Light and Electromagnetic Waves Honors Physics
Electromagnetic Waves EM waves are a result of accelerated charges and disturbances in electric and magnetic fields (Radio wave example here) As electrons fall from high energy levels to lower energy levels, a photon of a specific frequency and energy is emitted (Bohr example here) All EM waves travel at speed of light c = 3 x 10 8 m/s in a vacuum
Electromagnetic Spectrum
Frequency, Wavelength, Energy speed of light c = 3 x 10 8 m/s Planck s constant h = 6.63 x 10-34 J s v f Energy units are Joules or electronvolts (ev) E hf 1 ev = 1.6 x 10-19 J
Example problem A red light wave has a frequency of 4.5 x 10 14 Hz. How much time will it take the light wave to travel from NY to LA if the distance is 3,944,000 meters? What is the period of the red wave? What is the wavelength of this red light? How much energy does it carry?
Example problem A red light wave has a frequency of 4.5 x 10 14 Hz. How much time will it take the light wave to travel from NY to LA if the distance is 3,944,000 meters? What is the period of the red wave? What is the wavelength of this red light? How much energy does it carry?
Example problem A hydrogen atom with an electron initially in the n = 4 level transitions to the n = 2 level. This energy level change occurs because the atom has a) absorbed a 0.85-eV photon b) emitted a 0.85-eV photon c) absorbed a 2.55-eV photon d) emitted a 2.55-eV photon
Example problem A hydrogen atom with an electron initially in the n = 4 level transitions to the n = 2 level. This energy level change occurs because the atom has a) absorbed a 0.85-eV photon b) emitted a 0.85-eV photon c) absorbed a 2.55-eV photon d) emitted a 2.55-eV photon Electron dropped down to a lower energy level, so the energy was released as a light wave E = -0.85eV- -3.4eV E = 2.55 ev
Spectral Lines Each element has its own characteristic pattern of spectral lines as a result of unique electron transitions
Photoelectric Effect Light strikes a metal and ejects electrons (how solar power cells work) Evidence that light is a particle As frequency increases (not intensity), energy of ejected electrons increases.
Diffraction (Double slit interference) Diffraction the bending of waves around an opening (most diffraction with small opening, longer ) Light through 2 narrow slits exhibits constructive and destructive interference patterns Evidence light is a wave
Wave-Particle Duality Evidence light is both a wave and a particle (and for small particles too) What?!
Light and Color Additive primary colors for light Primary colors: red, green, blue For light Red + green = yellow Red + blue = magenta Blue + green = cyan All colors = white light
Why does a green shirt appear green? White light on a green shirt will reflect green and absorb all the other colors What if a red light shines on a green shirt? There are no green wavelengths to reflect, so it absorbs the red and appears black
Light on Pigment A pigment will only appear that color if that wavelength is present if that wavelength is absent the pigment will appear black
Opaque vs. Transparent Opaque materials have electrons that resonate with a frequency of an electromagnetic wave will absorb the wave Transparent materials will pass the EM wave to the next atom Notice glass is transparent to visible light, but opaque to infrared.
Polarization The transmission of light such that electric field waves are oriented in one direction Vertical light passes through vertical filter, but not horizontal filter
Doppler Effect with Light Motion toward observer causes observer to measure slightly higher frequencies ( blueshift ) Motion away from observer causes observer to see measure lower frequencies ( redshift )
Why is the sky blue? Scattering of light through particles of atmosphere Higher frequency light scatters more (why our sky appears blue) Lower frequency light scatters less (why sunsets appear red)
Reflection Diffuse reflection: off surfaces such that light comes off at random angles Specular reflection: off smooth surfaces that follows law of reflection
Law of Reflection Incident angle = reflected angle θ i = θ r Angles are measured with respect to the normal line
Index of Refraction Ratio of speed of light in vacuum to speed of light in that material n = c v The greater the index of refraction, the slower light travels through material
Refraction Bending of light rays when passing between transparent media because of a change of speed in the medium (frequency stays constant) Snell s Law n 1 sinθ 1 = n 2 sinθ 2 Fast to slow bends toward normal Slow to fast bends away from normal Higher frequencies will refract more than reds, separating colors into rainbows (called dispersion)
Example A ray of light strikes the boundary between air (n=1.00) and flint glass (n=1.66) at an incident angle of 40 What is the speed of light in glass? What is the refracted angle?
Example A ray of light strikes the boundary between air (n=1.00) and flint glass (n=1.66) at an incident angle of 40 What is the speed of light in glass? n = c v 1.66 = 3x108 m/s v v = 1.8x10 8 m/s What is the refracted angle? n 1 sinθ 1 = n 2 sinθ 2 (1.00)(sin40 ) = (1.66)sinθ 2 0.387 = sinθ 2 so θ 2 = 22.7