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Light

An understanding of light is crucial for both the Remote Sensing Project and the Northern Lights Project. The Remote Sensing Project uses red and near-infrared light to compare to gauge plant health. The Northern Lights Project uses an instrument to detect green light, since it is the most common color during an Aurora. Understanding light can help you understand how light is produced in the aurora, why we see certain colors as opposed to others, and why red and near-infrared are the most useful for remote sensing of plant health.

  • What is Light?
  • Properties
  • Electromagnetic Spectrum
  • Radiation

Light Wave

What is light? This is actually a very difficult question to answer. Sometimes light behaves like a particle (like a ball bouncing off a wall) and sometimes it behaves as a wave (like the crests and troughs of a water wave). Scientists have just had to accept this and say that light exhibits "wave-particle duality." We must use both points of view if we are to fully explain the behavior of light - neither one alone is enough.

 

Photon PacketLight as a particle: Photons

When light behaves like a particle, we say that it comes in little packets called Photons. If we think of light as photons, you can think of a bunch of little packets of light streaming out of the Sun or a lamp, hitting, say, a building, and then bouncing back to your eye, which is why you see the building. The particle nature of light is useful when thinking about the emission spectra of atoms.

 

Sun-Photon Diagram

 

Light as a wave: Electromagnetic waves

Water WaveWhen light behaves as a wave, you can think of it as oscillating electric and magnetic fields. The different wavelengths and frequencies (see properties tab) give the light its color. You can think of a bunch of waves rippling away from the Sun or a lamp, piling up when they encounter an object like a building, and repounding back to your eye, which is why you see the object. The wave nature of light is useful when thinking about the different colors of light.

Light is the fastest thing in the universe, traveling at 300 million meters per second (670 million miles per hour). All forms of light travel at this speed, so what makes the different kinds of light different?

Wavelength

Wavelength: Waves can be very different sizes, from the size of an atom (tenth of a nanometer) to the size of a building (meters). The wavelength is a measure of how "stretched out" the wave is. It is the distance from crest to crest or from trough to trough.

Frequency: Frequency describes how "bunched up" the wave is. It is the number of times a repeating element goes by in a certain amount of time. For example, if you sit on your front lawn and watch 20 cars go by in 1 hour, the frequency of cars on your street would be 20 cars/hr (read 20 cars per hour). When it comes to light, you can imagine recording how many crests go by in 1 minute, or even 1 second. Light moves very fast, so it has a high frequency and could be a large number like 540 Tera-cycles/sec, which is the frequency of green light. When the time interval is seconds, we call that measurement Hertz (abbreviated Hz). So green light has a frequency of 540 THz. A longer wavelength implies a lower frequency.

FrequencyHere the purple wave has the highest frequency and smallest wavelength, and the red wave has the lowest frequency and the biggest wavelength.

 

Energy: Light carries energy. The energy from light warms the planet and helps plants perform photosynthesis. The energy of light is determined by its frequency. A higher frequency light carries more energy, which is why high frequency light like ultraviolet light has enough energy to cause sunburn.

The Electromagnetic (EM) Spectrum showcases all the different varieties of light. You can see the huge differences in scale of frequency and wavelength, so scientific notation is used to give you an idea of the order of magnitude. The EM spectrum is continuous - there is no clear dividing line between the different kinds of light.

EM spectrum

Radio waves have the longest wavelength and the lowest energy. These waves are always around you transmiting radio and TV signals. They are also used for radar.

Microwaves are low energy light, used to heat food in microwaves and transfer signals to satellites.

Infrared (IR) light has a little less energy than red. It is usually emitted by objects with heat and is used in night vision. Your body has heat, so you are glowing in infrared light.

Near infrared (NIR) has a little more energy than IR, and is just outside the range that humans can see. It is strongly reflected by plants, making it a very useful measure of plant health with the NDVI index.

Visible spectrumVisible light consists of all the colors of the rainbow: red, orange, yellow, green, blue, indigo, violet. All of what humans can see is limited to this narrow wavelength regime of 380-750 nanometers (nm). White light is a combination of all the colors.

Ultraviolet (UV) light has more energy than violet, enough to cause sunburn. Much of the UV light from the Sun is absorbed in the atmosphere by ozone molecules, but it comprises 5-6% of all light that reaches the surface.

X-ray light is energetic enough to go through your skin, which is why it is used in medicine to picture bones. It is created by accelerating electrons or very hot places in the universe.

Gamma rays are the most energetic form of light, with the shortest wavelength and highest frequency. They are given off during nuclear reactions and decay, and are deadly to humans. All the gamma rays produced in outer space are blocked by our atmosphere, protecting us everyday.

Everything in the universe radiates light in some form or another. Stars shine from nuclear fusion in their cores, humans glow in infrared light from the heat in our bodies, and even cold objects such as ice emit very low intensity infrared light. There are different ways objects can radiate.

Black Body RadiationBlackbody Radiation:

Blackbody radiation occurs when a source emits a continuous spectrum of light at all wavelengths. If we plot the energy output of a blac-body radiator across all wavelengths (or frequencies), we can see a smooth curve with a peak. The peak corresponds to the strongest energy output and depends on the temperature of the object. For example, our Sun is a near-perfect blackbody radiator with a surface temperature close to 6,000 Kelvin. This corresponds to a peak output that is actually in the green spectrum. The other colors drown each other out, so the Sun is more of a peachy white, but appearing yellow to us on Earth. A star that is cooler than the Sun may have a peak of radiation closer to the lower-energy red side of the spectrum, therefore appearing red instead of a peach color. You have probably seen blackbody radiation in action as you heat up a stove. The burner starts out warm, but with no change in color, because its blackbody peak is in the low-energy, invisible infrared region. As it heats up, it starts to glow red, then orange, and then even white, meaning it is very hot.