Question

If the real image of a candle flame formed by a lens is three times the size of the flame

and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens?

A. -80cm

B. -40 cm

C. -40/3 cm

D. -80/3 cm​

Answer 1

Given info : the real image of a candle flame formed by a lens is three times the size of the flame  and the distance between lens and image is 80 cm.

To find : the candle should be placed from the lens ...

solution : we know,

magnification of a lens = $\frac{\text{height of image}}{\text{height of object}}=\frac{v}{u}$

⇒ -3 = $\frac{v}{u}$ [ negative sign indicates that image is real and inverted ]

⇒ v = -3u ...(1)

as it has given that the distance between the lens and the image is 80cm.

so image distance, v = 80cm

from equation (1) we get,

the object distance, u = -v/3 = - 80/3 cm

therefore the candle should be placed from the lens -80/3 cm i.e., the correct option is (D) -80/3 cm.

Answer 2

Answer:

D) -80/3 cm

Explanation:

GIVEN: the real image is three times the size of flame.

image distance= 80cm.

SOL.: we know,

magnification= height of image = v

height of object = u

-3= v/u

Therefore, v= -3u

since we know,

v is the image distance, and

v= 80 cm.

Therefore, -3u= 80

AND, u= -80/3

Thus, the candle should be placed -80/3 cm from the lens.

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