Optics

Refraction

The change in direction of a wave due to a change in optical density

Snell’s law

Refractive index (n) = sin(i)/sin(r)

Refractive index of air

1

Refractive index of air equation

n = speed of light in air/speed of light in material r

Critical angle

The angle of incidence that produces an angle of refraction of 90 degrees

Angle of incidence < critical angle

Ray is refracted with very small reflection

Angle of incidence = critical angle

Ray emerges along edge of block

Angle of incidence > critical angle

Ray is totally internally reflected

Critical angle and refractive index equation

n=1/sin(C)

Where n= refractive index

C= critical angle

Uses of optical fibres

Decoration

Communications

Endoscopes

Structure of optical fibres

Perspex with higher refractive index

Cladding with lower refractive index

Relationship between lens power and focal length

The shorter the focal length, the greater the lens power

Lens power equation

P = 1/f

Where f= focal length

Why are some values for lens power negative

The lenses are diverging

How are images described?

inverted/upright

Magnified/diminished

Real/virtual

Magnification equation

magnification=h_i/h_o=v/u

Where h_i= image height

h_o= Object height

v= distance between image and lens

u=distance between object and lens

polarised waves

transverse waves where the vibrations are confined to a single plane and are at right angles to the direction of energy flow

how can polarised light be produced using Polaroid filters?

crystals of quinine iodosulfate all lined up in the same direction in a sheet of nitrocellulose, producing plane polarised light by selective absorption.

how can polarised light be produced by reflection?

when unpolarised light falls on a non conductive surface (like water or glass) it is partly plane polarised

How is polarisation used in sunglasses?

reflected light is partially polarised, which causes glare

Polarising sunglasses remove much of this reflected polarised light and reduces glare

How is polarisation used for television and radio transmitters?

polarising waves are used to transmit waves at different planes to prevent signal interference.

how is polarisation used for stress analysis?

plastics can show changes to polarised light when under stress, which can highlight weak points in the manufacturing process

how is polarisation used for earthquake prediction?

s waves are polarised when they pass through cracks produced in the rock caused by the stresses of crystal movement.

Detection of polarised waves is often a precursor for earthquakes, so can be used as a method of prediction.