The axis of a polarizing filter is the direction along which the filter passes the electric field of an EM wave (see Figure 10.40).įigure 10.40 A polarizing filter has a polarization axis that acts as a slit passing through electric fields parallel to its direction. Thinking of the molecules as many slits, analogous to those for the oscillating ropes, we can understand why only light with a specific polarization can get through. Polarizing filters are composed of long molecules aligned in one direction. Polarized materials, invented by Edwin Land, act as a polarizing slit for light, allowing only polarization in one direction to pass through. Such light is said to be unpolarized because it is composed of many waves with all possible directions of polarization. The sun and many other light sources produce waves that are randomly polarized (see Figure 10.39). Vertical slits pass vertically polarized waves and block horizontally polarized waves. The first is said to be vertically polarized, and the other is said to be horizontally polarized. Thus, we can think of the electric field arrows as showing the direction of polarization, as in Figure 10.37.įigure 10.38 The transverse oscillations in one rope are in a vertical plane, and those in the other rope are in a horizontal plane. For an EM wave, we define the direction of polarization to be the direction parallel to the electric field. Waves having such a direction are said to be polarized. This is not the same type of polarization as that discussed for the separation of charges. Polarization is the attribute that a wave’s oscillations have a definite direction relative to the direction of propagation of the wave. There are specific directions for the oscillations of the electric and magnetic fields. As noted earlier, EM waves are transverse waves consisting of varying electric and magnetic fields that oscillate perpendicular to the direction of propagation (see Figure 10.37). Light is one type of electromagnetic (EM) wave. Polarizing sunglasses are particularly useful on snow and water. As a result, the reflection of clouds and sky observed in part (a) is not observed in part (b). Part (b) of this figure was taken with a polarizing filter and part (a) was not. This satisfies 1st point mentioned above.įor the 2nd point, let us consider \(\phi = 0\)Īs the intensity of an electromagnetic wave is proportional to the square of the amplitude of the wave, the ratio of transmitted amplitude and the incident amplitude is \(\cos^2\phi\).Figure 10.36 These two photographs of a river show the effect of a polarizing filter in reducing glare in light reflected from the surface of water. An ideal polarizing filter allows 100% of the incident unpolarized light to pass through, which is polarized in the direction of the filter’s polarizing axis.įrom the above two points, it can be assumed that, \(I = I_0 \cos^2 \phi\).When unpolarized light is incident on an ideal polarizer, the intensity of the transmitted light is exactly half of the intensity of the incident unpolarized light, regardless of how the polarizing axis is oriented.This law is useful in quantitatively verifying the nature of polarised light.Ĭoming to the expression of Malus law, let us first see two points
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