Refraction of Light
Refraction is the change of direction that occurs when light passes non-normally across a boundary between two transparent mediums
· No refraction takes place if the incident ray is along the normal
· At a boundary between two transparent substances, the light ray bends towards the normal if it passes into a more refractive substance and away from the normal if it passes into a less refractive substance
· The angle of refraction at P will always be less than the angle of incidence
If i₁ and r ₁ are the angles of incidence and refraction at point P then n = sini₁ / sinr₁. If i₂ and r₂ are the angles of incidence and refraction at point Q then 1/n = sini₂ / sinr₂
NB* n means refractive index.
Young's Double Slits
The first slit is used to help polarise the light from the lamp
· it then goes through the double slits which act as coherent sources of waves which emit light waves with a constant phase difference and the same frequency
· The interference is then shown on the screen producing alternate bright and dark fringes that are equally spaced and parallel to the double slits
· If the single slit is too wide then the dark fringes become narrower than the bright fringes and contrast between the two is lost
· Where bright fringes are formed, the light from one slit reinforces the light from the other slit, meaning they are in phase with each other
Here dark fringes are formed the light from one slit cancels light from the other, meaning they arrive 180° out of phase
Young's double slits ctd
· w = λD/s
· The fringes become more widely spaced if the D is increased, the wavelength is increased or the slit spacing is reduced
· NB* w is fringe separation, is wavelength, D is distance from the slits to the screen, and s is the slit spacing
Total Internal Reflection
· Total internal reflection can only take place if the incident substance has a larger refractive index than the other substance and if the angle of incidence exceeds the critical angle
· The angle of refraction is always 90° at the critical angle i.
· A communications optical fibre allows pulses of light that at one end of a transmitter to reach a receiver at the other end
· Fibres are often transparent to reduce absorption
· Cladding is put around the fibre and has a lower refractive index
· Total internal reflection occurs at the core-cladding
· The core must be very narrow to prevent multipath dispersion
Light used should also be monochromatic.
Sources are coherent - same frequency/constant phase difference
Light from two nearby lamp bulbs cannot from an interference pattern because the light waves are emitted at random, meaning there is not a constant phase difference. Points of cancellation and reinforcement change randomly so a pattern cannot be formed
• Vapour lamps and discharge tubes, are sources of monochromatic light, because their spectrum is dominated by light of one colour
• Light from a filament lamp or from the Sun is composed if the colour spectrum and therefore covers a variety of wavelengths from 350nm to 650nm.
• Light from lasers differ because
They are highly monochromatic, its beam power is concentrated in a very small area. Making the eye focus on the beam on a tiny spot in the retina and the intense concentration of light at that spot would destroy the retina.
It is also a convenient source of coherent light, meaning it doesn’t have to pass through a narrow single slit first. As a result photons in a laser are in phase with each other. The atoms in non-laser light emit photons at random.
• The central fringe is twice as wide as each of the outer fringe & much brighter
• The peak intensity of each fringe decreases with distance from the centre
• The outer fringes all have the same width and are much less intense than the central fringe
• The fringes become wider if the slit is made narrower
• W =2λD/a
• The width of each fringe is proportional to λ/a
NB* a is the width of a single slit, W is the width of the central fringe.
White Light Fringes
• The central fringe is white because every colour contributes at the centre of the pattern
• Inner fringes have a hint of blue on the inner side and red on the outer side. Red fringes are more spaced out than blue fringes due to wavelength, the fringes don’t overlap exactly.
• The outer fringes merge into white light becoming fainter as they move further apart from the centre, this is because where fringes meet, different colours reinforce and overlap.