3 3 Interference and Diffration Interference 12 January

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Title: Aims: Textbook pages: Key vocab: Key concepts: Interference Describe the origin of interference patterns using Young’s Double Slit experiment and explain the conditions for maxima and minima Contructive, destructive � When two (or more) waves meet, interference effects may be met. Key equations: Progress in topic: 1

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Demonstrating Interference �Laser Speckle – bright and darker patches of light �Bubble – only one colour is seen – the colour with a wavelength matching the thickness of the bubble �Tuning forks – each fork is a source of sound, so two waves are made. They can constructively and destructively interfere �Two sound sources – what do you notice as you walk in front of the speakers? �Why does this happen? 2

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Review - Diffraction � Through a gap width less than the wavelength of the light, the light will diffract with a pattern as shown. � By considering lines from the gap to the screen at different angles, at a given point in time, the incident wave may be either at maximum or minimum displacement. 3

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Young’s two-slit experiment � Young’s two-slit experiment is perhaps one of the most famous experimental arrangements in physics. � It was inspired by Young’s discovery of interference that he related in May 1801: “Given a pond with a canal connected to it. At two places in the pond waves are excited. In the canal two waves superpose forming a resultant wave. The amplitude of the resultant wave is determined by the phase difference with which the two waves arrive at the canal. ” (resourcefulphysics. org) 4

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Young’s Double Slit experiment: Interference – how it happens 5

3. 3 Interference and Diffration 12 January 2022 Interference Textbook pages: Path difference Difference lengths, by simple trigonometry. Red = 5λ Blue = 5. 25λ � Path difference is the difference between the distance (measured in m) travelled by two waves meeting at a point. � The path difference is a measure of how much ‘out of synch’ one wave is to another, expressed as a proportion of a wavelength, or a distance in meters � It is similar to phase difference in that by determining the path difference the conditions for constructive or destructive interference can be identified. 6

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Young’s two-slit experiment: maxima �Why is there a bright central ‘fringe’? ◦ The light has travelled the same distance, therefore there is no path difference and so the light arrives in phase. Their displacement add to give a bright fringe. �What do the next bright fringes represent? ◦ The light has travelled different distances, with a path difference equal to . When the path difference equals the wavelength, the phase difference is 0 again. (resourcefulphysics. org) 7

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Young’s two-slit experiment: minima �Why is there a dark fringe in between? ◦ One ray has travelled /2 further than the other, so they are out of phase and interfere destructively. ◦ Destructive interference means they ‘cancel out’. (resourcefulphysics. org) 8

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Conditions for minima and maxima �For maxima, the path difference should be: �nλ where n = 0, 1, 2, 3, …, n �For minima, the path difference should be: �(n-½) λ where n = 0, 1, 2, 3, …, n (resourcefulphysics. org) 9

3. 3 Interference and Diffration 12 January 2022 Interference Textbook pages: The Equation w s D � 10

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Young’s two-slit experiment �What happens if… ◦ We use slits with a smaller separation? ◦ We move the slits closer to the screen? �How could this experiment be used to measure the wavelength of light? (resourcefulphysics. org) 11

3. 3 Interference and Diffration Interference 12 January 2022 Textbook pages: Young’s two-slit experiment �A modern version of the Young’s Two Slit experiment was voted the ‘most beautiful experiment in physics’ in a Physics World readers’ poll in 2002. It still forms the basis of ongoing research into the fundamental quantum nature of matter. � As early as 1909, it was established that fringes were found even if the source was so faint that only one photon at a time was in the apparatus. � Fringes can also be seen using de Broglie (or matter) waves. � The most massive particles used to generate fringes to date (March 2005) are fluorinated buckyballs C 60 F 48 (i. e. 1632 mass units). (resourcefulphysics. org) 12