If an object has to coincide with its image, then the ray have to retrace its path, hence, it is concluded that object is at the focus of the combination.
Focus of equivalent combination.
`(1)/(F)=(1)/(f_(g))+(1)/(f_(1))`
`(1)/(f_(g))=(1.5-1)(2)/(R)==(1)/(R)`
`(1)/(f_(1))=(mu-1)(-(1)/(R)-(1)/(oo))=-((mu-1))/(R)`
`(1)/(F)=(1)/(R)-((mu-1)/(R))=((2-mu)/(R))`
Since, `F=D`
`:. mu=2-(R)/(D)`
![image](https://learnqa.s3.ap-south-1.amazonaws.com/images/161040673813599948321610406738.png)