lens away from the screen to focus the image because on moving the candle towards the lens, the image distance increases.

(b)The size of the image increases when the object is moved towards the lens.

(c) Intensity of the image decreases.

(d) When the candle is moved very close to the lens, no image is formed on the screen. A virtual image is formed behind the candle on the same side of the screen.