How do you Compare red light and blue light using the term energy?

Answer 1

This is a good question, and one that is of interest to those concerned with the environment. To understand energy in light, we need to understand waves and wavelengths.

Consider waves on an ocean - each wave has a high peak and a low trough, and the ocean is an endless cycle of peaks and troughs. If we were to measure the distance from one peak to the next peak, that measurement would describe the length of one wave. This measurement is appropriately called the wavelength. On the ocean, we would likely measure wavelengths in feet or meters.

Light travels in waves too, but unlike ocean waves, light waves are so small that we cannot see individual light waves. In fact, light waves are so infinitesimally tiny, we describe light wavelengths in nanometers (a nanometer is one billionth of one meter) or often in Angstroms (1 tenth of a nanometer, or one ten-billionth of one meter).

Visible light ranges in wavelength from approximately 4000 Angstroms (blue) to 7000 Angstroms (red). Blue light is therefore carried by waves that are shorter than red light.

Ultraviolet light has even shorter waves than blue light (100 to 4000 Angstroms), while at the other end of the spectrum, infrared has even longer waves than red (7000 to 10,000,000 Angstroms).

The amount of energy in light is inversely proportional to its wavelength. In other words, as the wavelength of light becomes shorter (more blue), the energy carried by that wave becomes higher. Specifically, the energy calculation for light is:

E (hc)/λ,

Where h is Planks Constant, C is the speed of light, and λ is the wavelength

While we will not concern ourselves with the mathematics here, the following statements help illustrate the relationship between light color and energy:

• Blue light (4000 Angstroms) has 75% more energy than red light (7000 Angstroms) when both lights are exactly the same brightness
• Ultraviolet light (1750 Angstroms) has four times more energy than red light of the same brightness

As an aside, the relationship between light color and energy is what has so many people concerned about the ozone layer. The ozone layer is a very high, very thin layer of ozone (O3), and one interesting property of ozone is that it filters out ultraviolet light. Without the ozone layer, high energy ultraviolet light penetrates the atmosphere, and reaches us on the Earth's surface. Because ultraviolet light carries so much more energy at the same brightness than visible light, it has the potential to cause more damage to our bodies, including cancer.