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position of candle 26cm and position of lens 50cm and position of screen 74 what is the focal

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Rohit focused the image of a candle flame on a white screen using a convex
Rohit focused the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle , screen and lens as under : Position of candle = 26.0 cm Position of convex lens = 50.0 cm Position of screen = 74.0 cm i) What is the focal length of the convex lens? ii) Where will the image be formed if he shifts the candle towards the lens at a position of 38 cm? iii) Draw a ray diagram to show the formation of the image in case (ii) as said above?
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Position of candle = 26cm
Position of convex lens = 50cm
Position of screen = 74cm



(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 .

image

Answer 2

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