what is the focal length of double concave lens kept in air with two spherical surface of radio R1=30cm and R2=60cm.Take refractive index of lens as n=1.5.
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lens maker formula
1/f=(nl/nm-1)(1/r1-1/r2)
1/f=(0.5)(1/60-1/30)
1/f=1/2(-1/60)
f=-120
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