Question

The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is $80cm$, at what distance should the candle be placed from the lens ?

• A. $50cm$
• B. $-36.67cm$
• C. $-26.67cm$
• D. $80cm$

Answer 1

Answer: (C)

Step 1: Given

Image distance$\left(v\right)=80cm$

Size of image(hᵢ) = Three times size of object(h₀)

Let u and m are object distance and magnification respectively.

Step 2: Formula used

$m=\frac{h₀}{hᵢ}=-\frac{v}{u}$

Step 3: Finding the magnification

Magnification(m) $=-\frac{v}{u}=3$

Step 4: Finding the object distance

$m=3=-\frac{v}{u}$

Object distance, $u=-\frac{v}{3}=-\frac{80}{3}=-26.67cm$

Hence, the object distance is -26.67 cm and option B is correct.