## Saturday, September 17, 2011

### CBSE 12th Std Physics Numerical from Optics

A convex lens of crown glass(1.5) has a focal length of 15 cm. The lens is placed in (a) water(1.33) and carbon bisulphide(1,65).determine in each case whether the lens behaves as a conveging or a diverging lens and determine its focal length.

## Formula used$\LARGE \frac{1}{f}=\left ( \mu _{2} -\mu _{1}\right )\left [ \frac{1}{R_{1}}-\frac{1}{R_{2}} \right ]$

For the lens to be converging, f must be positive and for it to be diverging, f must be negative

(a) In water the lens will be converging as the refractive index of glass is greater than that of  water.

(b) In Carbon disulphide the lens is diverging as the refractive index of glass is less than that of CS2