e spectral data is simply invisible to us on Earth.
             The Earth receives electromagnetic radiation of all wavelengths from all directions in space, but most of the electromagnetic spectrum is blocked out by the atmosphere well above the Earth's surface, where our eyes and instruments are mostly based. However, wavelengths from only two regions of the electromagnetic spectrum are able to penetrate the atmosphere. These two spectral windows in the atmosphere through which we can observe the Universe are called the optical window-which allows the visible wavelength region through; and the radio window-which includes the wavelength region from about 1 mm to 30 m. The telescopes used by astronomers on the ground are therefore classed as optical and radio telescopes. Optical telescopes work by either reflecting or refracting light, using lenses or curved mirrors to focus the light from a subject to form an image. Radio telescopes consist of a parabolic reflector and receiver on which the waves are focused. The gathering and resolving power depend on the diameter of the antenna. Radio observations are unaffected by the weather or time of day, and because of the larger wavelength of radio waves, dust in space and atmospheric convection currents are not a problem. Radio astronomy is used in the chemical analysis of elements (by emission and absorption spectra); to detect the motion of bodies due to the Doppler effect; and in investigation into the early Universe and the Big Bang. We can analyse radio waves from the centres of galaxies, including our own.
             Despite the radio window, there are still wavelengths that do not penetrate the atmosphere. Some radio waves are reflected from the ionosphere, part of the thermosphere, where streams of charged particles from the sun ionise gas molecules: this is photo-ionisation. Ultra-violet radiation, X-rays and gamma-rays are also absorbed at this layer.
             Absorption of the electromagnetic spec...