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Lenses are constructed by a material of refractive index 1.50. The magnitude of the radii of curvature are 20 cm and 30 cm. Find the focal lengths of the possible lenses with the above specifications.

Answer»

The possibel CASES are as follows:R1=20cm 'R2= 30cm n=1.5 LENS MAKERS fromula : 1/F= (n-1) [1/R1-1/R2] For Double Convex lens :R1=+20cm R2= - 30cm1/f= (1.5-1)[1/20+1/30]=0.5 x [ 3+2/60]=0.5 x 5/60= 5X5/ 10 x 60=1/24f= 24cmFor Double concave lens :R1= - 20cm R2=+30cm1/f =(1.5-1)[-1/20 -1/30]=0.5x [ -3-2/60]f= -24cmFor Concave Convex : R1= +20cm R2= +30 1/f= 0.5[ 1/20-1/30] = 0.5 x [ 3-2/60] =5/10x60 f= 120cm For concave concave lens : R1= -20cm R2= - 30cm 1/f = 0.5x[ -1/20+1/30] = 0.5 [ -3+2/60] f= -1 20cm



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