Infrared and ultraviolet

Continuing in the theme of the electromagnetic spectrum from my last blog, what exactly is infrared and ultraviolet?

Electromagnetic spectrum

The above picture was taken from the Nasa website and illustrates the different wavelengths of the electromagnetic spectrum.

Infrared (IR)

British astronomer William Herschel discovered infrared light in 1800, in an experiment to measure the difference in temperature between the colors in the visible spectrum.

Infrared wavelengths are longer than visible light and shorter microwaves at between 1 and 100 microns. It has frequencies from about 3 GHz up to about 400 THz and wavelengths of about 30 centimeters (12 inches) to 740 nanometers (0.00003 inches), although these values are not definitive.

Infrared is invisible to the human eye but is detected as heat. Infrared is one of the 3 ways heat is transferred along with convection and conduction. Everything above minus 450 degrees Fahrenheit or minus 268 degrees Celsius emits IR radiation. Half of the sun’s total energy is emitted as IR.

Infrared astronomy

The California Institute of Technology (CalTech) describes infrared astronomy as “the detection and study of the infrared radiation (heat energy) emitted from objects in the universe.”

Comets, asteroids and dust clouds have been detected by IR as they can detect objects that are too cool to emit visible light. Charged coupled device (CCD) detectors can be used to detect cold gas molecules and establish its chemical makeup.

Due to it’s longer wavelength IR passed through objects such as dust and gas clouds, where visible light is scattered. This allows astronomers to see objects such as newly forming stars in a nebulae at the centre of the Milky Way.


The earths atmosphere prevents most IR from reaching the earth so most experiments are conducted using satellites. The Infrared Astronomical Satellite (IRAS), produced images of the sky at wavelengths of 12, 25, 60 and 100 micrometers (µm). Its imaging sensor had to be cooled to 2 K (minus 456 F) using 161 lbs. (73 kilograms) of superfluid liquid helium, which limited the satellite’s mission to only 10 months. It still completed a survey covering 96 percent of the sky, and identified several asteroids, comets and interstellar dust clouds, producing the first images of the galactic core.

JAXA launched the  AKIRI   satellite, which had an improved cryogenic system, allowing its mission to extend to 18 months. AKIRI also had more sensitive higher-resolution sensors than IRAS, resulting in images containing a wealth of new data.

Ultraviolet (UV)

Ultraviolet is the wavelength responsible for our summer tan despite only making up approximately 10% of sunlight. In 1801, Johann Ritter conducted an experiment to investigate the existence of energy beyond the violet end of the visible spectrum. Knowing that photographic paper would turn black more rapidly in blue light than in red light, he exposed the paper to light beyond violet. Sure enough, the paper turned black, proving the existence of ultraviolet light.

Ultraviolet (UV) light falls in the range of the EM spectrum between visible light and X-rays. It has frequencies of about 8 × 1014 to 3 × 1016 cycles per second, or hertz (Hz), and wavelengths of about 380 nanometers (1.5 × 10−5 inches) to about 10 nm (4 × 10−7 inches). UV is generally divided into three sub-bands:

  • UVA, or near UV (315–400 nm)
  • UVB, or middle UV (280–315 nm)
  • UVC, or far UV (180–280 nm)

Ultraviolet astronomy

Young stars emit most of their light as UV, however this is difficult to view since our atmosphere blocks most of the UV light. Observations are generally conducted using high-altitude balloons and orbiting telescopes equipped with specialized imaging sensors and filters for observing in the UV region of the EM spectrum.

CCD can determine the surface temperatures of the hottest stars and reveal the presence of intervening gas clouds between the Earth and quasars. The Hubble Space Telescope is fitted with instruments that can split the electromagnetic spectrum into its individual ranges. Hubble has captured many amazing images in the ultraviolet range.

Screenshot 2018-06-03 09.15.03

The above image was taken from the Nasa website and is one of Hubble’s many images. Below is a comparison of 3 galaxies shown in visible light bottom row and ultraviolet top row.

Screenshot 2018-06-03 09.20.53



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