Diffraction – light as a wave

1. Diffraction – light as a wave Examples of wave diffraction: Water waves diffract through a small opening in the dam. Sound waves diffract through a crack on the wall. Light diffracts. Huygens’ principle of diffraction: Wave front made of spherical wavelets travel to the small opening, those wavelets passing through this opening result in a spherical wave front with coherent wavelets.

2. Single-slit diffraction -- concept Constructive interference, bright fringe Light source Destructive interference, dark fringe So light diffracts because the wavelets interfere!

3. Single-slit diffraction -- formula

4. Single-slit diffraction -- example Coherent monochromatic light is incident on a slit whose width is 0.0211 mm. The diffraction pattern is viewed on a screen that is placed 3.12 m from the slit. The distance along the screen from the middle of the central maximum to the first dark fringe is 10.1 cm. What is the wavelength of the light? 10.1 cm m = 1, a = 0.0211 mm. Solve for ,  = 683 nm. 3.12 m θ a

5. Diffraction by a circular aperture and resolving power When light passes through a circular aperture with a diameter comparable to the light wavelength, one sees diffraction patterns shown on the right photograph. The angle θ to the first minimum is:

6. Rayleigh’s criterion, resolution of a circular aperture (lens) Diffraction by a circular aperture and resolving power

7. Example on resolving power

8. Bragg’s law: 2d sinθ = m d = distance between planes of atoms θ = angle between ray, plane of atoms m = order of maximum  = wavelength. Another example of diffraction -- grating X-rays reflected from a regular structure of a crystal creates diffraction pattern that can be used to measure the crystal structure. Like this diffraction pattern of a DNA molecule.