# i wanted to know about first principal focus and second principal focus .

### Asked by Shagun Rastogi | 14th Mar, 2015, 07:05: PM

**First principal focus**:

First principal focus of a lens is that point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity.

It is denoted by F_{1}.

_{1}and v= in the above equation, we get,

**Second Principal Focus**:

_{2}.

_{2}in the above equation, we get,

### Answered by Faiza Lambe | 16th Mar, 2015, 09:49: AM

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