Question

Determine the image distance and image height for a 5.00-cm tall object placed 20.0 cm from a concave mirror having a focal length of 15.0 cm.

a. di = 60.0 cm and hi = -15.0 cm

b. di = -60.0 cm and hi = 15.0 cm

c. di = 30.0 cm and hi = -15.0 cm

d. di= -30.0 cm and hi=15.0 cm

Answer 1

The equation for the lens:

$$\frac1f=\frac1o+\frac1i,\\\space\\ d_i=i=\frac{fo}{o-f}=\frac{15·20}{20-15}=60\text{ cm}.$$

Image size:

$$h_i=-mh_o=-\frac{i}{o}h_o=-\frac{60}{20}·5=-15\text{ cm}.$$