Light Wave
Light can be described as both a particle or as a wave. Light shows properties of the two. For example, light defracts like a wave but is able to travel through a vacuum as only particles can seemingly do.
This strange property of light is thought to be due to of the effects of quantum mechanics. Light is usually treated as a wave but is sometimes drawn as a ray (line showing the direction of the wave front).
Light is a transverse wave. A transverse wave is a wave that causes the individual particles of a media to move up and down as the wave passes through it. For example, water particles move up and down as a water wave travels through them.
Figure 1.0 - Light reflecting.
Reflection of Light
Law of reflection: i = r
The angle at which the light hits the mirror (angle of incidence) i is equal to the angle of reflection r [Figure 1.0]. The angle is measured from the normal which, in the graphic above, is represented by a dotted line.
When light hits a very smooth surface, such as a mirror, most of the light is reflected back in the same direction. However, when light hits a rough surface such as a desk, the light is reflected in many different directions. This is called diffused reflection. This is why you can.t see your reflection in a desk as you can in a mirror. Be aware that the law of reflection still applies for diffused reflection.
Figure 2.0 - Light refracting.
Refraction of Light
Refraction is when light bends as it travels through the boundary between two transparent substances [Figure 2.0]. The angle of the light ray before it reaches the boundary between the two substances (incidence ray) and the angle of the light after it has been refracted (angle of the refraction) is measured between the light ray and the normal (90 ° from boundary of the two substances).
To figure out the angle of refraction this formula is used:
n1 x sin(i) = n2 x sin(r)
Rearanged for sin(r): (n1 x sin(i))/n2 = sin(r)
Where i is the angle of incidence, r is the angle of refraction, n1 is the refractive index of the substance the light starts from and n2 is the refractive index of the second substance.
For example,
If light was traveling from air, which has a refractive index of 1, into glass, which has a refractive index of 1.5, at an angle of 32 °:
(1 x sin(32))/1.5 = sin(r)
sin-1(0.35327) = 20.69
Therefore, the light is refracted at an angle of 20.69 ° from the normal.
Why Light Bends
Different colors, or wavelenths, of light travel at different speeds when traveling through non-vacuum media. As some of the light speeds up more than other light it veers at a different angle than it started at.
Total Internal Reflection
If light travels from a substance with a high refractive index, into another substance that has a lower refractive index, the light will always bend away from the normal. The refracted ray will be bigger.
Now that you know about refraction of light you might be thinking: "What happens when the refracted ray is reflected at an angle greater than 90° from the normal?". The answer is total internal reflection.
When total internal reflection occurs, all the light is reflected instead of some of the light being reflected and some refracted. Total internal reflection is what makes optical fibres work, letting the light bounce back and forth along the inside of the cable without losing any light.
Here is an example movie of total internal reflection through a straight and a bent optical fibre.
The minimum angle needed for total internal refraction to occur is called the critical angle. To work out if the light is totally reflected, use this formula:
n1/n2 = sin(C)
Where n1 is the angle of incidence, n2 is the angle of refraction and C is the critical angle.