Transcript wave
Electromagnetic Waves Electromagnetic Waves • An Electromagnetic wave travels like a wave, but can interact with matter like a particle. • It has a “dual nature”, behaving like a wave at times and behaving like particles (of NO mass!) at times. When it is behaving like a particle, we call these massless bundles of energy photons. Wave Behavior Electromagnetic waves reflect, refract, and diffract! Polarized waves If there are many waves and ALL the waves are vibrating in the same plane, they are said to be “polarized” Polarized waves Electromagnetic waves are transverse waves and can be polarized. Longitudinal waves, like sound waves, cannot be polarized. • In a pure vacuum, all electromagnetic waves travel at the speed called “c”, which we call “the speed of light”, however the average speed slows down a little when it travels through substances such as water or glass as it interacts with the molecules. Albert Michelson is known for making an definitive measurement of the speed of light in the late 1800’s to early 1900’s, using a rotating mirror apparatus between 2 California mountains 22 miles apart. His value: 299,796 km/s. Accepted value today: 299,792 km/s He was the first American to win the Nobel Prize in physics. c = 3 x 108 m/s As is true of ALL waves, Velocity = wavelength x frequency So, for electromagnetic waves, c = lf Now, you try one…. • What is the frequency of purple light, which has a wavelength of 450 nm? 1 nanometer = 10-9 m Don’t just sit there! Get out a calculator and find the frequency. c = lf c = 3 x 108 m/s l = 450 nm (1 nm = nanometer = 10-9 m) What is f, the frequency? f=c÷l f = 3 x 108 ÷ (450 x 10-9 ) f = 6.67 x 1014 Hz How to make an electromagnetic wave: Something must be vibrating to produce a wave. What IS vibrating to produce an electromagnetic wave? A vibrating (accelerating!) electric charge creates a wavy electric field that produces a wavy magnetic field that produces an electric field that produces a magnetic field……… … Electromagnetic waves are transverse waves of intertwined electrical and magnetic fields. “Visible Light” is only a VERY small part of the ELECTROMAGNETIC SPECTRUM. It is the only part of the EM spectrum that is VISIBLE to us. The entire spectrum, in order from long wavelength to small wavelength, is Rattlesnakes May Inject Venom Under Xtreme Grumpiness “Light” Our color spectrum is only a very small part of the electromagnetic spectrum. “Visible Light” is the part of the electromagnetic spectrum that is in RESONANCE with the rods and cones in our retinas. All electromagnetic waves are the same kind of wave. Blue light is the same type of wave as X-rays. Their names (microwave, infrared, radio, etc.) are based only on a particular range of wavelengths (or frequency). All electromagnetic waves travel at the “speed of light” in a vacuum. Radio waves: used by all TV and Radio stations and garage door openers, have very long wavelengths so that they are not easily blocked by objects like buildings. 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? (90.1 x 106 Hz) Using c = lf, l = ? l = 3.3 m Microwaves have an energy that is easily absorbed by water molecules. This is why they are used in “microwave ovens”. What happens is that when the water in your TV dinner absorbs the microwaves, it makes the water molecules vibrate faster. The energy of the microwaves is converted into heat. Cell phone calls, radar and “bluetooth” are all transmitted using microwaves. Some people are frightened that the waves coming in and out of their cell phones are short enough (energetic enough) to cook their brains. (They aren’t!) Infrared: These waves have a shorter wavelength than microwaves, but is still longer than visible light. IR waves are used in remote controls for your TV Infrared radiation is associated with HEAT. Every object that emits heat is emitting IR. Our bodies can “feel” IR if the source is close enough and hot enough. Animals like the pit-viper have infra-red detectors so that they can find their prey in the dark. You have been warned !! The Visible Spectrum Roy G Biv: Red, Orange, Yellow, Green, Blue, Indigo, and Violet are the colors of visible light. Wavelength range from about 700 - 400 nm 1 Nanometer = 10-9 m Ultraviolet: These waves have very high energy and very short wave lengths- shorter than visible light. Too much exposure to UV can cause skin cancer- too little exposure and we don’t produce enough Vitamin D! UV is sometimes used to sterilize tools, glassware, and kill bacteria on food. Some animals like honey bees can see ultraviolet waves. Even shorter wavelength: X-Rays have so much energy and such a short wavelength that they can go right through you. They also mess with your DNA… that’s bad However, they cannot get through bone as easily as they can get through muscle. This is because your bones contain so much Calcium. The very shortest, highest frequency: Gamma rays These are nasty ones. They have very high energy and will even go through metals if they are not thick layers. Some radioactive materials produce gamma rays (on Earth) as do super nova explosions and pulsars (out in space). Gamma rays and X-Rays can cause cancer, but gamma rays can also be used to destroy cancer cells: this is radiation therapy.