a convex lens made of glass(RI=1.5) is put in water (RI=1.7). what will happen to its focal length?1. decrease and change sign2. increase and change sign3. decrease and same sign4. increase and same sign

The power of a lens depends on the differences in the refractive indices of the environment and the lens material. A glass lens immersed in water will loose power (a positive lens will focus at a greater distance). (This is for normal glass with refractive index greater than water - there could be glasses with refractive index less than water in which case the lens would gain power)

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