Refraction

• Refraction is where a wave changes direction as it enters a different medium as a result of it changing speed, this happens because the wave interacts with the particles in the new material.
• Light refracts because different materials have different optical densities. 
• When a wave travels into a more optically dense material: Speed reduces, wavelength redues, frequency stays the same. 
• n = c / cs
• n = absolute refractive index of a material, has NO UNITS!
• c = speed of light in a vacuum (3*10^8)
• cs = speed of light in the substance
• When light travels from a LESS optically dense material to a MORE optically dense material it bends TOWARDS the normal.
• When light goes from a MORE optically dense material to a LESS optically dense material it bends AWAY from the normal. 
• When light hits a material at 90 degrees to the surface it doesn't change direction, this is called hitting the surface normally.

Snell's Law

• n1 * sin theta 1 = n2 * theta 2
• Where n1 is the refractive index of material 1, and theta 1 is the angle of INCIDENCE.
• Where n2 is the refractive index of material 2, and theta 2 is the angle of REFRACTION.
• The angles must be measured from the normal.
• If n1 is smaller than n2, then theta 1 is bigger than theta 2 because it's travelling into a more optically dense material, thus refracts towards the normal. 
• If n1 is bigger than n2, then theta 1 is smaller than theta 2 because it's travelling into a less optically dense material, thus refracts away from the normal. 

Total Internal Reflection

• We observe total internal reflection if the angle of incidence when travelling from a large refractive index to a low refractive index is increased.This happens at and above a certain angle known as the critical angle.
• The critical angle is the angle of incidence when the refraction is 90 degrees when travelling from a more optically dense medium to a less optically dense medium. 
• ONLY HAPPENS when travelling to a lower refractive index e.g from glass to air. 
• when the angle of refraction is 90 degrees, the ray no longer emerges from the surface, it's reached the critical angle. Then all of the light is internally reflected. 
• sin thetac (critical angle) = n2 / n1 

Fibre Optics

• fibre optics or optical fibres are thin flexible glass or plastic…