Colours in the Sky
We’ve all been stopped in our tracks by
spectacular sun rises and sets. Gradually changing from yellow, to bright
orange, and finally culminating in a fiery red ball, the surrounding mountains
seem to glow. Although it borders on heresy to attempt to break down a sunset
into simple physics, there’s a symphony going on behind the scenes.
Interesting to note is that the sun looks
yellow at all – pictures from space show a white and not a yellow sun. When
light travels through the atmosphere, it comes into contact with a materials
that scatter and filter out certain parts of the spectrum. These include atoms
of oxygen and nitrogen. As light moves through the atmosphere, the blue end of
the spectrum is gradually filtered out leaving a predominance of yellow light.
This causes the sun to appear yellow from the earth's surface. As the sun
travels greater distances through the atmosphere, more and more of the blue
light will be removed. If it travels far enough, only the red end of the
spectrum will remain visible. From overhead (i.e. at noon), the light only
travels far enough to provide a yellow sun.
As day slips into evening, a number of things
change. The most important change can be found in the angle of the sun. As it
drops down lower in the sky, the light beams must now skim along the surface of
the planet rather than shine straight down on us. As a result, the light travels
much further through the atmosphere and more of the blue spectrum is
scattered – slowly changing the colour to orange and then red.
Of course there's much more to a good sunset
than just a red ball of light. The sunset varies greatly in intensity depending
upon a number of factors. If there is a lot of water moisture or dust in the
air, the entire sky begins to take on the same colour as the globe of the sun
itself. This is often enhanced by some cloud cover (as long as it doesn't
obscure the show). These factors lead to the endless variety of sunsets
experienced over the years.
This scattering of blue light also results in another atmospheric phenomenon – a blue sky. When we look upwards, we are actually seeing the scattered blue light – the light that didn't reach the earth's surface. It may seem hard to believe that the same process that causes the sunset to be red can also cause the sky to appear blue, but that is precisely what happens.
All Material © Ward Cameron 2005