Microwaves
Microwaves are measured in centimetres because they are so big. They range in size but it is the larger ones that we use to heat or food in a microwave oven.
Microwaves are good for transmitting information from one place to another because microwave energy can penetrate haze, light rain and snow, clouds, and smoke.
Shorter microwaves are used in remote sensing. These microwaves are used for radar like the Doppler radar used in weather forecasts. Microwaves, used for radar, are just a few inches long.
Weather pictures from the Doppler radar in the US →
Radar is an acronym for "radio detection and ranging". Radar was developed to detect objects and determine their position by transmitting short bursts of microwaves. The strength and origin of the "echoes" received from objects that were hit by the microwaves is then recorded.
Infrared Light
Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Infrared light has a range of wavelengths; just like visible light has wavelengths that range from red light to violet.
"Near infrared" light is closest in wavelength to visible light and "far infrared" is closer to the microwave region of the electromagnetic spectrum.
The longer, far infrared wavelengths are about the size of a pinhead and the shorter, near infrared ones are microscopic.
Far infrared waves are thermal. In other words, we experience this type of infrared radiation every day in the form of heat! The heat that we feel from sunlight, a fire, a radiator or a warm pavement is infrared. The temperature-sensitive nerve endings in our skin can detect the difference between inside body temperature and outside skin temperature. Infrared light is even used to heat food sometimes - special lamps that emit thermal infrared waves are often used in fast food restaurants!
Whereas we cannot even feel the shorter, near infrared waves. These shorter wavelengths are the ones used in TV remotes.
Since the primary source of infrared radiation is heat or thermal radiation, any object, which has a temperature, radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiate visible light, it will emit most of its energy in infrared. For example, a hot coal may not give off light but it does emit infrared radiation, which we feel as heat. The warmer the object, the more infrared radiation it emits.
Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns. (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter.) This image shows a man holding up a lighted match! The warmest areas in this picture are white and the coldest are blue.
To make infrared pictures special cameras and film are used, that detect differences in temperature, and then assign different brightness’ and colours to them. This provides a picture that our eyes can interpret. It is such an effective way of detecting people the police are now using infrared cameras to find criminals in the dark.
The image at the left shows a cat taken with an infrared camera. The orange areas are the warmest and the white-blue areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture.
Humans may not be able to see infrared light, but snakes like rattlesnakes, have sensory "pits", which are used to image infrared light. This allows the snake to detect warm-blooded animals, even in dark burrows. Snakes with two sensory pits are even thought to have some depth perception using infrared!
Many things besides people and animals emit infrared light - the Earth, the Sun, and far away things like stars and galaxies do as well! For a view from Earth orbit, whether we are looking out into space or down at Earth, we can use instruments on board satellites.
Satellites that look at the Earth have special sensors aboard them to record data about the amount of infrared light reflected or emitted from the Earth's surface; this information is used to give a clear picture of weather fronts, clouds and energy wastage.
Visible Light
Visible light waves are the only electromagnetic waves we can see. We see these waves as the colours of the rainbow. Each colour has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they make white light.
When white light shines through a prism or through water vapour like this rainbow, the white light is broken apart into the colours of the visible light spectrum.
Cones in our eyes are receivers for these tiny visible light waves. The Sun is a natural source for visible light waves and our eyes see the reflection of this sunlight off the objects around us.
The colour of an object that we see is the colour of light reflected. All other colours are absorbed.
Satellite Pictures & Visible light
There are two types of colour images that can be made from satellite data - true-colour and false-colour. To take true-colour images, satellites use sensors to record data about the red, green, and blue visible light waves that reflect off the earth's surface. The data is then combined on a computer to produce an image. The result is similar to what our eyes see.
A false-colour image is made when the satellite records data about brightness of the light waves reflecting off the Earth's surface. These brightness’ are represented by numerical values - and these values can then be colour-coded. It is just like painting by number! The colours chosen to "paint" the image are changeable, they can be chosen to either make the object look realistic, or to help emphasise a particular feature in the image.
The pictures below show the planet Uranus in true-colour (on the left) and false-colour (on the right).
