Calculate the focal length of the lens shown in the figure.

Explanation

`1/f=(mu_(l)/mu_(s)-1)(1/R_(1)-1/R_(2))`
`rArr 1/f=(1.5-1)(+1/30 (+1/60))`
`rArr f=120` cm

If in the above case direction of rays is reversed.
`1/f=(mu_(l)/mu_(s)-1)(1/R_(1)-1/R_(2))`
`rArr 1/f=(1.5-1)(-1/60-(-1/30))`
`f=+120` cm
This Illustration shows that focal length does not depend on the incident ray direction.