Question

what is the focal length of double concave lens kept in air with two spherical surface of radii R1 30cm and R2 60cm take refractive index lens as n=1.5

Answer 1

I think so the answer might be like this way i am not sure
we know that ------    F = R by  2
where R is radius  and F = focal length 
 therefore      first concave lens it must be 
F = 30 by 2
ie = 15 cm 
and the second 
F =60 by 2
ie = 30 cm  

Answer 2

☘︎From the . - E-7 using sign convention

$r_{1} = - 30 \: . \: r_{2} = 60cm$

☘︎And also given that n= 1.5

☘︎using

$\frac{1}{f} =$

$(n - 1)( \frac{1}{r1} - \frac{1}{r2}$

$\frac{1}{ = f}$

$(1.5 - 1)( \frac{1}{ - 30} - \frac{1}{60} )$

☘︎Solving this,we get

$f = - 40$

Here minus indicate that lens is divergent