The component of the electric field in an EM wave perpendicular to these molecules passes through the filter, while the component parallel to the molecules is absorbed.įigure 10 illustrates how the component of the electric field parallel to the long molecules is absorbed. Long molecules are aligned perpendicular to the axis of a polarizing filter. It can be shown that reflected light is completely polarized at a angle of reflection θ b, given byįigure 9. Since the part of the light that is not reflected is refracted, the amount of polarization depends on the indices of refraction of the media involved. Sunglasses with vertical axes would then block more reflected light than unpolarized light from other sources. Horizontal polarization is like an arrow bouncing on its side and would be more likely to be reflected.
Vertical polarization would be like an arrow perpendicular to the surface and would be more likely to stick and not be reflected. The reasons for this phenomenon are beyond the scope of this text, but a convenient mnemonic for remembering this is to imagine the polarization direction to be like an arrow. Vertically polarized light is preferentially refracted at the surface, so that the reflected light is left more horizontally polarized. This is akin to arrows striking on their sides bouncing off, whereas arrows striking on their tips go into the surface.įigure 8 illustrates what happens when unpolarized light is reflected from a surface. After interaction with a surface, the vertical components are preferentially absorbed or refracted, leaving the reflected light more horizontally polarized. Unpolarized light has equal amounts of vertical and horizontal polarization. This implies the reflected light is partially polarized and cannot be completely blocked by a polarizing filter.įigure 8. As you rotate the sunglasses, you will notice the light gets bright and dim, but not completely black. You can check this for yourself by holding Polaroid sunglasses in front of you and rotating them while looking at light reflected from water or glass. Note that 71.6º is 18.4º from reducing the intensity to zero, and that at an angle of 18.4º the intensity is reduced to 90.0% of its original value (as you will also show in Problems & Exercises), giving evidence of symmetry.īy now you can probably guess that Polaroid sunglasses cut the glare in reflected light because that light is polarized. It is interesting that, at an angle of 45º, the intensity is reduced to 50% of its original value (as you will show in this section’s Problems & Exercises). This seems reasonable based on experimenting with polarizing films. Solving for θ yields θ = cos −1 0.3162 = 71.6º.Ī fairly large angle between the direction of polarization and the filter axis is needed to reduce the intensity to 10.0% of its original value. For EM waves, the direction of the electric field is analogous to the disturbances on the ropes.Ĭos θ = I I 0 = 0.100 I 0 I 0 = 0.3162 \displaystyle\cos\theta=\sqrt=0.3162\\ cos θ = I 0 I = I 0 0.100 I 0 = 0.3162 However, a vertical slit blocks the horizontally polarized waves. If a vertical slit is placed on the first rope, the waves pass through. Those in the other rope are in a horizontal plane and are horizontally polarized. The oscillations in one rope are in a vertical plane and are said to be vertically polarized. To examine this further, consider the transverse waves in the ropes shown in Figure 3. Thus we can think of the electric field arrows as showing the direction of polarization, as in Figure 2.
For an EM wave, we define the direction of polarization to be the direction parallel to the electric field. (This is not the same type of polarization as that discussed for the separation of charges.) Waves having such a direction are said to be polarized.
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 2). Light is one type of electromagnetic (EM) wave. The electric and magnetic fields are perpendicular to the direction of propagation. An EM wave, such as light, is a transverse wave.
0 kommentar(er)
