# Homework Help: Sign of biconcave lens

1. Aug 16, 2014

### somecelxis

1. The problem statement, all variables and given/known data
referring to the notes attached , r1 is negative beacuse it's concave to the incident light , r2 is positive bacuse it's convex to the incident light. ( incident light move from left to right. ) , but my another book states that the r1 is and r2 have the same sign , which is negative beacuse both are concave to the light ray. for r2 , assuming the light ray is travelling from right to left , so the r2 is also concave to the incident light which is also same with r1.
Which is correct? the sign for r1 and r2 shoud be different or equal ? namely (both the r1 and r2 is negative) or ( r1 is negative and r2 is positive)?

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### DSC_0069.JPG
File size:
94.8 KB
Views:
92
2. Aug 17, 2014

### rude man

Reason it out. You know that r1 is negative. Now, suppose the 2nd surface is convex to the right with |r2| = |r1|. Now there is just curved glass of uniform thickness. What is the effective f of such a piece of glass? Is it different from your window pane?

Having answered that question you should be able to figure out what the sign of r2 must be if the 2nd surface has the opposite curvature from the above example. Remember the lensmaker's formula has a minus sign in it somewhere.

EDIT: It's possible that one of your textbook's lensmaker's formula does NOT have a minus sign in it, in which case the sign of r2 would indeed be the opposite of the case where the minus sign is present. In other words, 1/f = (n-1)(1/r1 - 1/r2) is the same as f = (n-1)(1/r1 + 1/r2) if you swap the sign of r2. But in the image you furnished the convention includes the minus sign in the 1/f expression. That is the conventional way.

Last edited: Aug 17, 2014