As the term suggests, the visible spectrum can
be seen by the human eye. However, it is only within a narrow range of wavelengths,
frequencies, and energies. The wavelengths range from about 0.0007 millimeter (7 x 10-7
meter) for red to 0.0004 millimeter (4 x 10-7 meter) for violet, with a similarly narrow
frequency range of about 1014 or 1015 hertz. The sun radiates heat and it is passed in or
out readily through the atmosphere. Visible light is produced when electrons in the
outermost orbits of atoms in excited energy states return to their usual, unexcited
orbits. It consists of 7 colours , red , orange , yellow , green , blue , indigo , violet.
It is this visible spectrum that has allowed
some considerable amount of scientific information due to the fact that the human eye does
not require advanced equipment just to witness it. For thousands of years, the nature of
this visible light has been the central of scientific interest.
The speed of visible light was first
quantified in 1676 by the Danish astronomer Olaus Roemer. Basic principles of reflection,
refraction, and diffraction were well understood soon at the 19th century. The particle
and wave models were developed to account for the behaviour of light. Certain parts of the
spectrum was known to pass through different substances at different speeds.These
observations were made long before visible light is known to be in another much more
complex spectrum - the electromagnetic spectrum.
A note for all, the light in all later
sections will refer to the visible spectrum unless otherwise stated.
However, there are two more kinds of spectrum
worth mentioning.
Emission Spectra
Suppose you do this activity - Fill a glass tube up with some
atmoic or molecular gas and supply a suitable voltage across the tube, what happens? The
contained gas immediatelt glows with one colour. When you pass this light through a few
prisms in a spectrscope a spectrum is produced. The separation of the light into
different-coloured beams takes place. Many separate sets of frequencies that comes out of
the prisms at various angles will fall at different and distinct places on the
spectroscope film. This spectrum is called a bright-line or simply emission spectra.
Absorption Spectra
Repeat the above activity this time with white light. Do you
think you will obtain the same results ? A continuous white-light spectrum will results
which are interrupted by black lines at the same place as the emission spectrum's bright
lines. Frequencies can be absorbed by the system and this is reflected by the black lines.
In this case, we call this spectrum an absorption spectrum.
Different kinds of atoms and molecules produces a different emission of absorption
spectrum. Using this fact, unknown elements of gases which radiate and even faraway stars
by comparsion of its very own types of spectrum may be identified.
That's all for now. Proceed on to the
components of the visible spectrum...
