- discuss the nature of light
- describe the different types of light and their uses
- describe the relationship between the speed of light and distances
Light is an amazing thing. Scientists are still
struggling to understand the nature of light, and most people don’t
realize how complex it is, even though they use it everyday. If light
were alive, it’s “life” might look something like this: light is
produced by a source, travels in a straight line, sometimes bouncing or
bending depending on what it runs into, and eventually is absorbed by
In our everyday life, light typically manifests itself in one of these forms:
- Sun emits light - travels to Earth - perhaps bounces off an object - arrives in your eye
- Light bulb emits light - perhaps bounces off an object - arrives in your eye
Incandescent light bulb.
both cases the light entering our eyes allows us to see either its
source, or what it has most recently bounced off of. For instance, the
light that bounces off the tree allows us to see the shape and colors
of the tree.
Color is one of the
main characteristics of light. The sun produces what is called ‘white’
light. This means that all the colours of the rainbow are present at
the same time. If none of the colors were present then there would be
no light, and hence it would be black!
when sunlight hits something like a tree and bounces off of it to
arrive in our eyes, there is a actually number of things happening to
the light. Some of the sunlight is absorbed by the tree, giving it
warmth and energy. Some of the light is reflected into our eyes so we
can see the tree. This would be green (yellow + blue) light, reflected
off the leaves, and a combination of colours off the trunk to make the
bark look brown. This means that when we say that the chlorophyll in a
leaf is ‘green’ what we are actually saying is that it reflects
primarily green light. Whatever colours are not reflected, are absorbed.
The range of colours that the human eye can see is called the visible spectrum
of light. In fact, it is just a small piece of the whole spectrum of light. One way to describe to light is as an electromagnetic
wave. This is similar to a water wave with crests and troughs and a wavelength
. A wavelength is the length of one complete cycle of a wave, often measured from the top of one crest to the top of the next.
A basic diagram of a wave.
Did you know?
light from the Sun is not pure white light – an equal representation of
all the colours of the rainbow. The outer layers of the Sun contain
hydrogen and a few other gases that absorb some wavelengths of light.
Each element absorbs different patterns of wavelengths. These appear
as black lines or ‘missing parts’ of the rainbow of light from the
Sun. This called an absorption spectra
and it is different for each star depending on the composition of its
outer layers. By analyzing the light coming from other stars
astronomers are able to determine some of the elements that make up the
Light can be divided up into different types
depending on the length of its wavelength. Different colours have
different wavelengths. Red light has a wavelength of about 700
nanometres [nm] (that’s 0.00007cm), while violet light has a wavelength
of about 400nm (0.00004cm). But in fact light can wavelengths can
exist as long as 1,000,000,000m (or 1Gm=1 Gigametre) and as short as
0.000,000,000,000,01m (or 10fm=10 femtometres.) There is a huge range!
This gives us the full spectrum of light. We generally divide light into 7 different ranges. This is shown below in Table 1
in the image below. It might surprise you to know that all of these
types of waves are essentially the same thing – it is only the length
of the waves that makes them differ from each other.
|Type of Light||Wavelengths (m)||Frequency (Hz)||Energy Level||Technological Applications|
|Radio Waves||1 - 100,000,000||0.1-109||Extremely Low||Radios, TV|
|Microwaves||0.001 - 1||109-1012||Very Low||Microwaves, Cell Phones|
|0.000,001 - 0.001||1012-1015||Low||Infrared Goggles, Remote Controls|
|Visible Light||0.000,000,7- |
|0.000,000,4||~1015||Medium||Vision, DVD’s, Blue Ray Discs|
|0.000,000,1||1015-1017||High||Filtration Systems, Tanning Beds|
|X-Rays||0.000,000,000,01- 0.000,000,01||1016-1019||Very High||Medical X-Rays|
|Gamma Rays||0.000,000,000,000,01 -|
|0.000,000,000,01||1019-1023||Extremely High||Substance Analysis|
The Electromagnetic Spectrum of Light. Note that there are not Table 1
clear dividing lines between the types of light, but their ranges overlap.
also lists the frequency
level of the various kinds of light. Frequency is defined as the number
of wave cycles that occur in one second. Imagine an oscillating
pendulum that swings back and forth twice every second. This would mean
it has a frequency of 2 cycles per second, or Hertz (Hz). Radio waves
have the lowest frequencies of the various types of light but they can
still complete a million full cycles every second! This extreme
oscillating movement is a good example to help you imagine how energy
is related to light. A wave that oscillates a million, million times a
second has more energy than one that only oscillates 10 times a second.
different kinds of light have given rise to various fields and
developments in astronomy, but before we address these there are a few
more basic elements to discuss.
Did you know?
speed of light is now what defines the length of 1 metre. Originally it
was calculated as a 1/10,000,000 of the distance from the equator to
the North Pole through Paris. However, as scientists were able to
produce more accurate measurements of the speed of light, it was
decided that this fundamental value in the universe is a better
standard to use than the size of the Earth. In 1983, it was decided to
fix the value of the speed of light at 299,792,458 m/s and then adjust
the length of the metre to exactly match this quantity.
speed of light, written as c (as in Einstein’s famous equation E=mc2),
is one of the most important values in astronomy. The speed of light in
a vacuum is: c = 299,792,458 metres/second, where a vacuum
is a space completely devoid of matter. This is the fastest anything
can travel in the universe. Even so, it still takes millions of years
for light to reach us from the nearest galaxy. This makes it difficult
for humans to consider traveling to another solar system, as the
nearest star, Proxima Centauri, is still 4.22 light years away. If we
could somehow travel as fast as the speed of light, it would take us
4.22 years to get there.
Earth’s closest star, after the Sun, is Proxima Centauri, is seen as the red star
in the centre of this photo UK Schmidt Telescope. It is only visible by
telescope, primarily in the Southern Hemisphere.
The various types of light offer many advantages for astronomers. By
observing different types they can learn different qualities of the
object observed. Below is an example of this using the Sun, as observed
by the SOHO Space Telescope.
The first two images show the Sun at different wavelengths of ultra-violet light
while the third image shows the Sun in the visible wavelengths