Question

Rohit focused the image of a candle flame on a white screen using a convex lens he noted position of candle =26 cm position of convex lens=50 cm position of screen = 74 cm
1) find focal length of lens
2) where will the image be formed if he shifts the candle towards the lens at a position of 38 cm
3) draw ray diagram to show the image formation of the 2nd case

Answer 1

distance of candle from lens= 50-26 = 24cm
distance of screen from lens=74-50=24cm
since u = v
(i)therefore the candle is placed at 2f because at 2f image distance is equal to the object distance.
2f = 24
f = 12
(ii) u = 50-38 =12 cm
since the object is at focus therefore the object will form at infinity.


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Answer 2

Hi there!

Position of candle = 26cm

Position of convex lens = 50cm

Position of screen = 74cm

In attachment , position of all the given things

form here, it is clear that,

(i) object (candle ) distance from lens , u = 50cm - 26cm = 24cm

image (on screen) distance from lens ,v = 74cm - 50cm = 24cm

Here , v = u

it means object place at centre of curvature of lens

so, u = 2f , f is focal length

f = u/2 = 24/2 = 12cm

(ii) position of candle = 38cm

so, object (candle ) distance from convex lens = position of convex lens - position of candle = 50cm - 38cm = 12cm

∴ u = -12cm and focal length f = 12cm

Now, use formula,

1/f = 1/v - 1/u

1/12 = 1/v -1/-12

⇒ v = ∞ , hence image is formed at infinity .

(iii) shown in figure

Hope this would help you!!