Light or the electromagnetic spectrum

Visible light, and the others:

What we call light is generally a mixture of several colours. When the Sun’s light is scattered in space by a prism or a drop of water, we can see its component colours in the rainbow.

Visible light, between red and violet, is that detected by the human eye. But this light is only a very small part of the electromagnetic spectrum which, beyond red, comprises infrared, microwave and radio waves and, beyond violet, ultraviolet, X-rays and gamma rays.

These radiations are all of the same type. The different regions of the electromagnetic spectrum have different names because they correspond to different detectors and different emission mechanisms.

The spectra in astrophysics:

Astronomy studies the position and movement ovement of stars and planets, using images. To study the physical structure of objects, astrophysicists produce the spectrum of the star or the planet. To do this, they scatter its light into the different wavelengths . The spectrum is the quantity of light per colour, in other words the quantity of energy according to the wavelength like the green curve in the following figure which represents the spectrum of a star.

A spectrum can also be represented by a band of alternating light and dark lines as shown in the following figure. The spectrum below, between 370 and 410 nanometres (or 0.37 and 0.41 microns ^? ) corresponds to a very small interval in the ultraviolet of the above spectrum. It shows the extreme diversity of a star’s lines.

A star’s spectrum can be separated into two components, a black body spectrun which depends on its temperature (blue curve in the above figure), and absorption lines which depend on the composition of its atmosphere, which give the green curve. The countless spectral lines ^? reveal the star’s chemical composition, including hydrogène, its main component.

Observing the light from the sky:

Some parts of the electromagnetic radiation from celestial objects do not reach the surface of the Earth, because they are absorbed by the atmosphere. To observe the sky in these wavelengths, detectors must be sent out into space.

Optical:

As light is the fundamental source of information for astronomers, optical instruments are the ultimate tool

A few figures/units concerning light:

Light/electromagnetic radiation travels at the speed $c = 299,790 \space \text{km/s}$. Light takes 8 minutes and 22 seconds to go from the surface of the Sun to Earth.

The quantities defining light are:

• The frequency, $f$, is the number of pulses per second (the unit is the hertz).
• The period, $P = \frac{1}{f}$, is the duration of a pulse (the unit is the second).
• The wavelength $\lambda = c \times P$ (the unit is the metre or the micron ^? = $10^{-6}$ metre, or the nanometre = $10^{-9}$ metre).
• The energy $E = h \times f$ (the unit is the Joule) where $h$ is Planck’s constant: $h = 6.626 \times 10^{-34}$ joule.second.

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