Light Lecture Review

Light

Behaviors of Light

• Light exhibits both wave and particle behaviors.
  • As a wave: Demonstrates five wave behaviors.
  • As a particle: Demonstrates two particle behaviors.

Wave Behaviors of Light

1. Reflection

• Reflection: The bouncing of light.
  • Example: Mirrors.
• Law of Reflection:
  • Angle of incidence = angle of reflection.
  • Angles are measured with respect to the normal, which is a line perpendicular to the surface.
• Diagram illustrating the incident ray, reflected ray, angle of incidence ($\Theta<em>i$), and angle of reflection ($\Theta</em>r$), with the normal line.

2. Refraction

• Refraction: The bending of light.
  • Change in path due to change in speed.
  • Change in speed caused by change in medium light is traveling through.
  • Example: A pencil in a cup of water appearing bent.
• Diagram illustrating refraction as light passes from air to water.

3. Diffraction

• Diffraction: The spreading out of light.
  • Diffraction grating disperses light into its component colors.
  • Examples: Hologram on a credit card, colors seen on CD/DVD face.

4. Interference

• Interference: Constructive or destructive.
  • Results in patterns of light and dark.
  • Young’s double-slit experiment demonstrates interference.

5. Polarization

• Polarization: Light waves vibrating in one plane only.
  • Light from the sun is non-polarized.
  • A polarizing filter lets only one vibration through, blocking all others.
  • If a second filter is placed on top of the first, no light gets through if it's perpendicular.

Electromagnetic Energy

• Electromagnetic (EM) Energy Characteristics:
  • Radiant energy
  • Transverse waves
  • Can travel through empty space
  • $c = 3 \times 10^8 m/s$ (speed of light in a vacuum)
  • Made by exciting an electron from its ground state

EM Energy & Planck

• Maxwell Planck discovered the relationship between energy and frequency.
• $E = hf$
  • E = Energy (Joules)
  • f = frequency (Hz)
  • $h$ = Planck’s constant = $6.626 \times 10^{-34} J \\cdot s$

Example Problem:

• What is the frequency of a photon with an energy of $2.7 \times 10^{-19} J$?
  • E = $2.7 \times 10^{-19} J$
  • f = ?
  • h = Planck’s constant = $6.626 \times 10^{-34} J \cdot s$
• Solution:
  • $E = hf$
  • $f = E/h$
  • $f = (2.7 \times 10^{-19} J) / (6.626 \times 10^{-34} J \cdot s)$
  • $f = 4.07 \times 10^{14} Hz$

Electromagnetic Spectrum

• Regions of the electromagnetic spectrum (from high frequency/short wavelength/high energy to low frequency/long wavelength/low energy):
  • Gamma rays
  • X-rays
  • Ultraviolet (UVA and UVB)
  • Visible Light
    • Violet (400 nm)
    • Blue
    • Green
    • Yellow
    • Orange
    • Red (750 nm)
  • Infrared
  • Microwaves (cell phones)
  • Radio Waves

Particle Behaviors of Light

• Two Particle Behaviors of Light
  1. Atomic Line Spectra
  2. Photoelectric Effect

Photon

• Photon: A massless particle of light that has a specific amount of energy associated with it, depending on the frequency (color) of the light.

1. Atomic Line Spectra

• An electron is hit by a photon and absorbs its energy.
• The electron moves to a higher energy level.
• When the electron “decays” back to a lower energy level, it releases a photon.
• Released photons emit light, which we see as specific colors.
• These energy level jumps are called quantum leaps.

2. Photoelectric Effect

• When light shines on a piece of metal, electricity begins to flow.
• Electricity = flow of electrons.
• Electrons are matter, so wave behavior of light cannot explain this.
• Light must be made of particles called PHOTONS.
• A photon strikes the metal surface:
  • If the photon has enough energy, an electron will be freed and will flow through the metal.
  • If the photon does not have enough energy, an electron cannot be freed.
• The energy of the photon determines if it can free an electron.