2.2.1 Refraction and Snell’s Law
Refraction is the bending of light as it passes from one optical medium to another. Going from more dense to less dense, a ray refracts away from the normal. Going from less design to more dense, a ray refracts towards the normal. Refractive index is a scale of how optically dense a material is and it can be calculated from n = Cr Cm where n is refractive index, Cr is speed of light in a vacuum and Cmis speed of light in a material. Snell’s Law of refraction is:
n1v1 = n2v2
n1 sinθ1 = n1 sinθ2
Snell’s Law demonstrates another property of light which explains why it is considered a wave, di raction.
2.2.2 Total Internal Re ection (T.I.R.)
For T.I.R. to occur, a ray must be:
1. Going from a more dense to a less dense medium (n1 > n2)
2. Angle of incidence must be equal to, or exceed the critical angle
A main use of T.I.R. is in data transmission in optical bres, they are useful because digital data can be sent down them, the signal is sent as a combination of colours, but since the colours can take a di erent route (mode) through the bre, and travel at the same velocity, they can arrive at di erent times, this causes signal loss, it is therefore bene cial to produce mono-mode bres, and there are two ways to reduce the modes:
1. Make the bre thin, this reduces the number of nodes.
2. Clad the bre in a material of slightly lower refractive index, which makes C (the critical angle) very large so only nearly parallel rays can get down the bre which reduces mode distortion.
