What is the main reason for axial charomatic aberration in the formation of image by an lens ?

Explanation

Form the lens maker's formula,
`(1)/(f)=(mu-1)((1)/(R_(1))-(1)/(R_(2)))`
When the lens of is kept in a liquid medium having the same refraction index, `mu=1`
In the above relation, settiong `mu=1`, we have
`(1)/(f)=0` or `f=infty`