It is true that we are blind to many wavelengths of light. This makes it important to use instruments that can detect different wavelengths of light to help us to study the Earth and the Universe. However, since visible light is the part of the electromagnetic spectrum that our eyes can see, our whole world is oriented around it. And many instruments that detect visible light can see father and more clearly than our eyes could alone. That is why we use satellites to look at the Earth, and telescopes to look at the Sky.
Ultraviolet Light
Ultraviolet (UV) light has shorter wavelengths than visible light. Though these waves are invisible to the human eye, some insects, like bumblebees, can still see them.
Scientists have divided the ultraviolet part of the spectrum into three regions: the near ultraviolet, the far ultraviolet, and the extreme ultraviolet. The three regions are distinguished by how energetic the ultraviolet radiation is, and by the "wavelength" of the ultraviolet light, which is related to energy.
The near ultraviolet is the light closest to optical or visible light. The extreme ultraviolet is the ultraviolet light closest to X-rays, and is the most energetic of the three types. The far ultraviolet lies between the near and extreme ultraviolet regions. It is the least explored of the three regions.
Our Sun emits light at all the different wavelengths in electromagnetic spectrum, but it is ultraviolet waves that are responsible for causing our sunburns.
To the left is an image of the Sun taken at an Extreme Ultraviolet wavelength this picture was taken by a satellite and shows what the Sun was like on April 24, 2000.
Though some ultraviolet waves from the Sun penetrate Earth's atmosphere, most of them are blocked from entering by various gases like Ozone. Some days, more ultraviolet waves get through our atmosphere. Scientists have developed a UV index to help people protect themselves from these harmful ultraviolet waves.
We can study stars and galaxies by studying the UV light they give off and we can even study the Earth. Below is an unusual picture of Earth taken from a lunar observatory! This picture shows how the Earth glows in ultraviolet (UV) light.
In this picture you can see that the side of the earth facing the Sun is reflecting the most UV rays, but more interestingly there are still strips of UV rays being reflected off the side that is not facing the Sun.
UV rays are harmful to humans and that is why we wear sun cream and why we shouldn’t stay out in the Sun too long on a hot day.
X-Rays
As the wavelengths of light decrease, they increase in energy. X-rays have smaller wavelengths and therefore higher energy than ultraviolet waves.
We usually talk about x-rays as the medical term. When you have broken bones and things. When you get an X-ray taken at a hospital, X-ray sensitive film is put on one side of your body, and X-rays are shot through you. At a dentist, the film is put inside your mouth, on one side of your teeth, and X-rays are shot through your jaw. It doesn't hurt at all - you can't feel X-rays. The x-ray film “sees” the x-rays that pass through muscle and skin but it also “sees” shadows left by things the x-rays can’t travel through (like bones or metal).
X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident when experimenting with vacuum tubes.
X-rays can also be used in astronomy to look at things that emit x-rays such as black holes, neutron stars, binary star systems, supernova remnants, stars, the Sun, and some comets.
The Sun also emits X-rays - here is what the Sun looked like in X-rays on April 27th, 2000.
Gamma Rays
Gamma rays have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum. These waves are generated by radioactive atoms and in nuclear explosions.
Gamma rays can kill living cells, a fact which medicine uses to its advantage, using gamma rays to kill cancerous cells.
Gamma rays travel to us across vast distances of the universe, only to be absorbed by the Earth's atmosphere. Different wavelengths of light penetrate the Earth's atmosphere to different depths.
Instruments aboard high-altitude balloons and satellites like the Compton Observatory provide our only view of the gamma-ray sky.
Gamma rays are the most energetic form of light and are produced by the hottest regions of the universe. Things like supernova explosions (the way massive stars die), neutron stars and pulsars, and black holes are all sources of gamma rays.
Unlike optical light and X-rays, gamma rays cannot be captured and reflected in mirrors. The high-energy photons would pass right through such a device. So gamma ray astronomy took along time (in comparison to other forms of astronomy) to develop.
If you could see gamma rays, the night sky would look strange and unfamiliar.
This gamma ray moon picture just looks like a round blob - lunar features are not visible. In high-energy gamma rays, the Moon is actually brighter than the quiet Sun.
We are still discovering more about gamma rays and they are the most unknown waves of the electronic spectrum.
Lucy Moore 10SW
