## More on Waves

There are only a few properties of light.

• Frequency

• Wavelength

• Speed

• Energy

• Amplitude

• For light in a given medium (for example the vacuum of space), the speed is constant so Frequency and Wavelength are essentially different ways of describing the same thing.

Frequency (f) and wavelength ( ) are inversely proportional to one another - one goes up, the other goes down to keep their product constant.

• The ENERGY of an E-M wave depends on the amplitude and the wavelength. For a given amplitude, SHORTER (higher f) waves carry more energy.

There is an alternative description for E-M radiation in which the light energy is carried in Photons (like little bullets of light) that have associated energy, wavelength and frequency. We will use these descriptions interchangably.

## E-M waves of different frequency (wavelength)

It turns out that most of you are already familiar with a broad range of E-M waves.

• E-M radiation with meters can be detected by the cells in the retina of the eye and we call that ``light''

• E-M radiation with between 1000 and 0.5 meters is used to transmit radio and television signals.

• E-M radiation with meters (microwave) is absorbed by water molecules (``absorbed'' means the energy of the E-M radiation is transfered to the water molecules, they heat up and your leftover burrito gets hot in the microwave oven).

• E-M radiation with meters (infrared) can be sensed with your skin (but not eyes).

• E-M radiation with meters (ultraviolet) activates pigments in your skin which cause suntans and trigger skin cancer. Ozone in the upper atmosphere effectively blocks much of the UV radiation from the Sun.

• E-M radiation with meters (x-rays) can penetrate flesh, but not bones.

• Q. What is the wavelength of 810 Kilohertz on your AM radio dial?

``kilo'' ``1000''; ``hertz''

Michael Bolte
Wed Jan 14 23:06:17 PST 1998