# What are the differences between light from the Sun and light from a laser?

The sun and the laser both give off photons, but there is clearly going to be a difference(s) in the light I receive from both sources. What are those differences (energy, intensity, orientation of photons, phases etc... literally any difference you can think of)? Imagine that I put a universal measuring device (something that can detect anything imaginable) in front of light from sun and also one in front of the laser. What are the differences?

The only one I know of is that the energy and therefore the frequency of the photons coming from the sun will all be higher than the ones from the laser (standard laser, nothing too mega and nothing out of sci-fi).

• A photon is a photon is a photon. Two photons with the same energy, well, you can't tell where they came from. They are not labeled with 'Made in the Sun'. Apr 4 '17 at 20:00
• @JonCuster that's one of those trueisms that's useful only if the listener already understands what's going on. Apr 4 '17 at 20:03
• Apr 4 '17 at 20:12

The only one I know of is that the energy and therefore the frequency of the photons coming from the sun will all be higher than the ones from the laser

The sun emits a spectrum of frequencies that include the frequencies of lasers, so the above does not constitute a difference.

Classically, light is a wave, i.e. it has an amplitude and a phase. The amplitude is proportional to the electric field and magnetic fields that constitutes the electromagnetic wave, which can be decomposed according to the frequency of light.

When there is one source, the frequency and the phase (φ_0) is the same for the light emitted by a source. That light is called coherent, and is the one used in interference experiments, the double slit for example. They create a point source so that the light is coherent before the experiment.

Here is an animation of the way a classical electromagnetic field propagates"

This 3D animation shows a plane linearly polarized wave propagating from left to right. Note that the electric and magnetic fields in such a wave are in-phase with each other, reaching minima and maxima together

When light comes from lots of incoherent sources there are phase differences and the light is called incoherent.

Sun light comes from innumerable de - excitations of atoms and molecules in its plasma atmosphere, and these in no way are coherent, so sunlight is incoherent, has a small coherence length. Coherence can be imposed by passing sunlight through a small hole to create a point source and enforce coherence.

Laser light, utilizing the quantum mechanical nature of light at the source, is of a single frequency ( with a small width) and is coherent, i.e has one phase. That is why the plane wave it generates has a long coherence length and a laser beam can hit the moon.

By contrast, sunlight cannot be made coherent enough for such utilization.

So it is the classical electromagnetic behavior of light that shows a difference between sunlight and laser light. Both are many particle systems of photons, but the laser photons are organized/coherent and build up a classical coherent beam with great intensity, whereas the sun photons are incoherent so it is hard to extract a coherent subset intense enough for utilization in experiments and industry.

• Is it the incoherent nature of sunlight that causes us to have such low returns on solar cells? Apr 5 '17 at 12:43
• if the light were coherent there would be crystals with greater efficiency, but there are more factors entering in efficiency of solar cells.en.wikipedia.org/wiki/… Apr 5 '17 at 13:01

No difference between "single" photons, however for a laser you can think of the photons as in step with each other (technical term would be "in phase"). Light from the laser is generated in a special ordered way of (more or less) one wavelength. The light from the sun is generated in a higgledy-piggledy way consisting of a whole spectrum of frequencies.

• Also lasers would have polarized light and the light from the sun won't be polarized.
– MaxW
Apr 4 '17 at 22:38
• @MaxW that's one interesting and important difference that I was looking for. Apr 4 '17 at 23:26

Photons have several relevant characteristics:

• Energy, which is also directly related to frequency, wavelength and color.
• Polarization of the electromagnetic wave.
• Phase of the electromagnetic wave.

Sunlight is principally black body radiation, which is spread over a broad range of energies/frequencies, is unpolarized, and the relative phases of the photons are random or incoherent.

Laser light is monochromatic, i.e. the photons all have a characteristic energy or color, which is very precise. It is coherent, which means that the photons all have the same phase and are locked in step. It is, in general, polarized.

Solar pumped lasers can be made with quartz and lasing media, but I suppose that with a Fresnel lens with previous polarization or solar wavelength or bandpass filter, a solar ray can be better focused.