The Nature of Light

LET'S REVIEW:
Photons are massless, chargeless particles that travel at the speed of light in wavelike fashion and carry radiation (travelling energy). Radiation is broken down into several groups, one of which is visible light. Collectively these groups are referred to as the electromagnetic spectrum. Photons, like water waves, have energy, a frequency and a wavelength. However, photons have very defined (discrete) quantites of energy. A photon's energy is directly related to it's frequency: the higher the frequency, the higher the energy. The lower the frequency, the lower the energy.

Radiation is energy that travels and spreads out. Earth is constantly bombarded by radiation from the Sun and other stars in outer space. There are several different types (or groups) of radiation: radio waves, microwaves, infrared waves, visible light (yes! visible light is radiation!), ultraviolet light, x-rays and gamma rays. When we talk about all of these groups as a whole we refer to them as the "EM" or "Electromagnetic" Spectrum. Earlier we referred to electrons absorbing energy from photons. In this section we want to talk about photons and the nature of light.


Electromagnetic Spectrum

 

 

 

 

 

 

Radiation travels by particles that have no mass or electric charge and move in a wavelike pattern at the speed of light. These particles are called "photons". These photons carry a certain amount of energy (sometimes called packets of energy, or quanta). The only difference between the various kinds of radiation is the amount of energy in the photons. Because they travel in a wavelike manner, they have a certain energy, frequency and wavelength associated with them.

Think for a moment about waves. If you throw a rock into a pool of water a series of waves appear. They're a series of high peaks and low troughs. If we measure the distance from one high peak to the next, the distance is called the "wavelength". It doesn't have to be peak to peak, it can be trough to trough or any other place on the wave as long as you measure to the same place on the next wave. The "frequency" is the number of wavelengths that pass a position every second. Frequency is usually measured in Hertz (Hz) and means "per second". Wavelength is usually measured in meters (or fractions of a meter). Frequency and wavelength have a special relationship with each other; they are the reciprocal of each other. That means that frequency = 1 / wavelength. Conversely, wavelength = 1 / frequency.

 

 

 

  The orange line shows the wavelength of the waves

 

 

The velocity of a wave is the product of it's wavelength multiplied by it's frequency.
Velocity = wavelength x frequency
In a vacuum (like outer space), visible light travels at a constant velocity (known as the speed of light) which is about 186,000 miles per second or 3x108 meters per second.

Visible light falls between Infrared (IR) and Ultraviolet (UV) in the EM spectrum and is a very small portion of the spectrum. It (light) is actually made up of several colors. If we put visible light into one side of a prism we get it's component colors out the other side.

 

 

 
Prism
Visible Light Spectrum

 

 

 

The main colors are: red, orange, yellow, green, blue and violet. There are more colors, but these listed are the main colors that are separated out. Each color group blends into it's neighbor next to it. Each wavelength in the visible part of the electromagnetic spectrum has it's own frequency and is associated with a color. On one end, the red color's wavelength is about 700 nanometers (700x10-9 m) in length. On the other end violet is about 400 nanometers (400 x 10-9 m) in length. Red has a longer wavelength and lower frequency. Violet has a shorter wavelength and higher frequency. Sometimes visible light is called white light because our eyes absorb all of the different colors at the same time where they are blended together and we perceive "white" light.

A photon's energy is directly proportional to it's frequency. Energy = hv, where h is known as Planck's constant (h = 6.62 x 10-34 Js or 4.13 x 10-15 eVs) and v is the photon's frequency. The equation tells us that the higher the frequency, the more energy the photon has. The lower the frequency, the lower it's energy.

Just an interesting side note ("lifted" from Nobelprize.org):
Max Planck was a German-born physicist that, about 1900, came up with a relationship between the energy and frequency of radiation. The idea was revolutionary at the time. He postulated that this radiated energy could have only discrete values (or quanta). It really marked a turning point in physics. At first it was not fully appreciated, but later had a far-reaching effect on classical physics. It gradually became overwhelming because it's application accounted for many discrepancies between observed phenomena and classical theory. His work later became known as the quantum theory.