Question

a double concave lens of refractive index 1.5 with its two spherical surfaces of radii R1=30cm and R2=45cm is kept in air. its focal length is ​

Answer 1

Explanation:

$\frac{1}{f} = (n21 - 1)(\frac{1}{R1} - \frac{1}{R2})$

$\frac{1}{f} = (1.5 - 1)(\frac{1}{30} - \frac{1}{45})$

$\frac{1}{f} = (0.5) (\frac{45 - 30}{45*30})$

$\frac{1}{f} = 0.5(\frac{15}{45*30})$

$\frac{1}{f} = 0.5(\frac{1}{3*30})$

$\frac{90}{0.5} = f$

$f = 180 cm=1.8 m$