Transmission of energy through a vacuum or using no medium is accomplished by electromagnetic waves, caused by the osscilation of electric and magnetic fields. They move at a constant speed of 3x108 m/s. Often, they are called electromagnetic radiation, light, or photons.
Did you ever wonder what is electromagnetic radiation? The word is somewhat complicated, but you are in contact with electromagnetic radiation all the time. Here is a diagram of the electromagnetic radiation spectrum that has appeared in many text books and websites. Electromagnetic radiation is caused by the disturbance of an electromagnetic field.
The last line of numbers in power of 10 gives the wavelength in m. The regions sometimes do not have a clear cut, because there is considerable overlap. For example, radio waves and microwaves boundary is very vague, but public regulation for their application (usage) is strict.
Electromagnetic waves are used to transmit long/short/FM wavelength radio waves, and TV/telephone/wireless signals or energies. They are also responsible for transmiting energy in the form of microwaves, infrared radiation (IR), visible light (VIS), ultraviolet light (UV), X-rays, and gamma rays. Each region of this spectrum plays an important part in our lives, and in the business involving communication technology. The list given above are in increasing frequency (or decreasing wavelength) order. Here again is the list of regions and the approximate wavelengths in them. For simplicity, we choose to give only the magnitudes of frequencies. That is we give log (frequency) (log(f)).
Region: Radio, FM, TV, microwave, IR, VIS, UV, X-rays, Gamma rays. Wavelength: 600 m 20 m 1 mc 1 mm 0.1 mm 1e-9 m 1e-12 m 1e-15 m log (f): 6 7 8 9 10 11 12 13 14 15 20 23Electromagnetic radiations are usually treated as wave motions. The electronic and magnetic fields oscillate in directions perpendicular to each other and to the direction of motion of the wave.
The wavelength, the frequency, and the speed of light obey the following relationship:
The Visible SpectrumWavelengths of the visible region of the spectrum range from 700 nm for red light to 400 nm for violet light.
red 700 nm orange 630 yellow 550 green 500 blue 450 violet 400There is no need to memorize these numbers, but knowing that the visible region has such a narrow range of 400-700 nm is handy at times when referring to certain light.
Photons - bundles of electromagnetic energyIn his research on the radiation from a hot (black) body, Max. Planck made a simple proposal. He suggested that light consists of photons. The energy, E, of each individual photon of a monochromatic light wave, is proportional to the frequency, n, of the light:
1 nm = 1E-9 m, nanometer: for wavelength of IR, visible, UV and X-rays.
1 pm = 1E-12 m, picometer: for X-rays and gamma rasy.
The Photoelectric EffectWhen photons shines on a metal plate, they free electrons. Energy is required to pull the electrons out of the metal surface, and this amount of energy is calle threshold. The excess energy of the photon appear as the kinetic energy of the electrons. Here is a simulation of the photoelectric effect, which gives a slightly different perspective. A photoelectric effect demonstration is also fun for you if you have the time.Einstein learned of Planck's proposal, and he wanted to perform experiments to show if the proposal is true. At that time, the photoelectric effect was known, and he measured the kinetic energy of electrons released by photons. He did find a linear relationship between the kinetic energy of the electrons and the frequency of light used, (see diagram below).
Furthermore, he found the light of minimum frequency needed to release electrons from a metal to be constant, and this energy must be overcome in order to take the electron out of the metal. This energy is called the threshold energy, W. The formula to descirbe photoelectron kinetic energyEk is