Behaviour of Light
Physics concepts aren’t the type of thing I’d normally publish here, but I figure I can write about anything on Medium, so here we go. This will be a slightly more formal article than what I usually publish.

Models of Light
There are three current models of light, all valuable in the explanation of light’s characteristics:
- Rays
- Waves
- Particles (photons)
In the ray model, light is described as travelling in straight lines, known as rays. Although light is most accurately modelled as a wave of perpendicular electric and magnetic fields, the direction of the overall wave can be considered as a straight line for simplicity.

One of the most immediately apparent properties of light is that it travels in a straight line as long as it is not interfered by external forces such as gravity or other forms of electromagnetism (electricity, light and magnets), in a uniform transparent medium (type of material) such as air.
Evidence of Rays
Evidence of this can be observed in shadows. When light is blocked by an opaque (non-transparent) body, a dark silhouette of the body is projected onto the surface behind it.

If the light travelled in a non-linear path, then the shadow cast by the ball would not have the same silhouette as the ball, perhaps an oval. In the example below, light bends outwards, creating an elongated and narrower shadow. This is not observed under normal circumstances.

If no light passes through my body, why isn’t my shadow completely dark?
If your shadow is being cast by a single light source, the light that reaches the surface behind someone is the result of rays being reflected in that direction from nearby objects.
Absorption-Emission
When particles of light, known as photons interact with atoms, they excite electrons. The energy of the proton is absorbed by the electron which is elevated to a higher energy state (absorption); from here the electron eventually returns to its ground state, releasing its energy in the form of photons (emission).
When absorption and emission happen at a larger scale, the atomic structure of an object determines its optical properties.
Blackbodies, Transparent Objects and Mirrors

Blackbodies are objects that retain the energy of incoming light. The electrons stimulated by photons vibrate within the object, rather than transmit any of it back out; the object absorbs light gaining a higher temperature (average kinetic movement of particles). However in practice, energy absorbed by objects is eventually released in some form of heat transmission (conduction convection and radiation).
The atoms within a transparent material are able to pass on the energy of incoming photons, electron by electron, to each other until the energy reaches the other side and is emitted back out. This is known as transmission.
In mirrors, atoms on their surfaces are unable to pass on nor retain the energy of photons, releasing back out in the form of light; light is reflected by mirrors.
Real world objects share a combination of all three properties, absorbing, transmitting and reflecting light in different proportions.

Reflection
Similar to the trajectory of a billiard ball, when light is reflected by a surface, is described by the Law of Reflection which states that:
The angle of an incident ray with the normal equals the angle of reflected ray with normal
The normal line is perpendicular to the reflecting surface. The incident ray, normal and reflected ray all lie on the same 2D plane and also pass a single point on the surface.

Depending on the smoothness of the surface of an object, the collection of rays as a whole will act differently.

Specular reflection occurs on smooth, flat reflective surfaces; incident rays that are parallel to each other produce parallel reflected rays.
Diffuse reflection occurs on rough, uneven reflective surfaces; parallel incident rays produce scattered reflected rays. It is worth noting that each ray abides by the Law of Reflection; each point the incident ray hits on the surface has a differently angled normal, producing differently angled reflected rays.
The Light Bender
Refraction is the bending of light due to the transition between between two mediums of varying optical density.

Changes in velocity
In terms of absorption and emission of light, different atoms react differently to light. The time taken between photons exciting electrons and said electrons releasing photons as they return to ground is varies by atom and the atomic structure of a medium will determine how quickly it is able to pass on the energy through it. This means that in different mediums, the velocity of a light ray is different. Although each photon travels at the “speed of light in a vacuum” between atom to atom, the effective velocity of a ray as a whole changes.
Materials that allow light to travel at a higher velocity, or advance, is described as optically lighter. Materials that allow light to travel at a lower velocity, or retard, is described as optically denser.
Relation to Mass Density
It is worth noting that optical density and mass density do not share correlation. Air and kerosene both have lighter mass densities than water. While air is optically lighter than water (refractive index of 1.000277 vs 1.330), Kerosene has a higher optical density than water (1.39 vs 1.33).

Refractive Index
Refractive indices are a unitless measure of the optical density of a medium, determined by the equation:

Optically denser objects have a higher refractive index.
Analogies to explain bend

A light ray can be represented by a car and photons by its wheels. The car travels faster on pavement than mud. As the car transitions from the pavement to the mud, the right wheel slows down while the left wheel continues at a higher speed. The imbalance makes the car turn towards the normal.
This analogy is also compliant with Fermat’s principle, or the principle of least time:
The path taken between two points by a ray of light is the path that can be traversed in the least time
An analogy for the refraction of light is a life guard rescuing a drowning person.

The life guard is able to run faster than he can swim so he will not take a straight path.
Snell’s Law
The bent path that light travels through two different mediums is described by Snell’s Law:

- θi is the angle of incidence
- θr is the angle of refraction
- n1 and n2 are the respective refractive indices of the two mediums
Similar to the description of reflection of light, incoming ray approaching the barrier between two mediums is called the incident ray while the outgoing ray beyond the barrier is called the refracted ray; a normal line perpendicular to the barrier at the point where the incident ray hits the barrier is also labelled.
Therefore refraction of light when it passes from a fast medium (Substance 1) to a slow medium (Substance 2) bends the light ray toward the normal and away from the normal if it passes into a faster medium. However, if the incident is normal to the barrier, the angle of incidence and refraction will both be 0 degrees and light will not be bent as it changes velocity.
Special cases
In the diagram below, light is travelling from a slower medium (water) to a faster medium (air).
According to Snell’s Law, the angle of refraction will be greater than the angle of incidence. By changing the angle of incidence, the angle of refraction can be adjusted to 90 degrees, at which no light escapes into the faster medium. This angle of incidence is known as the critical angle.
Any incident angle beyond the critical angle (but below 90 degrees) will cause total internal reflection of the light.
Footnote
This article is a draft for my allocation of work on a poster on light for an assessment:
