Question

The diagram below shows an object located at point P, 0.25 meter from a concave spherical mirror with principal focus F. The focal length of the mirror is 0.10 meter. How does the image initially change as the object begin to move from point P toward point F?

• A. Its distance from the mirror decreases
• B. The size of image decreases
• C. Its distance from the mirror increases
• D. The size of images increase

Answer 1

Answer: (C), (D)

The correct options are
C The size of images increase
D Its distance from the mirror increases

From mirror equation we know

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$=constant

As the object is moving towards F, u is increasing(using sign convention, u is negative of the distance between P and the mirror)

Hence from mirror equation, v must decrease.

According to sign convention, v is negative of the distance between image and mirror and is decreasing. This means the distance between mirror and image is increasing.

Magnification=$-\frac{v}{u}$

v is increasing in magnitude and u is decreasing in magnitude.

Hence the magnitude of magnification increases. Hence the size of image increases.