Wave sources and receivers Energy Transmission and Reception using Waves

Outline Review: the wavelength/frequency spectrum Wave energy Wave sources and receivers The Energy vs. frequency spectrum Sound waves Electromagnetic waves The Blackbody Spectrum and Energy transmission from the sun to the earth.

Sound Wave Energy Sound waves are longitudinal, pressure waves Sound waves can do work Can compress air – an acoustic refrigerator. Shock Waves.

Making Sound Waves Demonstrations: Tuning Forks Singing Rods Vibrating Strings Resonators To make sounds, something has to vibrate and make the air move back and forth to set up the sound wave.

Hearing Sound Waves Need some instrument sensitive to pressure variations. Microphone – a diaphragm connected to a magnet surrounded by a wire coil. The diaphragm vibrates, causing the magnet to move and sets up an electric current which can be detected. Ear drum – a diaphragm which sound waves cause to vibrate. Its motion is detected by nerves and sent as electrical signals to the brain. To hear sounds, something has to vibrate in response to the pressure waves.

Resonators Mikes and eardrums are sensitive to pressure variations over a large range of frequencies. Resonators are instruments which can vibrate only at certain frequencies. Even though a sound may contain many frequencies, it will respond only to the resonant frequencies. Use resonators to determine Sound energy vs. Frequency.

Wave energy vs. Frequency Making sound – A monkey playing a piano. Each sound consists of energy being transmitted over a range of frequencies. Resonators are used to detect the amount of energy in the sound at a particular frequency.

Energy vs. Frequency spectra

Electromagnetic Waves Making EM waves Need vibrating charges. Antennae, MW transmitter, bremstrahlung. Receiving EM waves Need vibrating charges. Antennae, retinae, photographic plates, etc. EM energy is being sent at various frequencies, just like sound waves.

EM energy vs. frequency spectra Pure tones = Lasers (light at ONE frequency) Harmonics or musical tones = Atomic and molecular spectra (Neon lamps) (Light at several distinct frequencies) Noise = Blackbody spectra (Light distributed over a range of frequencies).

Blackbody radiation Blackbody radiation depends on the temperature (internal energy) of the object. The total amount of energy increases with increasing temperature. As temperature increases, the peak energy of the spectrum shifts to shorter wavelengths/higher frequencies.

The Blackbody Spectrum

Earth and Sun

Summary The sun’s energy comes to us in the visible spectrum Peak at λ = 5 x m or f = 5.5 x Hz The earth tries to emit energy in the Infrared Peak at λ = 1 x m or f = 3 x Hz The earth’s atmosphere doesn’t resonate in the visible range, but it does resonate in the IR range. Atmosphere passes visible light, but blocks the IR from leaving, keeping the earth’s surface about 58 C warmer than it would otherwise be.