What is incident light?
Understanding the meaning of incident light is fundamental to understanding the nature of light. So-called 'incident light' refers to light that is not coming directly to you from the original source that created the light. Everything that we see around us in our daily lives is made possible to see because of incident light.
As light can never bounce or be reflected, what happens instead is that the light is absorbed into things around you, and then brand-new incident light is emitted in its place. You can think of incident light as replacement light because all incident light is light that replaces directly-created light, i.e. replaces directly-created light that has been absorbed. So incident light is in effect a replacement light.
When light is absorbed into a material, medium or object it is gone forever by being converted to heat and other forms of energy.
For example, when daylight hits a red car, the photons of the daylight are absorbed into the atoms located below the red paint and then new incident photons are emitted. Those incident photons travel to our eyes and we see a red car. The incident photons are not somehow encoded with the colour red or the image of a car, so how do we end up seeing a red car?
This is what happens. The incident light comes out of the red car as streams of incident photons and go in all directions in straight lines. But those streams of incident photons have a journey-time that is a little bit slower than the journey-time of the speed of light. Why so? Because although each incident photon moves at the speed of light, a small time-interval occurs between each photon streaming out of the car. This time-interval is caused by the time it takes the electrons in the atoms of the car to absorb photons and then emit newly created (different) photons.
This means that the stream of incident light coming from the red car to your eyes has a journey-time that is a little slower than the normal speed of light. To be clear, each photon as such does not slow down, but the stream as a whole slows down. Here's a somewhat more technical description of incident light:
The amount by which light is slowed down as a result of absorption and emission is called the refractive index. and the process itself of absorption & emission is called attenuation. Light has no existence except as a photon travelling at speed 'c' (the constant speed of light). Light is absorbed by the first layer or so of atoms of a material or medium it comes upon. The incident light is then 'reconstructed' in accordance with the characteristics of the atoms of the material receiving the light. Some materials take longer to attenuate light.
More specifically, the incoming photons that hit electrons will over-energise such electrons and make them unstable. When this happens, the electrons are compelled to release their excess energy in the form of newly created photons.
The absorption and emission of photons takes the electrons a moment of time to accomplish. This puts a particular distance between each emitted photon. This distance determines the journey-time of light for any given light ray or group of photons. The greater the distance between each moving photon, the greater the journey-time of that whole light ray. So although any individual photon always moves at the constant speed of light, the journey-time of a given group of photons can vary. More about this throughout the book.
The key point here is that all photons in the universe are identical and every photon carries the same energy. When such energy is absorbed by an electron, the electron will release a new photon with exactly the same energy as the amount absorbed.
There are many studies showing this to be so, as in the following example:
"When a photon is absorbed into an electron, the electron is energised making it change levels. In doing so the electrons in the atom emit photons. The photon is emitted with the electron moving from a higher energy level to a lower energy level. The energy of the photon emitted has the exact same energy as the absorbed photon, i.e. the electron loses the exact energy received by moving to its lower energy level" (source: Photon Emission, Dept of Physics, Kansas State Univ).
Note: A photon cannot literally be absorbed into an electron. This is a figure of speech to explain how an electron temporarily absorbs the energy of a photon.
Coming back to incident light, it was just mentioned that such light, once absorbed and emitted, can have a journey-time that can vary depending on the physical distance between each moving photon in a light ray. Light that is absorbed/emitted is referred to as 'incident light' or 'refracted light'.
The type of material or medium receiving the light very much influences the time taken for the photons to be absorbed and then emitted as new incident light. This is why rays of incident light vary tremendously, one from one to another, in their mentioned journey-times.
The mentioned physical distance between any two moving photons is referred to as the wavelength of light. This wavelength (i.e. distance) is what tells the eyes and brain to see the colour that we are looking at.
There are many millions of different journey-times of incident light rays, and each different journey-time determines the colours we see and the overall energy of a given light ray. As the photons enter the eyes, the physical distances between the photons trigger the eyes and brain to see specific colours:
"It has to do with the special parts of the eye called rods and cones. These are what make the eye act much like a spectroscope when measuring absorption and transmittance of light into and out of a substance" (source: K. Sundeen, Spectroscopy, University of Pennsylvania MCEP).
So just about everything we see (trees, streets, people, books, food, etc) is not direct light or reflected light, it is incident light that occurs as a result of absorption and emission, into-and-out-of the objects we see. This incident light arrives at our eyes as light rays with different journey-times. Each light ray will have its own mix of distances between moving photons (i.e. own mix of wavelengths), mapping out the panorama of colours and shapes that we see. This explains why light never bounces or reflects off anything.
To avoid confusion in terminology the following image shows some of the terms used in contemporary physics regarding light:
In the above image the phrases in column A are interchangeable and they all mean exactly the same thing. They all refer to the same process by which photons are absorbed and emitted from electrons inside atoms.
Equally, the six phrases in column B mean exactly the same thing. They refer to light that has been created, for example in a candle or the sun, but has not yet been absorbed and emitted from the atoms of some object, medium or material, i.e. it is disorganised light that carries a mix of different wavelengths.
The many different phrases referring to exactly the same kind of light have gradually arisen as a result of a poor understanding of the nature of light, and also because of the 'Big Misunderstanding of Light' as explained in this book.
The phrase 'white light' causes endless confusion, so here is a clarification. The colour white, such as white paint or a white sheet refers to a colour that looks white. It looks white because when you have an equal mix of red, green and blue, the result is white. For example, if you mix red, green and blue lights for illuminating a football stadium you will have so-called white light, giving a good approximation to daylight. This kind of white light is incident light with a fixed combination of wavelengths (a fixed 'recipe') giving the colour white.
But sometimes non-incident light can also look white. For example, sunlight shining through the clouds can look white. Or some kinds of laser light or torchlight can look white, yet such light is incoherent, it is non-incident light, hence the confusion. More about this later in the book.
To finish on the subject of light attenuation it should be mentioned that the rate of attenuation varies tremendously. The 'rate of attenuation' refers to the percentage of light absorbed that is successfully emitted out as incident light.
For example, a pair of shoes may have a 52% rate of attenuation, meaning that for every 100 photons absorbed into the shoes, only 52 photons are emitted as incident light. The other 48 photons absorbed into the shoes were changed into heat or into other types of particles. A very good quality mirror may have a 99.9% rate of attenuation, meaning that nearly all the photons going into a mirror were emitted as incident light. Lead has nearly a 0% rate of attenuation, meaning that when you shine light onto lead, virtually all the photons that go into lead are not re-born as incident light (i.e. virtually no incident light comes out of lead).
Regarding planets, it's a similar situation. The moon has an 11% rate of attenuation, meaning that only about 11% of the sunlight absorbed into the moon is 'reflected back out' (attenuated) as incident light. For the Earth it's about 30%, for Mars 25% and so on.
The scientific name given to the mentioned rate of attenuation is the 'Bond albedo' effect. Here is a chart showing the Bond albedo effect for various planets in our solar system:
For example, in the above chart, the Bond albedo effect for Earth is 0.306. This means only 30.6 % of sunlight absorbed into Earth is attenuated and sent out into space as incident light. The other approximate 70% of this...
