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Do you guys still remember optics in AS/A-level Physics?
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 (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 cancels light 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
- w is fringe separation, λ is wavelength, D is distance from the slits to the screen, and s is the slit spacing
Diffraction
Diffraction is an important chapter in AS/A-level Physics.
- 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
- 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 500linesmm-1. The situation can be analysed in the same way as Young’s two slit experiment.
- if b–c is then d–e will be 2 , f–g will be 3 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
White Light Diffraction
- 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.
This is the end of the topic!