- #1

- 970

- 3

If I attach a pipe (of the same radius as the opening of the flashlight) to the aperture so that light has to go through the pipe, then geometrically the circle of light on the wall should be as big as the pipe diameter.

I'm thinking it could be diffraction, that you treat the aperture of the flashlight as a circle filled with continuous sources, and the equation for how big the light should appear on the wall should go roughly around:

[tex]R=\frac{\lambda}{D} L [/tex]

where R is the radius of the light formed on the wall, lambda the wavelength of light, D the diameter of the aperture, and L is the distance to the wall. But just an order of magnitude estimate doesn't produce a reasonable answer, as it'll take a huge L to get a big R.

But strictly speaking that equation only holds for coherent light sources, when the sources in the aperture are uniformly in phase. For noncoherent light, shouldn't R be infinity, since there can be no interference effects? Each source in the aperture propagates spherical waves that can reach any point on the wall and won't interfere with neighboring sources.

I'm also a little bit confused about spatial coherence. Two noncoherent finite-size light sources (or even just one), cannot interfere (with itself in case of one). Yet why is it said that a light bulb has a coherence area, albeit small? I know that coherence area can be taken to mean how far two slits have to be to produce interference fringes from a source (incoherent or coherent), but it is really the two slits that cause interference and not the incoherent sources. So when you give incoherent light a coherence area, does it just mean that it'll produce interference if two slits are within the area? Or is there a more general definition of coherence area that doesn't involve double slits?

Also, given that the wavelength of light it very small, shouldn't the light just go straight through the slits geometrically and form no diffraction at all, so that the trajectory of the light from the source would look like a less than sign < )?

Also, just collimating an incoherent source shouldn't produce spatially coherent light, right? Within a cross-section of the beam, all the phases at each point in the cross-section should still be random? I heard in a laser each point in the entire cross-section is in phase. Is this the difference between a laser and collimated light passed through a colored piece of glass? What can you do with a laser that you can't do with monochromatic collimated light? What's so good about having all the light in a cross-section have uniform phase?