3.2.3

why is there refraction:
two materials of different optical densities, light will travel at different speeds in each

from less optically dense to more optically dense, light bends toward the normal

from more optically dense to less optically dense, light bends away from the normal


refractive index: a measure of optical density

nx = c/cx

where
 nx is the refractive index of material x
 c is the speed of light in a vacuum (ms^-1)
 cx is the speed of light in material x (ms^-1)

n>1, as if n<1 the speed of light in x is greater than the speed of light in a vacuum, which isn't the case

the greater the value of n, the more optically dense, as light is being slowed more

refractive index of air is approximately 1


snell's law of refraction:

n1sin𝛳1 = n2sin𝛳2

where
 n1 is the refractive index of material 1
 sin𝛳1 is angle 1
 n2 is the refractive index of material 2
 sin𝛳2 is angle 2


critical angle:

sin𝛳c = n2/n1

note: this is only for more dense to less dense i.e. n1 > n2

fibre optics:

optical fibre: a thin flexible transparent fibre used to carry light pulses from one end to the other

uses: communication, endoscopes

cladding:
 prevents crosstalk
 provides protection
 stops signals escaping
 needs a lower refractive index than the core

ideally:

the core is thin to reduce the effects of MODAL DISPERSION

use monochromatic light to reduce effects of MATERIAL DISPERSION

modal dispersion: the lengthening of a light pulse as it travels along an optical fibre, due to rays that repeatedly undergo total internal reflection having to travel a longer distance than rays that undergo fewer total internal reflections

material dispersion: caused by using white light. all wavelengths of white light travel at slightly different speeds and so leads to pulse broadening