- WAVES IN GENERAL:
A wave is a cyclical (periodic) phenomenon that fluctates back and forth repeatedly with time.
All waves can be reflected by bouncing off reflective surfaces. (EXAMPLE: Light waves bound off of mirrors.)
The height of a wave crest is called its amplitude.
The distance between each crest and the next is called the wavelength, generally measured in distance units.
The Greek letter lambda, λ, is the traditional symbol for wavelength.
As a wave moves, the time between each crest and the next is called the period.
The number of crests or periods per unit time is called the frequency. The energy of a wave is proportional to its frequency.
Frequency is measured in waves per second; 1 wave/sec = 1/sec = 1 Hertz (abbreviated Hz).
AM radio frequencies are measured in thousands of Hertz (kiloHertz or kHz); FM radio frequencies are measured in
millions of Hertz (megaHertz or mHz)
IMPORTANT: The velocity (speed) of a wave is always equal to its frequency times
its wavelength: v = f λ
- Material or physical waves:
Throw a stone into a pond and it makes ripples, or waves, that spread out across the surface.
These are transverse waves because the up-and-down motion of the water is perpendicular (transverse) to the direction of the wave's travel.
Sound is also a physical wave. When you make a sound, it compresses the air molecules and they quickly bounce back, again and again.
This produces a compression wave, also called a longitudinal wave because its back-and-forth motion
is in the same direction as the wave's travel.
High-frequency sounds have a higher pitch (like a flute or whistle); lower-frequency sounds have a lower frequency (like a bassoon or a dog's growl).
The speed of sound varies with the density of the material it travels through. Sound travels faster through water and faster still through rocks.
The speed of sound in air is about 343 meters per sec (m/sec), or 767 miles per hour. It varies with atmospheric conditions like barometric pressure.
Because the speed varies with density,
scientists can use the speed of sound through underground rocks to estimate their density.
Sound and other physical waves need a medium to travel through; they cannot travel through a vacuum.
- Conduction: Conduction is the transmission of wave energy or heat energy through materials. The speed of conduction usually varies with density.
Convection: Convection is the transmission of heat or other energy by the movement of fluids such as water or air. Hot air rises and cooler air sinks, and this
results in both air movement and a generally upward flow of heat. The same happens in bodies of water from small (tea kettles) to large (oceans).
- Electromagnetic radiation and the electromagnetic spectrum:
Light is a form of electromagetic radiation or energy. All hot objects give off electromagnetic energy (they glow).
The different forms of electromagnetic energy form a spectrum. Visible light is just one small part of this spectrum.
In order from low frequency to high, the electromagnetic spectrum includes radio waves, microwaves, infrared,
visible light, ultraviolet, x-rays, and gamma rays.
Low frequencies also have low energy, and high frequencies have high energy.
Low frequencies also have the longest wavelengths (radio waves), and high frequencies (gamma rays) have the shortest wavelengths.
All forms of electromagnetic energy travel at the speed of light, or about 3.0 x108 m/sec = 186,000 miles/sec
Electromagnetic radiation can travel through the vacuum of space; it does not need a medium to travel through.
- SAMPLE PROBLEM:
If a certain green light has a frequency of 5.45 x1014 Hz (= 545 gigaHertz or 545,000,000,000,000 waves/sec),
find the wavelength.
Answer: v = f λ, so λ = v / f = 3.0 x108 m/sec
/ 5.45 x1014 (1/sec)
= (3.0 / 5.45) x 108-14
= 0.550 x 10-6 = 0.550 x 1000 x 10-6 / 103 = 550 x 10-9 m or 550 nm
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