(a) At what position a candle of length 3 cm be placed in front of a convex lens so that its image of length 6 cm be obtained on a screen placed at distance 30 cm behind the lens ?

(b) What is the focal length of lens in part (a) ?

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Solution

Height of the candle object =3 cm
Height of the candle image=6 cm
Image distance =30cm

(a)
The formula for magnification of a lens
m=h'/h=v/u
6/3=30/u
u=15cm

(b)
1/f=1/v-1/u
1/f=1/30- (1/-15)=3/30
f=+10 cm

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(a) At what position a candle of length 3 cm be placed in front of a convex lens so that its image of length 6 cm be obtained on a screen placed at distance 30 cm behind the lens ? (b) What is the focal length of lens in part (a) ?

Height of the candle object =3 cm Height of the candle image=6 cm Image distance =30cm (a) The formula for magnification of a lens m=h'/h=v/u 6/3=30/u u=15cm (b) 1/f=1/v-1/u 1/f=1/30- (1/-15)=3/30 f=+10 cm

(a) At what position a candle of length 3 cm be placed in front of a convex lens so that its image of length 6 cm be obtained on a screen placed at distance 30 cm behind the lens ?

(b) What is the focal length of lens in part (a) ?

Height of the candle object =3 cm
Height of the candle image=6 cm
Image distance =30cm

(a)
The formula for magnification of a lens
m=h'/h=v/u
6/3=30/u
u=15cm

(b)
1/f=1/v-1/u
1/f=1/30- (1/-15)=3/30
f=+10 cm