MCQ practice · SSC Physics Board questions · Chapter 8: Reflection of Light

Mymensingh Board 2021

How many times will the image of the object AB be magnified?

Think it through first, then pick an option.

a 0.33
b 1.5
c 3
d 24

Answer and explanation

Correct answer: c 3

In the figure C to F is 12 cm and CF = f, so f = 12 cm. A is 8 cm from the mirror, inside the focus. 1/v = 1/12 − 1/8 = −1/24, v = −24 cm. The magnification is m = 24 ÷ 8 = 3.

Why the other options are wrong

a 0.33: This is not the answer: 0.33 comes from 8 ÷ 24, the fraction upside down. The magnification is image distance ÷ object distance = 24 ÷ 8 = 3. In the figure C to F is 12 cm and CF = f, so f = 12 cm. A is 8 cm from the mirror, inside the focus. 1/v = 1/12 − 1/8 = −1/24, v = −24 cm. The magnification is m = 24 ÷ 8 = 3.
b 1.5: This is not the answer: 1.5 comes from 12 ÷ 8, which is f ÷ u. The magnification is v ÷ u, and v = 24 cm. In the figure C to F is 12 cm and CF = f, so f = 12 cm. A is 8 cm from the mirror, inside the focus. 1/v = 1/12 − 1/8 = −1/24, v = −24 cm. The magnification is m = 24 ÷ 8 = 3.
d 24: This is not the answer: 24 is the image distance v in cm, not the magnification. The magnification is 24 ÷ 8 = 3. In the figure C to F is 12 cm and CF = f, so f = 12 cm. A is 8 cm from the mirror, inside the focus. 1/v = 1/12 − 1/8 = −1/24, v = −24 cm. The magnification is m = 24 ÷ 8 = 3.