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Abhipsa Mishra Grade: 12
        

Convex side of a thin concavo-convex lens(μ=1.50) has a radius of curvature of 20cm. The concave surface has a radius of curvature 60cm. The concave part is filled with water of μ=1.33 and the convex side is silvered. What is the effective focal length of the optical system?

7 years ago

Answers : (1)

Askiitians Expert Bharath-IITD
23 Points
										

Dear Abhipsa,


We know the lens makers formula...


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5939-1362_8384_Untitled1.jpg


 


5939-2198_8384_Untitled3.jpg


 


Now the given case can be considered as combination of


1. A thin lens of Radius of curvatures given by R1= infinity and R2= -60 cm


and μ = 1.33 refractive index of water


it is a thin lens


thus focal length of this lens is f1= 181.81 cm.


2. A concavo convexlength of  R1= -60 and R2= -20 cm


and  μ = 1.5


thus focal length of it for the case of thin lens is f2=60 cm


and


3. A concave mirror of focal length f3= -R/2 = -10 cm


where R= 20cm


thus resultant focal length becomes 1/f = 1/f1 + 1/f2 + 1/f3


 as distances between the lenses are zero


thus resultant focal length is -12.85 cm.


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Askiitians Experts


Adapa Bharath

7 years ago
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