LIGHT NOTES

CP PhysicsMs. Morrison

ELECTROMAGNETIC WAVES

Electromagnetic Waves

Moving charged particles create magnetic fields

Changing motion of charged particle creates expanding and collapsing magnetic field which generates and expanding and collapsing electric field

Travel at 3.0 x 108 m/s through empty space ( c = speed of light)

Travels more slowly through mediums Energy depends on frequency

Electromagnetic Spectrum

7 Types of Electromagnetic Radiation

1. Radio Waves2. Microwaves3. Infrared waves4. Visible light5. Ultraviolet light6. X-rays7. Gamma rays

Radio Waves

Lowest frequency Longest wavelength Least energy of spectrum Carry sound waves

Microwaves

Frequencies match natural frequencies of molecules (fats, proteins, etc.)

Causes food molecules to vibrate making the food cook itself

Icebergs give off microwaves Match natural frequencies of some

pacemakers

Infrared waves

Heat waves – all animals give off infrared Uses:

Heat lamps Night vision Alcohol breath test Remote controls

Visible Light

Only 1/1,000,000 of EM spectrum Different frequencies seen as colors (ROY

G BIV) Red light

Lowest frequency (least energy) Longest wavelength

Violet light Highest frequency (most energy) Shortest wavelength

Ultraviolet Light

Causes sunburns, damages tissues, and can kill bacteria

UV-B rays more dangerous than UV-A because they have higher frequency

X-rays

Produced by high speed electrons slamming into a metal plate

Uses: Medical X-rays (bones, teeth) Airport security

Gamma Rays

Highest frequency of EM spectrum so highest energy waves of spectrum and greatest penetrating power

Shortest wavelengths of spectrum Produced by nuclear particles and occur

with every nuclear reaction Used in radiation therapy (kill healthy

cells, but kill cancer cells)

VISIBLE LIGHT

General Characteristics

Frequencies that eye is sensitive to c = 3 x 108 m/s c = λf Transverse wave Interaction with materials

Transparent – all light transmitted, ex. Clear glass

Translucent – scatters light transmitted, ex. Frosted glass

Opaque – does not transmit any light, ex. Brick

Wave Behavior

Behaves like wave when traveling through empty space or through a medium when it does not interact with the medium’s particles

Behave like all other waves: Reflects Refracts Diffracts (ex. Prism – see colors of light) Interferes

Constructive – produces light bands Destructive – produces dark bands

Particle Behavior

1900s – discovered that light can act like a particle when it interacts with matter

Certain colors of light produce photosynthesis when light absorbed by green leaves

Certain colors of light shown on metal plates cause electrons to jump of the atoms and create electric current (ex. Solar calculators)

Electrons jump to higher energy levels in atom when absorb energy, when return to ground level will emit specific color of light

Quantum Theory

Explains how light interacts with matter Photon = quantum of light energy

(packet of light energy) Photon’s energy depends on its

frequency – red photons have less energy than violet photons

Dual Theory of Light: Light moves through space as a wave and interacts with matter as a particle

Production of Light

Illuminated objects = reflect light, ex. Moon Luminous objects = emit light, ex. Sun Different methods to produce light based

on how they excite electrons: Heating gases Heating metal filaments Electricity Ultraviolet radiation Chemoluminescence Bioluminescence Phosphorescence

Polarization of Light

Only occurs with transverse waves Polarized materials have molecules that

only allow EM waves of one direction pass through them = polarizing axis

Blocks waves perpendicular to polarizing axis

Example, polarized sunglasses reduce glare of light reflecting off a variety of surfaces