Reflection and Refraction
The intensity of a wave is proportional to its amplitude squared.
Intensity ∝ (Amplitude)2
Laws of Reflection:
- Angle of incidence = Angle of reflection
- The image formed by a plane (flat) mirror is the same distance behind the mirror as the object is in front.
- The image is laterally inverted and virtual
Rules of Refraction:
A wave speeds up or slows down when it enters a different medium.
If the wave slows down it bends towards the normal.
If the wave speeds up it bends away from the normal.
Frequency, Wavelength and Speed
Any part of the electromagnetic spectrum has a frequency that decides what type of wave it is. This frequency does not change when the wave is refracted. If the speed of the wave is reduced the wavelength of the wave must therefore also be reduced as:
Speed = frequency x wavelength
So the wavelength of blue light in air will be slightly longer than the wavelength of blue light in glass!
Snell's Law and the Refractive Index
So we know that waves slow down when they enter optically denser materials, and bend towards the normal line.
But can we predict how far waves will change direction?
If we label the angle of incidence as i and the angle of refraction as r, then it can be shown that when travelling from a vacuum into a material, the ratio remains constant for all values of i and r. This is Snell's Law.
We call the constant from Snell's law the refractive index, n.
Relative Refractive Index
So, what happens if you are travelling from a material that isn't a vacuum into another material?
Well its simple:
If the wave is travelling from material 1 into material 2, the ratio of the sine of the angles is still constant, but now we use the relative refractive index, 1n2.
Total Internal Reflection
If the angle of incidence is greater then the critical angle, C for that boundary the wave will totally internally reflect.
The refractive index of a material is given by:
You should know how total internal reflection is used in optical fibres and reflecting prisms.
Uses of TIR:
There are many uses for TIR starting from the simple replacement of mirrors with prisms in periscopes to the complicated world of fibre optics.
Optical fibres use TIR to send light pulses down glass fibres.
Transverse waves can oscillate in any plane. Polarisation is the process by which the oscillations are made to occur in one plane only.
This is done by passing the waves through a 'grid' so that only the waves that can fit through the slits can continue through:
Longitudinal waves cannot be polarised.
Reflection and Refraction
n = refractive index
i = angle of reflection or refraction (depends on the equation)
r = angle of reflection or refraction (depends on the equation)
C = the critical angle
c = speed of light, ms-1
λ = wavelength, m