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Colors

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.


Figure 2.1:  Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is the wavelength. [NASA]

Figure 2.2:  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. [NASA]

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).


White

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 appear white.

Figure 2.4:  Multiple frequencies of colors (will appear white to a human observer). [NIST]

Solar Spectrum

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]


Atmospheric Attenuation

The solar spectrum depicted above is for sunlight in space. Some of that sunlight is dispersed or reflected by Earth’s 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).


Response Curves

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 photopic vision.

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:

Figure 2.9:  Kerosene emission curve. [MikeMachala]

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.


Photosynthesis

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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