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 Radar

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)normal 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 θ1θ1 θ2θ2

Index of Refraction n = c / v n air = n water = 1.33 n vacuum = 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

Constructive and Destructive Algebraic Addition  Constructive (in phase) Destructive (180° out of phase)  Partially Constructive (somewhat out of 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