Our view of the sky at night is possible because of the emission and reflection of light. 'Light' is the better-known term for the electromagnetic spectrum, which includes waves in the visible, ultra-violet, infra-red, microwave, radio, X-ray and gamma-ray regions. The scale of the spectrum is so large that no region is distinct, several overlap each other.
Each of these regions in the electromagnetic spectrum represent transverse waves, travelling as electrical and magnetic fields which interact perpendicularly to each other, with different ranges of wavelength. The magnetic field oscillates vertically and the electric field horizontally, and each field induces the other.
By the end of the nineteenth century, Maxwell gave a realistic value for c, the speed of light:
c = __1__ = 3 x 108 ms-1
The relationship between the speed of all electromagnetic radiation, wavelength (l) and frequency (f) is shown to be c = l f.
Because the Universe is so vast, interstellar distances are so great that light emitted can take upwards of millions of years to reach us. Such large distances are often measured in ‘light-years’; one light-year (ly) is the distance travelled by a wave of light in a year. Because of the massive speed of light and distances, the light arriving at us would have left the object many years ago, so that looking at a far away star is much like looking back in time.
Scientific observation of the stars is difficult because of the distorting effect of the Earth's atmosphere. One problem is atmospheric refraction-where light is bent. Turbulent air currents cause varying refractive indices, as there is no uniform air density. This causes an effect called scintillation, where stars appear to twinkle. The effect on regions of the electromagnetic spectrum other than the visible part, such as the absorption of certain frequencies by atmospheric chemicals, and the reflection of waves by charged molecules in the ionosphere, means that some 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...