# Convergence of light rays and Optical Path Length (OPL)

1. Dec 17, 2007

### pardesi

Why do all rays coming from far off distance from a source(may be planar)... converging at a point need to travel the same Optical Path Length(OPL)

2. Dec 17, 2007

### mgb_phys

They don't have to - it depends on what you want to do with them at the point.

3. Dec 17, 2007

### arunbg

Usually they don't travel the same OPL.

4. Dec 17, 2007

### Gokul43201

Staff Emeritus
For a truly planar object with its plane is normal to the direction of the rays, then you can estimate the maximum difference in the OPL from its largest dimension (say 2d) and the distance to the point of convergence (call this D):

$$\delta L= D[(1+(d/D)^2)^{1/2}-1] \approx d^2/2D$$

This number is tiny compared to D, but it's much harder to make it small compared to the wavelength of the light.

5. Dec 18, 2007

### pardesi

but is it true if they start from the same point source