Dispersion of Light
The phenomenon of splitting of white light into its constituent colours is called
dispersion of light. The band of seven colours obtained from the splitting of white
light is called the spectrum.
All the constituent colours of a white light have same velocity in vacuum, but
when it passes through a transparent medium like glass prism, their velocity
changes. At the first face of the prism, different colours are deviated by different
angles. Violet having the minimum speed gets deviated by maximum angle and
red having the maximum speed gets deviated by minimum angle. On reaching
the other face of the prism, these colours are refracted according to the laws of
refraction. Thus, the dispersion of white light occurs on entering a prism.
EM Spectrum
The electromagnetic spectrum is the series of waves that are created by
disturbances in electric and magnetic fields. They are arranged in order of increasing
frequency and decreasing wavelength. Every wave on the spectrum is technically
‘light’; they all travel at lightspeed, and only a small part of the spectrum is visible to
us.
All electromagnetic waves travel at the speed of 3 x 108 m/s.
Radiowaves have the longest wavelengths and so are used in radio and television
communications. The long wavelengths and low frequencies of radiowaves cannot
be easily absorbed and scattered, allowing them to be transmitted long distances.
They should be captured by using a roof antenna.
Microwaves are used in satellite television and telephone communications. The
higher frequency of these waves allow them to transmit more information, allowing
higher quality communication. They travel easily through earth’s atmosphere and
reach broadcasting satellite.
Infra-red waves are used in electrical appliances like remote controllers, televisions
and burglar alarms. They are also used in optic fibres. Note that infrared waves are
also known as the ‘heat carriers’ – most hot objects emit infrared waves and infrared
waves are absorbed by the surface of most objects too, causing them to warm up.
As infrared waves are easily absorbed, and can’t penetrate many things, they are
used in numerous systems – e.g. in burglar alarms, a beam of infrared is directed at
a receiver. When the alarm is on, if something intercepts the beam, it no longer
reaches the receiver, causing a signal to go off and the alarm to be activated.
Visible light waves are the light waves emitted by bulbs, lamps etc that is visible to
us. Visible light is also used in optic fibres.
Ultraviolet waves are emitted by the sun, among other sources, and exposure to it
can cause skin diseases. There is one type of UV light that is safer and often used in
tanning booths. Many fluorescent substances only fluoresce upon being illuminated
by UV waves .Therefore, they are often used to detect forged bank notes (real ones
often have markings only visible under UV light), as security markers (again, markers
only visible under UV light, that tells us that the object it is present on is authentic
and not a fake), etc. UV light is also used to disinfect water.
X-rays are used in imaging – they are used in security check when scanning
luggage, for medical diagnoses, etc.
Gamma rays are produced by radioactive materials. It has a high frequency and so
can penetrate through most materials and cause gene mutation and cancer.
Alternatively, when controlled, it can be used in the sterilisation of medical
equipment. It can also be used to detect the presence of cancer and its treatment.