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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 otherEach 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 otherBy the end of the nineteenth century Maxwell gave a realistic value for c the speed of light The relationship between the speed of all electromagnetic radiation wavelength l and frequency f is shown to be c l fBecause 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 timeScientific observation of the stars is difficult because of the distorting effect of the Earths 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 EarthThe Earth receives electromagnetic radiation of all wavelengths from all
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