What is the wavelength?
100 to 400nanometers.
Does it need a medium?
What is it? (400 words)
Ultraviolet rays are invisible rays that are part of the energy provided by the sun. UV rays contain a lot of energy, too much energy. The more energy different types of radiation has, the shorter its wavelength would be, so UV rays has a shorter wavelength compared to other rays. For example, the energy contained in the infrared rays causes the molecules of the substance it hits to vibrate back and forth. However, the energy contained in ultraviolet rays is much higher, so instead of just causing the molecules to shake, it actually can knock electrons away from the atoms, or causes molecules to split. This results in a change in the chemical structure of the molecule. This change is especially detrimental to living organisms, as it can cause cell damage and deformities by actually mutating its genetic code.
UV levels are not constant all the time. There are a few factors for this. An obvious factor is the position of the sun in the sky. At noon, the electromagnetic waves emitted from the sun travel a much shorter path through the earth’s atmosphere then they would in the afternoon, so during noon, the intensity is stronger. The second factor determining UV ray levels at the ground is the amount of ozone present in the stratosphere. Low ozone correlates with much UV. However, there are many other features of the environment that contribute to UV radiation variability. Clouds are another important factor. On cloudy days, UV levels are usually lower than during clear skies as clouds can deflect rays up into space. However, clouds can also lead to increased UV levels. This happens when the sun is not obscured by clouds but clouds in the vicinity of the sun reflect additional radiation to the ground. So we are not safe from UV radiation just because it's cloudy.
The amounts of UV rays one is exposed to also has to do with altitude. UV levels increase about 4% for every 1,000 foot gain in altitude. This increase has nothing to do with being closer to the sun as any elevation you might gain would be minuscule in comparison to the distance from the earth to the sun, and so would have an insignificant outcome on UV levels. Instead, the increase is the result of a thinner atmosphere with a smaller number of molecules being present to absorb or scatter the UV rays. Examples of such molecules are tropospheric ozone, which is commonly associated with smog, and aerosols, molecules that remain suspended in the air. Aerosols can be a multitude of substances such as dust, soot, sulfates, etc. These aerosols absorb and scatter UV rays, and so cut down on the ultimate UV irradiance.
Other factors that have an influence on UV levels are the physical features of the land such as sand, snow, and water all tend to reflect UV rays. This phenomenon is called albedo. Some of the ultraviolet rays reflected off the ground encounter scattering by air molecules, aerosols or clouds back down to the earth, thus increasing the total irradiance. When there is snow on the ground the amount of time it takes for sunburn to occur is therefore significantly reduced. Also, the closer one is to the equator, the more ultraviolet rays one is exposed to. This can be explained by the fact that the sun is usually higher at the sky at low latitudes. In addition, the ozone layer is thinner at the equator as it is over, for example the United States or Europe, and this also contributes to more UV.
What are the advantages and disadvantages? (100 words)
There are some advantages of UV rays. UV rays assist our bodies in making vitamin D, which strengthens bones and teeth and helps our bodies build immunities to such diseases as Rickets and colon cancer. UV rays are also used to treat psoriasis, where the rays slow the growth of skin cells, preventing the outbreak of the itchy patches for which psoriasis is known. UV rays have various commercial uses as well, including sterilization and disinfection. Some animals can see UV rays, and UV vision helps bees to collect pollen from flowers. UV rays can also be used to study stars and galaxies.
In spite of their positive uses, UV rays remain very harmful for anyone who spends a large amount of time in the sun without proper protection. The most common effect of exposure to UV rays is sunburn. Sunburn is the damage caused to skin cells when they have absorbed too much energy from UV rays. Sunburn causes blood to rush to the affected area as the body tries to cool the burn, and the damaged skin would peel away in the end. In addition to the burning pain on the skin, sunburn can also cause skin cancer.
Sources: http://www.wisegeek.com/what-are-uv-rays.htm, http://uv.biospherical.com/student/page3.html, http://science.hq.nasa.gov/kids/imagers/ems/uv.html,
Done by Jeremy