Barry Rice

- doing science on a terrestrial planet -

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The physics of light

Light is a very strange thing. You can think of it as consisting of pebbles of pure energy (photons) that race about at an extremely high speed. On the other hand, you can also think of light as waves of energy rippling through space. Physicists tend to think of light as simultaneously being both particles and waves; this concept of particle-wave duality has delighted philosophers and frustrated students for decades. In studying remote sensing, we deal both with light's particle-nature and its wave-nature.

Being massless, photons are allowed by Einstein's laws of special relativity to travel at the speed of light. They consist of pure energy, stored as an electric field and a magnetic field (hence the synonym for light, electromagnetic radiation). The energy is constantly shuffled between these two fields as the photon races through space. The only thing that distinguishes one kind of photon from another is exactly how much energy is stored in those fields. A photon's frequency indicates how many times per second the photon shuffles its electrical and magnetic fields back and forth. As you might guess, high-energy photons have comparably high frequencies. Extremely high-energy photons (such as gamma rays) tend to behave very particle-like, while low energy radiation (such as radio waves) tends to behave very wave-like. Remember, though, that all photons travel at the same speed, the speed of light, indicated in equations by the lowercase letter "c".

Another aspect of a photon is its wavelength. The wavelength, very roughly, can be thought of as the size of the photon. The higher a photon's energy, the smaller its wavelength. Gamma rays and X-rays are tiny photons with high frequencies that can sneak between the molecules of human bodies easily, while ponderous radio waves are big distended entities that snag easily on television and radio antennae. The three factors of wavelength, frequency, and speed of light, are related by the simple equation:


c = speed of light
= frequency
= wavelength.

Note that SONAR is a remote sensing technique that uses sound energy, and not photons. Radar and LiDAR, however, do use photons.


4 February 2009