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Refraction of Light
In A2/A-level Physics you will be asked to know the refraction - 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 (if it slows down) and away from the normal if it passes into a less refractive substance (if it speeds up)
- n₁ sin i = n₂ sin r
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 cancelslight from the other, meaning they arrive 180° out of phase
- 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, λ is wavelength, D is distance from the slits to the screen, and s is the slit spacing Young's Double Slits
Total Internal Reflection
- Total internal reflection can only take place if the incident substance has a larger refractive index than theother 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 refractiveindex
- Total internal reflection occurs at the core-cladding
- The core must be very narrow to prevent multipath dispersion
- Light used should also be monochromatic
- sin θc = n2 / n1
Diffraction
- 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.
Diffraction Grating
- A diffraction grating has thousands of narrow apertures. Consider a typical grating having 500 lines mm-1. The situation can be analysed in thesame way as Young’s two slit experiment.
- if b–c is λ then d–e will be 2 λ, f–g will be 3 λ and so on. Therefore, waves from hundreds of slits will interfere constructively, producing a well defined maximum of the diffraction pattern, called a diffraction image.
- Therefore, to find the angular positions of the maxima, where n (= 1, 2,3 etc) is now called the order of the image of the diffraction pattern:
- d sin θ = nλ
- https://getrevising.co.uk/resources/optics_as_physics_a
- https://www.hebergementwebs.com/image/a6/a68c4efe9d16af999f8d7ac73f53beae.jpg/principles-of-fiber-optic-communications.jpg
And we're all done for today!
Drafted by Kin Ho (Physics)