EM 1110-2-2907
1 October 2003
The equation for energy indicates that, for long wavelengths, the amount of energy will
be low, and for short wavelengths, the amount of energy will be high. For instance, blue
light is on the short wavelength end of the visible spectrum (0.446 to 0.050 m) while red
is on the longer end of this range (0.620 to 0.700 m). Blue light is a higher energy ra-
diation than red light. The following example illustrates this point:
Example: Using Q = h c/λ, which has more energy blue or red light?
Solution: Solve for Qblue (energy of blue light) and Qred (energy of red light)
and compare.
Calculation: λblue=0.425 m, λred=0.660 m (From Table 2-2)
h = 6.6 10-34 J s
c = 3.00 108 m/s
* Don't forget to convert length m to meters (not shown here)
Blue
Qblue = 6.6 1034 J s (3.00x108 m/s)/ 0.425 m
Qblue = 4.66 1031 J
Red
Qred = 6.6 1034 J seconds (3.00x108 m/s)/ 0.660 m
Qred = 3.00 1031 J
Answer: Because 4.66 1031 J is greater than 3.00 x 10-31 J blue has more
energy.
This explains why the blue portion of a fire is hotter that the red portions.
(2) Implications for Remote Sensing. The relationship between energy and wave-
lengths has implications for remote sensing. For example, in order for a sensor to detect
low energy microwaves (which have a large λ), it will have to remain fixed over a site for
a relatively long period of time, know as dwell time. Dwell time is critical for the collec-
tion of an adequate amount of radiation. Conversely, low energy microwaves can be de-
tected by "viewing" a larger area to obtain a detectable microwave signal. The latter is
typically the solution for collecting lower energy microwaves.
j. Black Body Emission. Energy emitted from an object is a function of its surface
temperature (refer to Paragraph 2-4c and d). An idealized object called a black body is
used to model and approximate the electromagnetic energy emitted by an object. A black
body completely absorbs and re-emits all radiation incident (striking) to its surface. A
black body emits electromagnetic radiation at all wavelengths if its temperature is above
0 Kelvin. The Wien and Stefan-Boltzmann Laws explain the relationship between tem-
perature, wavelength, frequency, and intensity of energy.
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