Question

Lenses are constructed by a material of refractive index 1.50.Themagnitude of the radii of curvature are 20 cm and 30 cm. Choose the focal lengths of all the possible lenses with the above specifications:

• A. + 24 cm
• B. - 24 cm
• C. + 120 cm
• D. - 120 cm

Answer 1

Answer: (A), (B), (C), (D)

The correct options are
A

+ 24 cm

B

- 24 cm

C

+ 120 cm

D

- 120 cm

$R_1=20cm,R_2=30cm$

Case 1 and 2

$\frac{1}{f}=(1.5-1)(\frac{1}{\pm 20}-\frac{1}{\pm 30})$

$=\pm \frac{1}{2}\times \frac{50}{600}\Rightarrow f=\pm 24cm$

Case 3 and 4

$=(1.5-1)(\frac{1}{\pm 20}-\frac{1}{\pm 30})$

$f=\pm 120cm$