The Nature of Light
Page 2: Colors
Light is packets of energy, called photons, transmitting through space.
Each photon has a fixed amount of energy that corresponds to
a constantly repeating electromagnetic wave.
That fixed amount of energy, or wavelength/frequency, is a color.
Wavelength is the distance from a particular point on a repeating wave
to the same point on the next repeating wave, for example peak to peak,
trough to trough, etc.
Frequency is how many waves pass a given point per second
(as if the waves were passing a point). The shorter the
wavelength, the greater the frequency and the greater the energy.
Electromagnetic waves have crests and troughs similar to those of ocean
waves. The distance between crests is the wavelength.
Moving along the electromagnetic spectrum from long to short wavelengths,
energy increases as the wavelength shortens. Consider a jump rope with
its ends being pulled up and down. More energy is needed to make the
rope have more waves.
Light is the portion of the electromagnetic spectrum
that is visible or almost visible.
Figure 2.3 shows the electromagnetic spectrum
with increasing energy toward the right:
Figure 2.3: Electromagnetic spectrum. [NIST]
The visible colors in Figure 2.3 are approximately
(in increasing order of energy)
Red, Orange, Yellow, Green, Blue, Indigo and Violet
(abbreviated ROY G. BIV in introductory science courses).
The color we call white is not actually a color
on the electromagnetic spectrum, rather it is the combination
of many visible colors added together.
When we see a color we call white,
we are actually seeing many colors at once
that when viewed together appear as white.
For example, bright sunlight appears white because
the Sun is randomly emitting visible colors that together
Multiple frequencies of colors (will appear white to a human observer). [NIST]
The Sun emits visible colors which together appear white.
Fig. 2.5 shows the colors of sunlight, with decreasing energy toward the right:
Figure 2.5: Solar spectrum. [NASA]
The solar spectrum depicted above is for sunlight
in space. Some of that sunlight is dispersed or reflected
by Earths atmosphere.
Fig. 2.6 shows atmospheric attenuation of the colors of sunlight:
Figure 2.6: Atmospheric attenuation of sunlight.
The yellow curve in Fig. 2.6 depicts extra-terrestrial sunlight (at zero atmospheres),
corresponding to the red curve in Fig. 2.5.
The green curve in Fig. 2.6 depicts sunlight at 1.5 atmospheres
(sunlight path distance through the atmosphere 1.5 times the vertical height of the atmosphere,
since the Sun is not usually directly overhead).
A response curve shows how sensitive a sensor is to different colors of light.
The sensor may be any sensor, for example a human eye, a solar collector, etc.
The following figure shows a response curve for a consumer-grade
solar collector that converts sunlight into electricity:
Figure 2.7: Consumer-grade (silicon) solar cell response curve.
This type of material mostly absorbs longer wavelength light.
It is less sensitive to much of what humans see as visible light.
What we call visible light is light that is visible to
the human eye for forming images (for seeing).
The process of the human eye forming images for seeing is called
Fig. 2.8 shows the human eye response curve for photopic vision
in bright sunlight:
Figure 2.8: Photopic vision response curve.
The human eye mostly sees green and yellow.
Much of the blue of bright sunlight is not seen by the human eye.
However, the human eye does see some blue for photopic vision.
Fig. 2.9 shows the emission curve of kerosene lighting
superimposed over Fig. 2.8, showing how much better
sunlight is for photopic vision:
Most of the light from a kerosene lamp is longer wavelengths
that are not visible to human eyesight, even if the lamp is bright,
making it not well suited for photopic vision.
Figure 2.10: Kerosene lamp.
Plants that appear green or red are actually reflecting green or red light while
using (absorbing) other colors of light.
Figure 2.11: Solar spectrum and chlorophyl/carotenoid absorbance.
The orange and red curves in Fig. 2.11 above show
the solar spectrum at zero and 1.5 atmospheres respectively.
The green curve shows light absorption of
plant chlorophyl and carotenoids.
Fig. 2.12 shows separate absorbance curves for
chlorophyl and carotenoids:
Figure 2.12: Chlorophyl and carotenoid absorbance.
Chlorophyl absorbs blue and red (reflecting light blue, green and yellow).
Those parts of plant appear light blue, green and yellow,
because those are the colors reflected (not absorbed).
Carotenoids absorb blue and greeen (reflecting yellow and red).
Carotenoids appear yellow and red because those are the colors reflected (not absorbed) by the carotenoids.
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