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  4. A convexo-concave lens has faces of radii 3 and 4 cm respectively, and is made of glass of refractive index 1.6, its focal length is

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Q. A convexo-concave lens has faces of radii 3 and 4 cm respectively, and is made of glass of refractive index 1.6, its focal length is

Ray Optics and Optical Instruments

Solution:

$\frac{1}{f}=(n-1) \left[\frac{1}{R_1}-\frac{1}{R_2}\right]=(1.6 -2 ) \left[ \frac{1}{-3}-\frac{1}{4}\right]$
Both $R_1\, and \, R_2$ have same sign.