Electromagnetic Waves

The source of Electromagnetic (EM) waves Electromagnetic waves are caused by the vibration of electric charges. Their vibration causes an electric field. A changing electric field causes a changing magnetic field.

Dual nature of light (EM) waves Remember, waves carry energy. EM waves can behave like a wave or a particle. The bundles of energy carried by EM waves are called photons.

EM waves can travel without a medium through empty space. They can also travel through matter. Longitudinal waves cannot travel through empty space (or a vacuum). The wave equation: c = f velocity = frequency X wavelength applies to all EM waves

The speed of light, c = 3 X 10 8 m/s All EM waves travel at the speed of light, c, in a vacuum and somewhat slower in mediums such as air or glass. Albert Michelson found the speed of light and became the first American to win the Nobel Prize in Physics in 1907.

Polarized light is light that only travels in one plane, either vertical or horizontal Where is the turtle? Turning the polarizing lens eliminates the glare. If many waves bounce off a reflective surfact like water or shiny metals, they become polarized.

Polarized light

Only transverse waves can be polarized. Longitudinal waves like sound waves cannot be polarized.

The electromagnetic spectrum includes a range of waves of different wavelengths and frequencies. Many EM waves cannot be seen. Visible light is only a small part of the spectrum.

“Visible Light” is the part of the electromagnetic spectrum that is in RESONANCE with the rods and cones in our retinas.

From longest to shortest Radio waves Microwaves Infrared Visible light Ultraviolet X-rays Gamma rays Rattlesnakes May Inject Venom Under X-treme Grumpiness

Relative sizes of EM waves

Radio Waves Longest wavelengths Can travel around structures Used by TV and radio stations Frequencies: AM radio stations broadcast in kHz. FM stations broadcast in MHz. TV stations broadcast in Mhz. What is the wavelength of our public radio station, FM 90.1? c = f, = ? (3 X 10 8 ) / (90.1 x 10 6 Hz) = = 3.3 m

Microwaves Microwave ovens “excite” water molecules in food, causing them to knock into other molecules to increase the temperature of food. Cell phones and radar transmit and receive microwaves.

Infrared IR radiation is associated with heat Some reptiles use it to locate prey TV remote controls use it to transmit signals

Visible Light Different frequencies appear as different colors. Roy G. Biv – Red, orange, yellow, green, blue, indigo, and violet Wavelength range from about nm 1 Nanometer = m

Ultraviolet High energy, can cause sunburn and skin cancer. Necessary for humans to produce vitamin D Used to irradiate (kill germs) on food and instruments Insects use it to locate food. Click to see How bees see the flowers

X-rays High energy and tiny wavelength Can penetrate our bodies and alter DNA (harmful) Denser bones and teeth composed of calcium can be diagnosed (beneficial)

Gamma Rays Extremely high energy with sub atomic size wavelength Can penetrate paper and even thin layers of metal Luckily, our atmosphere protects us from much of the radiation from space. Generated by radioactive decay of elements (on Earth) or present in cosmic radiation from outer space (high energy explosions such as hypernovae) It can cause cancer, but also can kill cancer cells as radiation treatment.