Revision Tips – Key areas Final Countdown 15th May You will need paper, pen and a calculator Use the chat pod to ask questions

Lots to cover – focus on light and waves Most other topics requested have been covered through the year so use the watch back facility. We’ll concentrate on path difference and interference, and refraction. If we have time I’ll try and fit in some relativity. Use the chat pod to ask questions

Path Difference I am assuming you know! What diffraction is, what coherent means and terms such as wavelength, crest and trough

Young’s Slits This experiment was first carried out by Thomas Young in 1801. Young split a beam of sunlight into two and showed that the two coherent beams that were produced formed a series of bright and dark lines or 'fringes' on the opposite wall of the room. This was the first time that light had been shown to form an interference pattern, proving that light showed wave properties. As the light passes through the slits S1 and S2, circular diffraction patterns are formed and where they overlap an interference pattern is formed. What happens in that interference depends on how far each of the waves have travelled and their wavelength.

Path difference 2 The interference pattern on a screen opposite would look like this, and a graph of irradiance against position is shown below. The brightest fringe occurs at the centre of the interference pattern as this point is the same distance from both slits, and so the waves arrive exactly in phase. The first dark fringes occur on either side of this when the path difference between the beams is exactly half a wavelength. This is followed by the next bright fringe, due to a path difference of exactly one wavelength, and so on.

Path difference 3 In general, a bright fringe occurs when the path difference between the two beams is mλ, where m = 0, ±1, ±2... The central bright fringe corresponds to m = 0. Path difference = mλ for maxima In a similar way, a dark fringe occurs when the path difference between the two beams is (m+1/2) l where m is again 0, ±1, ±2... Path difference = (m+1/2) l

Example A microwave transmitter is placed in front of a metal plate that has two slits A and B as shown. A microwave detector is moved along the line from C to D. The zero- order maximum of radiation is detected at C and the first-order maximum is detected at D. AD = 0·52 m and BD = 0·55 m.

Calculate the path difference between paths AD and BC. 0.55 - 0.52 = 0.03 cm What is the wavelength of the microwaves? Path difference = ml, 0.03= 1 l, l = 0.03m Calculate the path difference from slits A and B to the second-order maximum. Path difference = ml, pd = 2 x 0.03, pd = 0.06m

Calculate the path difference from slits A and B to the minimum of intensity between C and D. Path difference = (m+1/2) l, pd = (0+1/2) 0.03, pd = 0.015m Calculate the path difference from slits A and B to the next minimum after D. Path difference = (m+1/2) l, pd = (1+1/2) 0.03, pd = 0.045m What is the path difference from slits A and B to point C Central maximum always pd = 0 m

Your turn Diffraction happens when waves go past the end of a barrier. a particle goes through a gap in a barrier. waves reflect off a barrier. a particle passes close to the edge of a barrier. waves pass from one medium into another. The Young's double slit experiment proves that light is carried by waves because the experiment shows absorption of waves. coherence of waves. interference of waves. reflection of waves. refraction of waves.

In the diagram, X is a point in an interference pattern produced by waves from two coherent sources S1 and S2. The wavelength of the waves is 4 cm and the distance S1X is 20 cm. Which two distances S2X would mean that X was a point of maximum interference? 22 cm and 23 cm 24 cm and 28 cm 24 cm and 26 cm 22 cm and 26 cm 26 cm and 28 cm

Refraction What you need to know What refractive index means Total internal reflection and critical angle What TIR is used for

Refractive index Refraction happens when a wave goes from one medium into another. If, for example, a ray of light travelling in air meets a glass block, the ray of light slows down when it enters the glass. This change in velocity may cause the direction of the ray of light to change (as long as it was not travelling along the normal) so that its new direction is closer to the normal. Refractive index, n, is unusual in that it is just a number, it has no unit. The refractive index, n, of a medium is the ratio where θ1 is in a vacuum, and θ2 is in the medium. In practice, there is very little difference in the value of n when θ1 is in air from that when θ1 is in a vacuum.

Refraction and frequency The frequency of a wave is determined by the source that generated the wave. As a consequence of this, no matter what happens to a wave after it has been generated, its frequency does not change. This means that, when a light wave passes from one medium into another (when it is refracted) its frequency is unaltered. However, the refractive index of a medium does depend on the frequency of the incident light. Since wavelength and colour of light also depend on frequency, it follows that light of different wavelengths and therefore different colours will be refracted by different amounts. If a beam of white light is refracted by a triangular glass prism, it is found that the white light is spread out to form a spectrum. The red end of the spectrum is deviated least from the original direction, while the violet end is deviated most.

Total Internal Reflection and Critical Angle We have already seen that when a ray of light travels from air into glass, its direction is changed to be closer towards the normal at the point of incidence. Since rays of light obey the principle of reversibility, then when a ray of light travels from glass into air, its direction will be changed to be further away from the normal at the point of incidence. This can be shown using the same semi-circular glass block that was used earlier, only this time with the ray of light directed at the semi-circular face. If the ray of light is directed towards the centre of the flat face of the block (i.e. along a radius of the semi-circle) its direction does not change when it enters the block. Because the angle of refraction in the air is always greater than the angle of incidence in the glass, there is a maximum angle of incidence that allows the ray to refract out of the glass.

Your turn The diagram shows the refraction of a ray of monochromatic light of frequency 4 x 104 Hz The ray makes an angle of 40° to the normal in the water. (The diagram is not drawn to scale.) The refractive index of water for the light used is 1.33 Calculate angle Q. Calculate the critical angle for the light in the water. Calculate the wavelength, in nm, of the light in air. Calculate the wavelength, in nm, of the light in the water.

Your Turn 2 The critical angle for light emerging from ice into air is 50°. What is the refractive index of ice? 1.13 1.19 1.31 1.56 2.00

Uses of Total Internal Reflection optical fibres used for communication car number plates and some reflective road signs Binocular and periscopes

Good Luck !!

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