Question

what is the focal length of double concave lens kept in air two sperical surfaces of radii R1 =30cm and R2=60cm ,take refractive index of lens as n=1.5​

Answer 1

Answer:

Focal length of the lens = - 40 cm

Explanation:

Lens formulae :

$\frac{1}{f} = (n - 1) [ \frac{1}{R_{1} } - \frac{1}{R_{2} } ]$

R1 = - 30 cm            

R2 = 60 cm

1/f = ( 1.5 - 1 ) [ - 1/30 - 1/60 ]

1/f = ( .5 ) [ -1/20 ]

f = - 40 cm