# Sign convention in geometric optics.

1. Jan 7, 2012

### PrakashPhy

Until today I learned in geometric optics that
1. Object distance +ve for real object else -ve
2. Image distance +ve for real image else -ve
3. Radius of curvature +ve for if light comes to the surcace from the side lying center of curvature else -ve

On the basis of this the lens formula would be
1/u + 1/v = 1/f [u=obj distance v= image distance f= focal length]

But recently, the new text book i follow writes that. "All distances towards the left of the lens are taken -ve and all other to the right are positive"

For a convex lens if the object is beyond 2f at a finite distance the new sign convention would give 1/v - 1/u = 1/f ;

I know it will not cause problem if i stick to one convention only but i wish to follow universal convention. So i would like to know if there is anything like "Universal sign convention" for geometric optics

2. Jan 7, 2012

### tommw

No there isn't. :-(
In fact every textbook still may use its own definition. On the other hand that's not so bad, because it forces students to really think about the problems and not only learn formulae by heart.
Actually there is a ISO-Norm for geometric optics which is mostly used by technicians but this is not implemented in physics education.

3. Jan 9, 2012