# Size and distance of different wavelengh photons?

1. Sep 28, 2009

### crockman1

a few questions for my own research (not too sure how to ask):

maxwells equations predicted that the speed of light should be the same watever the speed of the source. it follows from this that if a pulse of light is emitted at a particular time at a particular point in space then as time goes on it will spread out as a sphere of light whose size and position are independent of the source. after one millionth of a second the light will have spread out to form a sphere with a radius of 300 meters; after two millionths of a second, the radius will be 600 meters.
-stephen hawking

(a)
1.would a group of photons of different wavelengths/energy levels travel at different speeds in relation to photons of different wavelengths/energy levels?
2.would a group of photons of different wavelengths/energy levels expand at the same rate in relation to photons of different wavelengths/energy levels?

(b)
1.would a group of photons of different wavelengths/energy levels travel at different speeds in mediums in relation to photons of different wavelengths/energy levels?
2.would a group of photons of different wavelengths/energy levels expand at the same rates in mediums in relation to photons of different wavelengths/energy levels?

and what would be the size (radius), speed, and distance of two or more groups of photons of different energy levels, at one millisecond. if not known, two frames of reference in referring to time would be appriciated.

thank you

Last edited: Sep 28, 2009
2. Sep 28, 2009

### James Leighe

What do you mean by expand?

3. Sep 29, 2009

### crockman1

Note; i have restated the question to resolve this statement, p.s. thank you james this was a valuable statement.

4. Sep 29, 2009

### Staff: Mentor

The questions don't make a lot of sense to me. You seem to ask if different wavelengths/frequencies of light move at different speeds even after apparently correctly noting earlier that the speed of light is constant.

Next, this "expand" thing, I don't understand that either. You seem to understand that light expands in a spherical pattern, at the speed of light, though...

5. Sep 29, 2009

### Born2bwire

He could be talking about wave packets or the group velocities. The speed part would be the group velocity while the "expand" would be the spreading of a packet of information. Like the speed of a Gaussian pulse and then the spreading of a Gaussian pulse.

In any case, in vacuum, all photons move at the same speed. In a medium, it depends. If the medium is isotropic, loss-less, and non-dispersive, all photons will travel at the same speed. However, once the medium becomes lossy or dispersive, the group velocities will be come frequency dependent. This means that waves at different frequencies will travel at different speeds. It also means that if we sent in a Gaussian pulse (in the time domain, well frequency too since it so neatly Fourier transforms), then the pulse will widen as it travels through the medium because of dispersion. An example (ignore all the stuff about matter waves): http://www.physics.ucdavis.edu/Classes/NonclassicalPhysics/dispersion.html .

6. Oct 1, 2009

### crockman1

would for example a radio photon expand, and or travel at a different speed compared to a visible light photon? do all electromagnetic energy waves aka photons travel at the same speed and expand at the same rates reguardless of their frequency/ relative position in space?

Last edited: Oct 1, 2009
7. Oct 1, 2009

### Staff: Mentor

In vacuum, all electromagnetic radiation travels at the same speed, regardless of wavelength.

8. Oct 1, 2009

### Bob_for_short

How many oscillations (periods) are contained in a photon? This determines its "length".

9. Oct 1, 2009

### crockman1

in a medium too?

it would lead me to conclude from this statement that all electromagnetic radiation also expands at the same rate? if a visible photon has a 600m radius at one millisecond then an infrared photon or a gamma photon will also have a 600m radius at one millisecond?

another question, how far does a photon travel in one millisecond in space?

thank you all for your support

10. Oct 2, 2009

### Born2bwire

Whoa there. I already stated in my own post that the speed is constant across frequencies in a vacuum and any isotropic, dispersion-less, lossless medium. Once you add in effects like anisotropy, dispersion, and/or loss then the speed will be frequency dependent.

HOWEVER, photons have no physical size. They do not expand, contract, or anything of that kind from what we know. They are simply energy quanta, discrete packets of energy.

11. Oct 2, 2009

### crockman1

ohhh hawking must have been referring to a sphere emitting light in all directions then.

i have more questions:

do photons effect photons? in other words if two or more photons travel paralel to each other in a vaccum would they repel each other over time? lasers spread over time. would this be attributed to an imperfection in the angle of the source of the laser?

Last edited: Oct 2, 2009