The electromagnetic spectrum
Figure 1.4 describes the electromagnetic spectrum.
Each part of the spectrum has a common name, and its range of wavelengths, frequencies and energies.
The borders between the ranges are not sharp and clear, but are defined according to the applications of radiation in that portion of the spectrum.
Figure 1.4: The Electromagnetic Spectrum
The most important ideas summarized in figure 1.4 are:
Electromagnetic waves span over many orders of magnitude in wavelength (or frequency).
The frequency of the electromagnetic radiation is inversely proportional to the wavelength.
The visible spectrum is a very small part of the electromagnetic spectrum.
Photon energy increases as the wavelength decreases. The shorter the wavelength, the more energetic are its photons.
Examples for electromagnetic waves are:
Radio-waves which have wavelength of the order of meters, so they need big antennas (The dimensions of an antenna are of the same order of magnitude as the wave).
Microwaves which have wavelength of the order of centimeters. As an example: in a microwave oven, these wavelengths can not be transmitted through the protecting metal grid in the door, while the visible spectrumwhich have much shorter wavelength allow us to see what is cooking inside the microwave oven through the protecting grid.
x-Rays which are used in medicine for taking pictures of the bone structure inside the body.اعلانات جوجل
Gamma Rays which are so energetic, that they cause ionization, and are classified as ionizing radiation.
The theoretical analysis of electromagnetic waves was the work of James Clerk Maxwell (1831-1879), and it is summarized in 4 equations (which are beyond our scope) bearing his name.
The discrete aspects of electromagnetic radiation is the result of Einstein’s work at the beginning of the 20th century.
Electromagnetic Radiation in Matter
Light Velocity in Matter
When electromagnetic radiation passes through matter with index of refraction n, its velocity (v) is less than the velocity of light in vacuum (c), and given by the equation:
v = c / n
This equation is used as a definition of the index of refraction (n):
n = (speed of light in vacuum)/(speed of light in matter) = c/v
Gases, including air, are usually considered as having index of refraction equal to vacuum n0=1.
The values of the index of refraction of most materials transparent in the visible spectrum is between 1.4-1.8, while those of materials transparent in the Infra-Red (IR) spectrum are higher, and are 2.0-4.0.
Wavelength in Matter:
We saw that the velocity of light in matter is slower than in vacuum. This slower velocity is associated with reduced wavelength: l = l0/n , while the frequency remains the same (see figure 1.5).
Figure 1.5: Change of wavelength in matter