Question

Ram, a science student, is trying to find the nature of a mirror he found in the lab. For doing that, he set up an experiment. He kept a candle in front of the mirror and a screen behind the candle. Now he started moving the screen until he got a clear image on the screen. Once the image is obtained, he measures and makes a note of the distances between the mirror, candle, and screen as shown.


For finding the focal length, Ram was advised to use the mirror formula. If he is using the mirror formula correctly, what value of focal length will he get?

[0.8 mark]

• A. -20 cm
• B. -30 cm
• C. -40 cm
• D. -50 cm

Answer 1

The correct option is B -30 cm

Given:
$v=-(80+40)=-120cm$

$u=-40cm$

$\frac{1}{f}=\frac{1}{-120}+\frac{1}{-40}$

From the mirror formula,
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

$\frac{1}{f}=\frac{1}{-120}+\frac{1}{-40}$

$f=-30cm$