Electromagnetic Radiation: Light

Electromagnetic Radiation: Light

1. Light Is A Wave

   • Characteristics of Waves
     • Wavelength: In a wave the distance between any given point and the same point in the next wave cycle. This can be crest-to-crest, trough-to-trough, null-to-null, etc.; represented by upside-down y (^)
       • inversely related by the speed of light: c = ^ x v
         • c = 3.00 x 10^8m s^-1
     • Amplitude: the displacement of a wave from zero; height of peaks
     • Peaks- Local Maximum; Troughs - Local Minimum
     • Frequency: measured in Hertz (H); how many times a determined point on a wave passes a given point per second; represented by mew (v)
   • Evidence:
     • 1. When light is passed through a slit, it diffracts (like a wave) and displays patters of interference -- Double Slit Experiment
   • Visible Light in order of Frequency/Wavelength
     • • • • • • • Lowest Frequency; Longest Wavelength; Low Energy
       • Highest Frequency; Shortest Wavelength; High Energy
   • Properties of Waves
     • Diffraction
     • Interference
   • So why can't we use ONLY the wave model?
     • According to the wave model of light, energy should increase with intensity, BUT IT DOESN'T -- instead higher frequency (shorter wavelength) causes light to have higher energy

2. Light Is A Particle

   • Evidence:
     • 1. Energy of light increases as frequency increases (wavelength decreases -- not related to amplitude)
     • 2. Photoelectric Effect
       • Many metals emit electrons when electromagnetic radiation shines on its surface
         • The light is transferring energy to the electrons, allowing some of them to "leave" the metal (if the energy is strong enough
           • Regardless of intensity, only certain wavelength's (colors) of light provide enough energy to the electrons to overcome the attractive force of the metal's nucleus
             • If an electron is ejected from a metals surface depends entirely on the frequency of the light emitted onto it
               • If the frequency of light is above the frequency threshold , electrons are emitted from the metal (current is created) i.e violet/blue light
                 • When the intensity (brightness) is increased, more electrons will be emitted
               • If the frequency of light is below the frequency threshold, electrons will not be emitted from the metal; i.e yellow/red light
               • The Frequency Threshold varies from element to element (i.e. metal to metal)

3. Light is a Wave AND a Particle: energy of a photon is quantized

   • E = hv
     • h = 6.626 x 10^-34 Js
   • Light comes in packets of energy

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