Ch. 03 – The Pond (Part 5)

The Pond

Near the beginning of the twentieth century, physicists developed a picture of energy as waves moving through something they called the “ether” – try to envision waves moving across a pond of water. Much of what they observed about the behavior of light and other forms of energy could be explained using this analogy.

If you drop two pebbles into a pond, the two expanding patterns of waves will interact with each other, forming completely predictable and fully understood interference patterns. It turned out that light emanating from two sources behaved in exactly the same manner. Just like the waves in the pond, the light “waves” interfered with each other, creating interference patterns. Indeed, even today the mathematical equations describing this behavior are the best way to deal with electromagnetic energy under certain circumstances.

Back in the pond, if you place an obstacle like a large stone in the path of the advancing waves, several things can happen. If the obstacle is smaller than half the distance between the wave peaks, the waves pass right by the obstacle with little or no interference. Once the obstacle’s size increases beyond half the distance between the wave peaks, however, it begins to interfere with the passing waves. Depending on its size, the waves may bend or refract as they pass the obstacle, which can lead to an interference pattern behind the obstacle. As the obstacle gets even larger, eventually the waves striking it will either be reflected or absorbed. If reflected, they will form an interference pattern in front of the obstacle.

During this discussion we have been referring to the size of the obstacle. We could just as well not have changed the obstacle’s size and instead changed the distance between wave peaks or, put another way, shortened the frequency of the waves. Remember when you took a bath as a child? You could make waves in the bathtub by moving your hand up and down in the water. If you moved your hand slowly, the waves you generated undulated slowly across the tub, and the distance between the waves was relatively large. If you moved your hand up and down swiftly, in other words if you put a lot of energy into your wave making, the resulting waves were larger, and the distance between them was very short. Waves with shorter wavelengths carry more energy – kind of obvious, really, since more wave fronts pass a point per time interval, and of course, they are higher.

Two things are important here. When the obstacle begins to interfere with the passing waves, some of the energy in the waves actually gets transferred to the obstacle. Until that size relationship exists, nothing happens. Once it exists, you can then increase the amount of energy absorbed by the obstacle either by shortening the wavelength or by increasing the size of the wave peaks, which are two sides of the same coin since they are linked. On the beach of an island in the ocean, this absorbed energy pounds beach rocks into sand. If the waves arrive more often and are larger, there is more pounding. The significant point, however, is that the obstacle absorbs some of the wave’s energy only if half the wavelength is shorter than the obstacle’s size.

When physicists work with electromagnetic energy, they use an appropriate mathematical description. When dealing with how electromagnetic energy affects living tissue, they use wave mechanics – what we have been discussing.

Let’s put it together. Cells essentially have a fixed size. If half the wavelength of arriving electromagnetic energy is about the size of a cell, then some of the energy will be absorbed by the cell. So if the frequency is high enough, that is if the wavelength is short enough, the cell will absorb some of the energy. The shorter the wavelength, the more energy will be absorbed. Put a pot of water on the stove and add heat. Sooner or
later the water will boil. Subject a living cell to electromagnetic energy of an appropriate frequency, and sooner or later it’s going to cook. But if the wavelength is long enough, or put another way, if the frequency is low enough, then it really doesn’t matter how much energy passes the cell, it won’t feel it.

Up until now we have been comfortable with the mathematical analogy of a wave in a pond to help us understand the nature of radiation. Radiation as we are using the word, however, also includes alpha and beta particles, neutrons, and protons. These are not electromagnetic energy. They are physical particles with size and mass. While it is true that in some esoteric quantum mechanical applications, it is still useful to use the wave-in-a-pond analogy when discussing particles, we are not going there.

Time to shift from the pond to the pool table.