Friday, February 25, 2011


X-rays are yet another type of electromagnetic radiation, which is radiation in the form of photons. X-rays are very high energy, and therefore have a very short wavelength and a very high frequency.

X-rays are the first portion of the electromagnetic spectrum to be ionizing, (high frequency ultraviolet rays also have this property) meaning that they have high enough energy to dislodge electrons from the outer shells of atoms, thus converting them into ions.

This part of the electromagnetic spectrum varies in wavelength from .01 nanometers to 10 nanometers, covering three orders of magnitude. X-rays are primarily used for medical purposes, although they can be carcinogens in larger doses. It is estimated that a common dental X-ray does not meaningfully increase cancer risk, but more invasive CAT scans may significantly increase the risk.

The first known instance of a medical X-ray, taken in 1895 by William Roentgen of his wife's hand. The X-rays penetrate skin but not bone, and the lack of X-rays passing through creates the image above. Note the ring present on the ring finger.

Sheets of lead are often used to contain this radiation, as it is the most cost efficient metal for doing so. Varied thicknesses are used, although 10 mm is sufficient for most X-rays.

In astronomy, X-ray telescopes are used to detect sources of radiation throughout the galaxy. The closest source of X-rays is our Sun, (these are often reflected off the moon) but the Sun predominantly emits light at higher wavelengths, and large sources of X-rays are relatively rare. The accretion disc of a black hole emits large amounts of X-rays as it is heated to extreme temperatures. These sources are particularly prominent when the black hole has a large source of material, e.g. a binary star companion.

An artist's conception of a binary star system in which one of the stars has become a black hole. The gravitational pull from the black hole pulls in material from its companion star, and this material rotates the black hole at extreme speeds, (often over a million miles per hour) causing it to emit X-rays. The most well-known example of a binary system containing a black hole is Cygnus X-1, named for being a strong X-ray source in the constellation Cygnus.

X-ray image of Cygnus X-1 (false-color). The blue supergiant companion star is not prominent in this part of the spectrum, and is therefore not visible.

Other astronomical sources of X-rays include very massive stars, and the supernovae they result in, as well as black holes at galaxy centers. In the former case, the massive stars are often very unstable, and shed material in terrific explosions periodically, culminating in a supernova, in which, huge amounts of X-rays are emitted in an extreme explosion. The latter case works on the same principle as stellar black holes: with accreted matter increasing in temperature and energy, and subsequently releasing X-rays before being sucked in to the black hole.


Thursday, February 17, 2011

Ultraviolet Rays

Ultraviolet Light is another type of electromagnetic radiation. It is known as ultraviolet because the frequency of ultraviolet rays are just larger than that of violet visible light (ultra=beyond).

This area of the spectrum has wavelengths as large as 400 nanometers (right on the cusp of visible light) and as small as 10 nanometers. This area is further subdivided into regions, the most well-known of them being UVA, UVB, and UVC, spanning 400-315, 315-280, and 280-100 nanometers, respectively. Unlike many of the former parts of the spectrum, ultraviolet light is primarily blocked from reaching the surface of the Earth. Of the ultraviolet light that does pass through the ozone layer, (perhaps 2% of the radiation that reaches the ozone layer from the Sun) most of it is UVA rays.

UVA rays are beneficial to health in small quantities, as they cause the production of vitamin D. In larger quantities, they cause tanning of the skin, and in excess, sunburn. UVA rays also are emitted from black lights, lights that are just on the edge of ultraviolet, making them partially visible. However, a majority of the light emitted is within the UVA region of the electromagnetic spectrum. and this radiation can cause chemical reactions, allowing a few substances to radiate a glow under UVA light.

An example of this is the security band on a typical U.S. $20 bill. This band is hard to duplicate, discouraging counterfeit.

UVB light (315-280 nm) is the next type (in increasing frequency) of ultraviolet light. It affects the skin in a more negative way, and is the rarer of the two types (UVA and UVB) that penetrate the atmosphere. Radiation in this part of the spectrum is more likely to cause cancer than UVA.

UVC light (280-100 nm) is blocked by the ozone layer, but is sometimes artificially produced on the earth's surface. This type of radiation serves as a disinfectant, as exposure of a microorganism to UVC rays damages its genetic material, resulting in mutations that cause infertility, and shortly after, death. In large exposure, these rays have harmful effects on humans as well. Despite their health hazard, they have practical applications in the disinfection of water and other materials.

