Two kinds of infrared radiation exist, the near infrared and thermal infrared radiation. Looking at the energy coming from the sun on some scale with the amount of energy called radiative intensity, the energy from the sun may have a peak at about 0.5 µm (Fig. 9.7). The light that we see is between 0.4 and 0.7 µm. Solar energy has its peak in the yellow of the visible light range. To the left of the visible at shorter wavelengths, solar radiation drops off quite quickly. This is the ultraviolet, labeled as UV. To the right of the visible, beyond where our eyes can see, is the IR, the near red IR. The wavelengths of the colors of the spectrum (violet, blue, green, yellow, and orange in the middle, and red at the right end) are also depicted.


Fig. 9.7 Radiant energy over the solar spectrum. Wavelength is micrometers to the left of the jagged green lines and meters to the right of the lines.

The amount of energy emitted by the sun at the 3 µm wavelength is very, very low. Solar energy at 1 µm is relatively high. This is the IR that can get through a camera lens, and this is the IR that we were seeing in our pictures of the vegetation taken by the satellites.

Infrared radiation, as you can tell by looking at the area under the curve, represents a great deal of the energy coming from the sun, about 37%. If a plant or crop absorbed all of the energy from the sun, it would heat up to a certain degree. If it only absorbed half of the energy from the sun, it would not heat up as much in the sun. How much of the energy from the sun does a plant absorb? Imagine a typical soybean leaf with sun shining on it. About one-fourth of the energy from the sun is reflected by the plant. About one-fourth passes through the leaves (called transmitted), and one-half is absorbed into the leaf to be used in heating the leaf and some for photosynthetic and photosensitive processes.

The percentage of energy from the sun that the plant absorbs is not uniform among wavelengths. What color are the plants? They are green. The fact that they are green means that they are doing something strange with the light. If they were treating all the light uniformly, the plant would be gray. Disproportionate absorption or reflection makes the leaf have some color to it. Healthy plants, as mentioned, appear green. One might think that plants reflect more green. It would be more accurate to say that healthy plants are absorbing the red. They are using the red wavelength for photosynthesis. The plants are taking the red out of the light that is in our environment and reflecting or transmitting most everything else. But because there is a lack of red, the plant appears to be green. So we're not really seeing green when we look at a plant, we're seeing lack of red.