When scientists speak of light they don’t just mean the light we see; they are referring to the range of photon energy packets that constitute the electro- magnetic spectrum. These packets have wide ranging energy levels and associated frequencies as in the diagram.
All photons would travel through space at the speed of light (186,000 miles every second) and in straight lines if space were particle free. But space isn’t particle free and photons are influenced by particles and particle energies are influenced by photon energies. Truth is all energy forms influence other energy forms and as we saw in the blogs on forces and magnetism photon energies interact with photon energies sothe concept of photon energies being fixed is like the concept of mass energies being fixed, incorrect.
Visible light that is repetitively absorbed, processed and emitted by the structures in our environment is most studied. It can be coherent or incoherent, reflected, refracted or diffracted and produce interference patterns. It is known to change its frequency and energy packet size when coming from distant structures moving rapidly away from or toward us on earth. Most of these features are related to photons in general.
When like photon energies are dispatched as regular pulses from like particle sources many photons will energy link together and set off in one of multiplicity of directions into space. We describe such light as cohered. The photons of visible light light that come to our eyes from the like atomic situation at a tiny distant point are cohered. The low energy photon pulses from a high voltage radio transmitter are cohered. As cohered light travels into space its photons occupy more space and becomes less intense.
Spatial coherence is about the sideways interactions between in step photons. Temporal (time) coherence is about the interactions between photons that are not in step as with delayed in time photons. Incoherent light is to do with more random photon emissions as happened with our tungsten light bulb but no light is entirely incoherent as all close photon energies interact.
We might think of cohered photons as linked together by energy exchanges in repeating wave fronts created by the pulsed emissions of the source. The periodicity of the waves is the frequency as set by those source vibrations and because the photons travel at light speed frequency determines the wavelength and distance wave fronts are apart. Frequency is related to photon energy packet size because source energy input levels are related to both particle agitations and energy emissions.
Light passes through glass but appears to do so at a delayed speed. The rigid particle structure of glass allows only limited electron movements. They attract visible light photons but absorb few of them. The photons have a wigging motion through the glass which is why their speed seems slowed.
Imagine a wave front of spatially cohered photons approaching an air to glass surface and at an angle to that surface. Some of them enter the glass whilst others are still in the air. The slowed photons in the glass are coherence linked to photons still in the air not yet slowed and a curving motion is the result. It’s like a slowing me hanging on to you as you move ahead of me. We stay connected but curve.
Once all the photons in the cohered bundle are in the glass they have a new and like direction and the wave front wiggles more due to increased particle attractions. Now our photon bundles come to the glass to air surface and it is as if I start speeding up before you but you cling on to me so we curve the other way. Refraction is about the behaviour of cohering photons under the changing influence of particle structures.
Young’s double slit experiment of 1801 convinced many scientists that light was not as described by Newton, corpuscular but had a wave nature. It showed that same frequency light passing through two close slits diffracted and produced a wave like interference pattern. But, why does light diffract (bend) on passing through a tiny slit or on encountering a structure edge?
It is because structure particles attract photons, more so those nearest them. The battles for photons between structure pulls and cohering pulls break the bundles up into smaller cohered bundles. Dark parts of the output pattern are where very few photons land. As with all light diagrams, think zillions of photons. When there are two slits, the cohered photon wave front photons from one slit cross the paths of cohered wave front photons from the other slit. On a screen they either support or oppose one another.
White light passed through a refracting prism breaks up into colours. Clearly violet light bends more than red light. That is because violet light is of higher energy and its cohering energy is much stronger than that of red light. With violet light the slowing me pulls more strongly on the overtaking you so we curve more rapidly and quickly re-cohere. In the case of red light I pull less firmly on you so we curve less severely.
The angle at which the light leaves the glass and re- enters the air is not as it was for refraction. The photons of light no longer have the opportunity to cohere back together into what we saw as white light when they passed through parallel air to glass and glass to air surfaces. Now, the different colours of visible light bend further apart so that we see them as rainbow colours.
We sometimes send single energy cohered light round two different routes and rejoin them. In such circumstance the light is temporally out of step and tries to cohere. Interference patterns like those shown above result. They show how the more powerful energy exchanges of violet light create more closely cohered energy bundles than the lesser cohering energy exchanges of green and red.
In an earlier blog we explained how electrons will collect and dispatch unwanted energies Rough structures emit photons in many directions whilst smooth structures emit lots of photons in the same direction behaving like an invisible trampoline.
When the angle of reflection of the emitted photons is the same as the angle of incidence of the absorbed photons the photon path lengths are near the same and they most easily cohere. Our eyes best see cohered and that is why we see the reflections we do off smooth surfaces like mirrors.
In a later blog we will look at the light coming from space and how we interpret it.