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 the motions of photons passing through it are influenced by the particle energies in space that have energy desires and process photons. Of course it follows that the motions of particle energies are influenced by photon energies and we have seen how routine particle vibrations and gravitational and electrical particle motions are examples of just such influences.
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. The concept of photon energies being fixed is like the concept of mass energies being fixed, incorrect.
In the above diagram we see a small range of photon energies and frequencies is the visible light we are able to see. Of all the radiations it has understandable been most studied. Science talks of it as being coherent or incoherent, refers to it as having undergone reflection, refraction or diffraction and sometimes describes it as producing interference patterns. When coming from distant galaxies moving rapidly away from or toward us on earth it can change its frequency and photon energy packet size in what is termed red or blue shifting.
I will try to explain some of these things in terms of visible light but much of what I say will apply to the whole of the electro magnetic radiation spectrum.
When like energy photons are dispatched as regular pulses from like particle sources they have a desire to energy link together and so exchange tiny energies before setting off on one of a multiplicity of space directions. We describe such point source light as cohered. The photons of visible light that come to our eyes from distant points in view 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 energy 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. However no light is entirely incoherent as all close photon energies interact.
We might think of cohered photons as linked together by energy exchanges in wave fronts created by the pulsed emissions of the source. The rate at which the wave fronts pass a point in space is the frequency of the photon pulses as set by the source particle vibrations. Because all photons travel at light speed this frequency will determine the wavelength or distance apart the wave fronts are. The reason that photon energy packet size is related to frequency in the above spectrum is because the agitation (vibration) of the source particles is determined by the energy of the photon exchanges they are making.
Why is it that when light passes through glass it appears to do so at a delayed speed? The rigid particle structure of glass is such that its electrons are held quite firmly in place. They can make only limited moves toward photons and so absorb only a small percentage of them. However the electrons still exert a pull on the photons that 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 wavefront photons 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 wavefront bundle are in the glass they have a new and like direction and the wave front wiggles more than it did in the air 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 corpuscular as described by Newton, but had a wave nature. It showed that a same frequency of 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 the structure particle 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.