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 ~ 10^-7meters 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 ~ 10^-3meters (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 ~ 10^-5meters (infrared) can be sensed with your skin (but not eyes).

• E-M radiation with ~ 10^-8meters (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 ~ 10^-9 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" -> 1/second

wavelength = c/f = 300,000(km/sec)/810,000(1/sec) = 0.37 km = 7839.5 ft(!)

VISIBLE LIGHT - Some Details

• The shortest wavelength (highest frequency) light our eyes can sense has ~ 4000 Å = 4 x 10^-7 meters.
• Our BRAIN interprets this as BLUE/VIOLET light.
• Move to longer wavelengths and we see Green, Yellow, Orange, Red.

• If a light source produces all the colors from red through blue, our brain interprets this as white light

It is easy to demonstrate that white light contains all the colors by "dispersing" the light through...

1. a prism

   

2. a raindrop
3. magic crystals obtainable from three-quarters of the stores on Pacific Avenue.
• The usual colors we see are due to the reflective properties of objects. This fact can also be used to demonstrate that white light contains all the colors of the rainbow.

• We have white light sources in the room. The color of all the objects in the room aside from the lights is determined by the wavelengths of light that the objects reflect.

• BLACK shirt absorbs all visible wavelengths (that is why black clothes are warmer).

• RED shirt absorbs all the wavelengths except for RED (so, the white lights must be emitting some Red light).

• BLUE shirt absorbs all the wavelengths except for BLUE (so the white lights must be emitting some Blue light).

Q. What wavelengths are reflected by a White shirt?

All wavelength are reflected by a white shirt.

Q. What color is a Yellow banana illuminated with Red light?

Since a yellow banana reflects only yellow light, a yellow banana illuminated by red light will appear black.

E-M radiation and the atmosphere

• The atmosphere only passes certain "spectral windows" (either way).

• The atmosphere is transparent to visible light (why do you think our eyes evolved to be sensitive to these wavelengths?), some parts of the radio spectrum, and some parts of the Infrared.
• Fortunately, UV, X-ray and Gamma ray radiation does not reach the surface of the Earth as they all cause various bad things to happen to humans, other animals and plants. This also means that if we want to make observations of x-rays from the Universe we need to launch x-ray observatories into space.