The remainder of the ultraviolet spectrum (100-10 nm) is mainly used for astronomical purposes, although the Universe looks fundamentally different in ultraviolet than in visible. The most prominent feature, or, in this case, lack thereof, is the dimness of most stars in ultraviolet. Only stars with surfaces at higher temperatures (very young stars and stars in the final stages of their evolution) appear brightly in this part of the spectrum. The interstellar medium, or the sparse material occupying the space between stars, can be best seen and studied with ultraviolet telescopes. These telescopes must be mounted in space, however, due to the low amount of the radiation that reaches the surface of the Earth.

An image (false-color) of the galaxy Messier 81 seen in ultraviolet light. To obtain colorful images of other galaxies, these images are often combined with the images in visible light to detect more features.


Wednesday, February 9, 2011

Visible Light

Visible light is a type of electromagnetic radiation, and is the only type observable through the naked eye.

Rather than being divided into different bands, visible light is divided into colors. The slight differences in the wavelengths of visible waves determine which color that the light is. Visible light is the "smallest" range of wavelengths of the seven main sections (logarithmically speaking) but it makes up everything we see.

Above: The division of visible light into a spectrum of colors. At the edges of the scale are near ultraviolet (the top) and near infrared (the bottom). The discovery of light as a composition of colors may have come in Isaac Newton's time (c. 1665), when the prism revealed sunlight as made up of light on many different wavelengths.

The prism is able to split visible light into its components because there are slight differences in the speeds of light through glass. Higher wavelengths are faster and have greater refraction angles, causing them to bend more and therefore appear at the bottom of the rainbow shown above. Red and other long wavelengths travel slower and refract at a smaller angle, causing them to be at the "top" of the rainbow above. Real rainbows work in a similar way, when water droplets disperse sunlight into the visible spectrum. This also proves that sunlight has many different wavelengths of light, by no means limited to visible light.

Visible, or optical astronomy, is by far the oldest type, and has been going on since antiquity. Only recently have observations in other parts of the spectrum expanded our knowledge of the heavens.


Tuesday, February 1, 2011

Infrared Rays

Infrared radiation is a type of radiation included in the electromagnetic spectrum. Infrared rays still have a longer wavelength than visible light, but one shorter than radio waves and microwaves.

Infrared is best known as being heat radiation. The shortest infrared rays are very close to being visible to humans, hence the name (Latin infra meaning below, "below red"). However, humans can sense this radiation as heat, and infrared makes up a majority of the radiation that the Earth receives at ground level. Animals absorb heat and emit it themselves, and these phenomena can be captured by thermographic images:

A thermographic image showing two people (false-color).

Infrared radiation is subdivided further into three zones, near-infrared (.7 to 5 microns, or .0000007 to .000005 meters), mid-infrared (5 microns to about 35 microns), and far-infrared (35 microns to about 300 microns). Each zone has separate uses, notably near-infrared for telecommunication, mid-infrared for heat-seeking missiles, and far-infrared for lasers.

The zones are labeled in this way because near-infrared is on the cusp of visible light, while far-infrared has a much longer wavelength. Some living things can naturally "see" into the infrared spectrum, such as some snakes that use a special heat-detecting sense to find live prey.

Infrared rays have many other uses, including night vision and the heating of objects. Infrared cameras are also a valuable resource in meteorology, as the surface of the earth emits more infrared radiation than that of clouds, creating an easy way to analyze cloud height and temperature.

An infrared image of the Atlantic Basin. Clouds of increasing height are brighter white. The image also shows Tropical Storm Richard in the Caribbean. Image taken 10/23/10 12:15 UTC.

Infrared astronomy was pioneered by the man who discovered infrared rays: William Herschel. He is also well known for discovering the planet Uranus, along with two of its moons. His discovery was made while studying the spectrum of visible light produced by a prism. He noted that there was a significant temperature increase outside the visible light spectrum, beyond the red. He performed more tests and concluded that it was indeed a new type of radiation, being absorbed and emitted just like visible light, and also being released in massive quantities from the Sun.

In modern times, near-infrared rays can usually be picked up with a normal optical telescope, as these telescopes often have larger visual ranges than the naked eye, seeing into both near-infrared and near-ultraviolet. As one progresses farther into the infrared spectrum, a vast majority of rays do not reach the surface of the earth. However, telescopes at high altitudes in dry environments can pick these rays up with good efficiency. Telescopes operating in infrared can detect objects behind interstellar dust clouds and within nebulas better than visible light, as they can see the heat emitted from the region.

A photo of the Orion constellation in visible (left) and infrared (right). Although the infrared provides little indication to the exact location of the stars, it detects gas clouds throughout the constellation and other features totally invisible in the optical spectrum.

Overall, infrared rays have important uses, such as telecommunication and lasers, as well as transmitting heat throughout the Universe.

Sources: National Hurricane Center,,