Electromagnetic waves

Laboratory work № 3

The polarization of light

Objective: studying the phenomenon of polarization of light, verification the law of Malus

Theoretical introduction

Electromagnetic waves

Maxwell’s equations show that a time-varying magnetic field acts as a source of electric field and that a time-varying electric field acts as a source of magnetic field. These and fields can sustain each other, forming an electromagnetic wave that propagates through space. Visible light emitted by the glowing filament of a light bulb is one example of an electromagnetic wave; other kinds of electromagnetic waves are produced by TV and radio stations, x-ray machines, and radioactive nuclei.

Figure 3.1 Electromagnetic wave.

Important features of all electromagnetic waves: 1. The wave is transverse; both and are perpendicular to the direction of propagation of the wave. The electric and magnetic fields are also perpendicular to each other. The direction of propagation is the direction of the vector product . 2. The wave travels in vacuum with a definite and unchanging speed:

c = = 3·10⁸ m/s (3.1).

3. The instantaneous magnitudes of E and B in an electromagnetic wave are related by the expression:

E =cB (3.2).

4. Unlike mechanical waves, which need the oscillating particles of a medium

such as water or air to transmit a wave, electromagnetic waves require no medium.

The electric and magnetic fields of a sinusoidal plane electromagnetic wave propagating in the positive x direction can be written:

E=E_max cos(kx - ωt)

B=B_max cos(kx - ωt) (3.3)

where ω is the angular frequency of the wave and k =2π/λ is the angular wave number. These equations represent special solutions to the wave equations for E and B. The wavelength and frequency of electromagnetic waves are related by:

c = λ·ν (3.4)

The electromagnetic spectrum encompasses electromagnetic waves of all frequencies and wavelengths. Figure 3.2 shows approximate wavelength and frequency ranges for the most commonly encountered portion of the spectrum. Despite vast differences in their uses and means of production, these are all electromagnetic waves with the same propagation speed.

The electromagnetic spectrum includes waves covering a broad range of

wavelengths, from long radio waves at more than 10⁴ m to gamma rays at less than 10^-14m.

We can detect only a very small segment of this spectrum directly through our sense of sight. We call this range visible light. Its wavelengths range from about 380 to 750 nm, with corresponding frequencies from about 790 to 400 THz. Light is produced by the rearrangement of electrons in atoms and molecules. The various wavelengths of visible light, which correspond to different colors, range from red (7,5·10^-7m) to violet (3.8·10^-7m). The sensitivity of the human eye is a function of wavelength, being a maximum at a wavelength of about 5.5·10^-7m. With this in mind, why do you suppose tennis balls often have a yellow-green color?

Figure 3.2. The electromagnetic spectrum. Note the overlap between adjacent wave types. The expanded view to the right shows details of the visible spectrum.

Ordinary white light includes all visible wavelengths. However, by using special sources or filters, we can select a narrow band of wavelengths within a range of a few nm. Such light is approximately monochromatic (single-color) light. Absolutely monochromatic light with only a single wavelength is an unattainable idealization.

Поиск по сайту: