Electromagnetic Radiation and Wave Behaviour 107
Table of radiation, wavelength and frequency of electromagnetic
radiation.
Type of wave Wavelength, λ (cm) Frequency, f (sec-1)
radio waves 103 to 10^4 108 to 10^9
microwaves 10 -3 to 10 109 to 10^12
infrared (heat) 10 -4 to 10-3 1013 to 10^14
visible light 4 to 7 × 10-5 1015
ultra-violet 10 -7 to 10-6 1016 to 10^17
x-rays 10 -9 to 10-7 1017 to 10^19
gamma rays less than 10-9 1019 and higher
The range of frequencies of visible light and the range of frequencies
that the Sun radiates with maximum intensity exactly overlap. The eye
has been biologically adapted through natural selection and evolution so
as to detect the electromagnetic radiation of the Sun. If human life had
developed on a planet orbiting a star, which emitted principally infrared
radiation then, through the process of evolution, one would expect visible
light for these people to be in the infrared range. The visual detection of
objects using infrared radiation has actually been achieved by the
military. They have developed a photographic film sensitive to infrared
rays, which they have used for aerial photo reconnaissance at night.
In addition to perceiving electromagnetic radiation, the eye is also
sensitive to the different frequencies in the visible light range. This
accounts for our colour vision. Each colour corresponds to a different
range of frequency in the visible range as is listed in the table below. The
order of the colours in the list is exactly the same as the rainbow. This is
no coincidence. The Sun radiates visible light of all frequencies and
hence, all colours. White light is merely a combination of all the colours.
When white light or sunlight propagates through water such as a raindrop
or glass such as a prism, the light ray is bent both upon entering and
leaving the medium of water or glass. The amount of bending a ray of
light experiences depends on the frequency of the ray; the greater the
frequency, the more it is bent. The different colours that compose light
are therefore separated when they propagate through a medium like
water or glass and hence, one observes a rainbow.
The knowledge that sunlight is composed of all the colours helps us to
understand why the sky is blue and why sunsets and sunrises are red.