A concave mirror has a focal length of 20 cm. What is the magnification if the object's distance is 100 cm?
A) 1/2
B) 1/4
C) −2
D) 4
E) −1/4

Question

Quizplus · Accepted Answer

The formula for magnification of a concave mirror is given as:
magnification (m) = -v/u
where,
v = image distance
u = object distance

Given, focal length (f) = -20 cm (since it is a concave mirror)
Using the formula for focal length: 
1/f = 1/v + 1/u
Substituting the values, we get:
1/-20 = 1/v + 1/100
Solving this, we get v = -25 cm (negative sign indicates that the image is virtual)

Substituting the values of u and v in the formula for magnification, we get:
magnification (m) = -v/u = -(-25)/100 = 1/4 (negative sign indicates that the image is inverted)

Therefore, the correct choice is E) −1/4.

A Concave Mirror Has a Focal Length of 20 Cm