The sun is the primary source of energy on earth. It has been said we are a star powered planet, and our star is the sun. Objects on Earth absorb some of that energy, and may reflect or radiate some of the energy. Light and heat are just two forms of electromagnetic radiation emitted by the sun. This energy is caused by nuclear reactions in the sun which cause it to make electromagnetic energy of all wavelengths. The sun is really just a mass of hot gasses that explode with energy in a way very similarly to a nuclear bomb. However in the sun, the explosions never stop. The sun explodes at a temperature of over 25,000 degrees Fahrenheit -- and has been doing so for five billion years! Each second the sun changes about four million tons of its mass into heat and light energy.
Solar energy radiates through space to the Earth. Some of the wavelengths are absorbed, scattered or reflected back into space by the Earth's atmosphere. What scientist's call "Atmospheric Windows" allow the rest, including some ultraviolet radiation, visible light, near-infrared energy, and some thermal or heat energy (sometimes called thermal infrared) to reach the surface of the Earth. From there it is either absorbed or reflected back from the Earth's surface or from clouds. This reflective radiation is measured by remote sensing instruments.
Scientists have made charts that show the characteristics of the electromagnetic wavelengths. This is known as the electromagnetic spectrum, which helps us to understand and "see" the invisible energy that is all around us. Some of the energy radiation, or rays, of the sun we feel as heat, some of them we as light, and some have to be detected with special instruments.
For example, at one end of the spectrum are high energy, short wavelength rays such as gamma rays and x-rays. Geiger counters can detect gamma rays stored in elements like uranium. X-rays are totally absorbed in the atmosphere so they never reach the Earth, but x-rays are generated in an x-ray machine in order to "remote sense" the bones and organs inside the body. Most ultra-violet rays are absorbed by the ozone in the Earth's atmosphere, but some get through. It is ultra-violet rays that cause sunburn and can kill living cells.
The next small section of the spectrum are the rays known as visible light -- from violet, to indigo, to blue, to green, to yellow, to orange, to red. Just beyond visible light, as the solar energy gets lower and the wavelengths longer, we find a large band of infrared.
Near infrared wavelengths are the invisible rays reflected by the chlorophyll in plants. Middle to Far Infrared radiation is also known as thermal infrared. The amount and wavelength of the energy emitted by a material is a function of its temperature and thermal properties. In other words, any object with a temperature above zero emits thermal energy -- or heat. A rock warmed by rays from the sun feels hot. Another rock, in the shade, may be a little cooler, but it is still emitting thermal infrared radiation too. A frozen rock at zero degrees would not emit thermal radiation.
Beyond the infrared we find very long wave very low energy waves such as microwave used in ovens, weather satellites and radar which generates its own microwaves. Beyond that are TV, and then radio waves which have wavelengths ranging from three feet to a mile.
Hold a prism so that a stream of sunlight will pass through it and will refract the band of color onto a piece of paper. Then, using two prisms, have the light pass through the first; place the second in a position to pick up the resulting colors. Pass them through the second prism and blend them back together into a stream of white light.
On a large piece of butcher paper draw the entire electromagnetic spectrum, leaving a small white space in the middle for the visible light produced by the prism. This will show how tiny the visible light portion is. Have students color in the visible spectrum.
Next have each student cut a circle or disk measuring 3 1/4" across out of a piece of stiff cardboard. Draw three straight lines across the disk and through the center, like you are dividing a pie into six equal pieces. Color each pie piece a different color of the rainbow -- in order: red, orange, yellow, green, blue, violet. Push the toothpick carefully through the center of the hole about 1/4". Spin the disk and observe the colors blurring together to create white!