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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. |
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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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