# Radius of curvature using spherometer

1. Jan 23, 2015

### leojun

which formula is correct? or

2. Jan 23, 2015

### A.T.

The one of the picture looks correctly derived. Where does the other one come from, and what is "a"?

3. Jan 23, 2015

### leojun

We use second formula instead of the first formula to find the radius of curvature using spherometer.
"a" in the second formula is the same as "b" in the first formula.
The first formula is correct but i don't get why we use second formula instead of first.
The two formula's are supposed to be the same r=R,but why are they different?

4. Jan 23, 2015

### A.T.

I see no "b" in the first formula.

5. Jan 23, 2015

### leojun

SORRY,"a" in the second formula is the same as "d" in the first formula

6. Jan 23, 2015

### Bystander

"a" is the side length of the triangle, or distance between two of the three points.

7. Jan 23, 2015

### A.T.

I don't think so. I think Bystander is right and the formulas are equivalent.