Optical- Scattering

Question 1

What happens when light traverses a boundary with a change in refractive index?

Answer 1

Some light is reflected. Remainder is refracted because of velocity change

Question 2

What is specular reflection?

Answer 2

Reflection from a polished flat surface

Question 3

What is diffuse reflection?

Answer 3

Reflection from a rough surface (so in many directions)

Question 4

Why is reflection needed?

Answer 4

The electric and magnetic vectors for incident and refracted rays do not match across the boundary. A third ray needed to make up difference (reflected ray). This may have change in phase as well as intensity.

Question 5

Reflection in polycrystalline materials

Answer 5

Some light reflected at every interface crossed. Interfaces are randomly oriented so net effect is to scatter light everywhere. The smaller the crystals (down to λ/2n for maximum scattering) the more boundaries and the more light is scattered. The bigger the refractive index difference the more the light is scattered.

Question 6

Titania paints

Answer 6

Opacifier in white paints typically TiO2 with highnrefractive index of 2.2 compared with resin in which it is embedded. More finely ground titania means more effective

Question 7

Why are polycrystalline ceramics generally opaque/translucent?

Answer 7

Because of light scattering at boundaries

Question 8

How come polycrystalline IR lenses can be made?

Answer 8

Use cubic symmetry materials because their r.i is independent of orientation (e.g ZnS, CaF2, NaCl not Al2O3). No r.i change means no refraction or reflection (scattering). Must eliminate porosity which causes scattering.

Question 9

Reflection at normal incidence formula

Answer 9

Ireflected/Iincident=(n2-n1)^2/(n2+n1)^2

Reflectivity independent of direction of light travel (high to low or low to high r.i)

Question 10

Problem with high refractive index glass

Answer 10

Can have reflectivities of 1/9 per surface or more. For several components, losses through reflection become large. Multiple reflections also add to background and spoils image.

Question 11

Antireflection coatings

Answer 11

2 interfaces at which reflection can occur. Make coating λ/4 thick so extra path of light reflected from coating/substrate interface is half a wavelength (strictly λ/4nc as nc affects wavelength). Makes two rays equal in intensity is appropriate choice of coating r.i. Total destructive interference should occur so no reflection.

Question 12

Ideal refractive index for coating

Answer 12

rt(nairxns)
nair is r.i of air
ns is r.i of substrate
Sometimes not many coating materials with low enough r.i exist

Question 13

Limitation of coating thickness

Answer 13

Coating can only have correct thickness at one wavelength and angle of incidence but fortunately reflectivity varies slowly with wavelength and angle

Question 14

Why are some coating slightly purple?

Answer 14

Designed for optimum performance in middle of visible spectrum and reflect more at the blue and red end giving net colour of purple.

Question 15

Solutions to limitations of coatings

Answer 15

Using 2 layers can generate 2 minima in reflectivity within visible range so giving a lower overall (greenish) reflectivity. Can use multiple coatings (low and high r.i material). Can use graded r.i mimicking a fly’s eye.

Question 16

Methods of application of multilayers

Answer 16

From vapour phase (off line in vacuum systems or on line).
By solution routes and evaporation.
Such deposition routes involve extreme conditions and costing material’s properties can be different to bulk properties.
Typically slow and costly processes
Can affect chemical and mechanical durability

Question 17

Reflecting layers

Answer 17

Make multilayer sandwiches with repeat distance of λ/2 thick so constructive interference occurs. Reflection can be enhanced if refractive changes are large. Can build up several layers to produce very high reflectivities. Can build in errors in sequence giving band pass filters transmitting some wavelengths and reflecting others.