In order to make a nearsighted glasses in the WATER, a convex-concave lens has r

Question

In order to make a nearsighted glasses in the WATER, a convex-concave lens has radius of 20cm and 15cm is used. And it's made of glass which has index n=1.66. The index number of water is 1.33. (A) Find the focal length of the nearsighted glass (be careful of the sign). (B) What kind of lens (converging or diverging) is it for a nearsighted glasses. Look in the book (chapter 36.7), or online, explain why it must be that kind for a nearsighted eye, what happens to a nearsighted eye. (C) Also explain the same question for a farsighted eye and glasses.

Explanation / Answer

ng = 1.66
nw = 1.33
R1 = +20 cm
R2 = +15 cm

1/f = (ng/nw - 1)*(1/R1 -1/R2)

= (1.66/1.33 - 1)*(1/20 - 1/15)

f = -241.82 cm

b) f is negative.so, it is a diverging lense

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