Question

Calculate focal length of given lens if the magnification is -0.5.

• A. 6.66 cm
• B. 5.44 cm
• C. 3.88 cm
• D. 1.38 cm

Answer 1

Answer: (A)

Magnification for lens = $\frac{f}{u+f}\Rightarrow -\frac{1}{2}=\frac{f}{-20+f}$
- 20 + f = - 2f
- 20 = -3f
f = 20/3
= 6.66 cm