A2/A-level Physics - Optics

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  

References:

1. https://getrevising.co.uk/resources/optics_as_physics_a
2. https://www.hebergementwebs.com/image/a6/a68c4efe9d16af999f8d7ac73f53beae.jpg/principles-of-fiber-optic-communications.jpg

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Drafted by Kin Ho (Physics)