Diffraction and Interference

Interference and Diffraction Distinguish Waves from Particles O The key to understanding why light behaves like waves is in interference and diffraction O Interference and diffraction are the phenomena that distinguish waves from particles: waves interfere and diffract, particles do not O Light bends around obstacles like waves do, and it is this bending which causes the single slit diffraction pattern O Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, X-rays and radio waves

Laser Interference

Assumptions Some assumptions must be made for the description of the single slit diffraction pattern: O The slit size is small, relative to the wavelength of light O The screen is far away O Cylindrical waves can be represented in 2D diagrams as circular waves O The intensity at any point on the screen is independent of the angle made between the ray to the screen and the normal line between the slit and the screen. This is possible because the slit is narrow

Difference Between the Interference and Diffraction O Richard Feynman wrote: No-one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them. O He suggested that when there are only a few sources, say two, we call it interference, as in Young's slits, but with a large number of sources, the process be labeled diffraction

When do we Have Diffraction O Diffraction of light occurs when the opening (an aperture) or obstacle (disk) are very small O We say that diffraction happens when the size of an aperture or disk are comparable to the wavelength of light

Diffraction Patterns from a Slits of Different Width

Intensity Distribution of Diffracted Light O A diffraction pattern consists of a bright central portion called the primary or central maximum flanked on both sides by secondary maxima O They are separated by dark sections known as minima O The secondary maxima decrease in intensity as their distance from the center

Interference O The principle of superposition of waves states that when two or more waves are incident on the same point, the total displacement at that point is equal to the vector sum of the displacements of the individual waves O If a crest of a wave meets a crest of another wave of the same frequency at the same point, then the magnitude of the displacement is the sum of the individual magnitudes – this is constructive interference O If a crest of one wave meets a trough of another wave then the magnitude of the displacements is equal to the difference in the individual magnitudes – this is known as destructive interference

Constructive and Distractive Interference O Constructive interference occurs when the phase difference between the waves is a multiple of 2 π, whereas destructive interference occurs when the difference is an odd multiple of π

Interference of Waves from Two Point Sources

Computational Model of an Interference Pattern From Two-slit Diffraction

Double Slit Interference

Diffraction Grating