3 3 Interference and Diffraction Textbook pages Title

3. 3 Interference and Diffraction Textbook pages: Title: Aims: 13 December 2021 Xx-xx Diffraction Describe diffraction, explore diffraction gratings derive formula, perform calculations Textbook pages: Key vocab: Key concepts: Grating, maximum, minimum, diffraction � Diffraction occurs when the gap a wave passes through tends to equal the waves wavelength. � Diffraction gratings can be used to diffract light. Key equations: Progress in topic: 1

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Today’s Activities… �Familiarise ourselves with the diffraction grating �Look through a variety of diffraction gratings to observe the phenomenon of diffraction �Derive the diffraction equation 2

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Observing Diffraction �Try observing diffraction using: ◦ Diffraction gratings (look like slides) ◦ Spectroscopes �Observe different lights: ◦ Overhead lights ◦ Projector light ◦ Sunlight (do not look directly at the sun) 3

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Diffraction with a LASER 4

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Why does diffraction matter… a lot � It has been argued as one of the most important experimental inventions. � It has been used to investigate the structure of matter, including DNA. � It has been used to determine the structure of the Universe from Doppler shifted lines. � The chemical composition of almost everything has been determined by diffraction. � Even Mr van Hoek has made discoveries about the structure of nano-crystalline Silicon Dioxide using X-ray diffraction. � http: //users. aber. ac. uk/ruw/res/papers/13. html 5

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx 6

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Diffraction with a Ripple Tank 7

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Deriving the diffraction equation 8

3. 3 Interference and Diffraction 13 December 2021 Diffraction Xx-xx Textbook pages: Deriving the diffraction equation � Constructive interference only occurs along a few precise directions, one of which is shown in the diagram. Light from A must be in phase with light from B, and this can only happen when the path difference is a whole number of complete wavelengths (even number of half-wavelengths). AC = nλ, where n = 0, 1, 2, 3. . . � � Parallel rays of a monochromatic light of wavelength l are incident on a diffraction grating in which the slit separation is d. If the grating has N lines per metre, the grating spacing is given by: Now, using trigonometry: AC = d sin θ ◦ where θ is the angle of diffraction. � So it’s easy to see that: d sin θ = n λ The term n is called the spectrum order. If n = 1, we have the first diffraction maximum. � sin θ can never be greater than 1, so there is a physical limit to the number of spectra that can be obtained. � 9

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx Note about diffraction gratings �Gratings are often specified as so many lines per metre (often on ‘old’ gratings as per inch!). �Separations as small as 100 nm possible, = 10 million lines per metre (equivalent to about 40, 000 lines per inch). 10

3. 3 Interference and Diffraction Textbook pages: 13 December 2021 Xx-xx 11