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Wave Behavior. Another McGourty-Rideout Production. The Physics of Waves. All waves follow the laws of physics no matter what type Waves can be reflected, refracted, diffracted, absorbed, scattered and experience interference . Reflection.
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Wave Behavior Another McGourty-Rideout Production
The Physics of Waves • All waves follow the laws of physics no matter what type • Waves can be reflected, refracted, diffracted, absorbed, scattered and experience interference
Reflection • Reflection happens when a wave bounces off an obstacle. Specular reflection: smooth surface • Angle of incidence = angle of reflection Diffuse reflection: rough surface • Reflection in all directions because angle of incidence varies over the surface due to its roughness
Law of Reflection angle of incidence= angle of reflection
Reflection • An echo - example of a reflection • Radar uses this principle to determine the size, characteristics of, and distance to an object
Refraction • Occurs as waves move from one medium into another or within a medium, like air, that varies in density or temperature • Waves bend toward the normal when they move from a less optically dense medium (faster) to a more optically dense medium (slower) • Waves bend away from the normal when the opposite is true
Snell’s Law of Refraction • Angles are measured with respect to the normal
Refraction θ2 θ1
Index of Refraction n = c / v • nair =1.0003 • nwater = 1.33 • nvacuum = 1.00 • Can you explain why “n” is a naked number? Can it ever be less than 1?
Index of Refraction Redux n=√εμ ε = electric permittivity μ = magnetic permeability These describe how the material interacts with electric and magnetic fields
Atmospheric Refraction • Causes gradual curve of light from stars and sun • Makes sun visible 2-3 minutes before sunrise and after sunset
Dispersion The index of refraction of real materials actually depends on the frequency of the light being bent. Dispersion is the explanation for rainbows: Each color has its own frequency Each gets slowed down differently Each comes out at a different angle
Diffraction • Waves that have longer wavelengths, or lower frequencies, diffract better than high frequency waves • Diffraction patterns are determined by both the size of the opening and the wavelength
Absorption • Absorption happens when the medium has the ability to absorb the energy of the wave • When the wave is absorbed, its energy is transferred to the medium and the wave is gone • Gradual absorption as the wave penetrates the medium is called “attenuation” • Absorption of only specific frequencies will leave “gaps” in the continuing wave spectrum called “spectral absorption lines”
Absorption • Absorption at the quantum level happens when an individual photon has the exact energy that corresponds to an energy gap between two energy states of the medium • The type of energy gap corresponds directly to the frequency of the photon
Scattering • If the photon is absorbed and then re-emitted immediately, it is said to be scattered • How the light is scattered is dependent on the frequency of the light and the size of the particle it is scattering from • Some of the energy of the light is absorbed by the scatterer and so the re-emitted light has a little less energy
Scattering • If the photon has a longer wavelength than the size of the scattering particle, it is called Rayleigh scattering • In Rayleigh scattering the very long wavelength light is hardly scattered at all but the shorter wavelength is much more strongly scattered • Since blue light is much ‘shorter’ than red, it gets more scattered by the molecules in the air: therefore the sky is blue!
Interference • When two or more waves come together, they “superimpose” or add together • The total amplitude is simply the sum (positive & negative!) of all the individual amplitudes • The extremes of what can happen are called constructive interference and destructive interference
S S S S Destructive (180° out of phase) Constructive and Destructive Algebraic Addition Partially Constructive (somewhat out of phase) Constructive (in phase) Non-coherent signals (noise)
Interference Fringes • Interference fringes are a series of bright and dark bands • Sometimes straight, sometimes circular, sometimes more complicated
Young’s Double Slit Experiment • Light diffracting through 2 slits produces fringes on a screen • Bright fringes are areas of constructive interference • Dark fringes are areas of destructive interference