Polarized Direct Reflection

A polarized direct reflection is so similar to an ordinary direct reflection that photographers often treat them as the same. However, these reflections offer photographers several specialized techniques and tools for dealing with them.

Like the direct reflection, only one viewer in Figure 3.8 will see the reflection. Unlike the direct reflection, an image of the polarized reflection is always substantially dimmer than a photograph of the light source itself. A perfectly polarized direct reflection is exactly half as bright as an unpolarized one (provided the light source itself is not polarized). However, because polarization is inevitably accompanied by absorption, the reflections we see in the scene are more likely to be much dimmer than that. To

Polarized Direct Reflection
3.8 Polarized direct reflection looks like unpolarized direct reflection, only dimmer.

see why polarized reflection cannot be as bright as an unpolar-ized direct reflection, we need to know a bit about polarized light.

We have seen that the electromagnetic field fluctuates around a moving photon. In Figure 3.9 we have represented this fluctuating field as a jump rope being swung between two children. One child is spinning the rope while the other simply holds it.

Now, let's put up a picket fence between the children, as shown in Figure 3.10. The rope now bounces up and down instead of swinging in an arc. This bouncing rope resembles the electromagnetic field along the path of a photon of polarized light.

Molecules in a polarizing filter block the oscillation of the light energy in one direction, just as the picket fence does to the oscillating energy of the jump rope. The molecular structure of some reflecting surfaces also blocks part of the energy of the photon in the same manner. We see such a photon as a polarized reflection or glare. Now suppose, not being satisfied with eliminating just a part of the children's play, we install a horizontal fence in front of the first, as shown in Figure 3.11.

3.9 The oscillating electromagnetic field around a photon represented as a jump rope. The child on the left is spinning the rope while the one on the right holds on.

3.10 When the children spin the rope through the picket fence, it bounces up and down instead of spinning in an arc. A polarizing filter blocks the oscillation of light energy the same way.

3.11 Because we've added a horizontal fence to the first, when one child spins the rope, the other will see no movement.

With the second fence in place, if one child spins the rope, the other sees no rope movement at all. The crossed picket fences block the transmission of energy from one end of the rope to the other. Crossing the axes of two polarizing filters blocks the transmission of light, just as the two picket fences do with rope energy. Figure 3.12 shows the result. Where the polarizers overlap with their axes perpendicular, none of the type is visible on the page. The transmission of light reflected from the page to the camera has been completely blocked.

A lake, painted metal, glossy wood, or plastic can all produce polarized reflection. Like the other types of reflection, the

3.12 The two overlapping polarizers have their axes perpendicular. They block light just as the two fences did with the energy of the jump rope.

polarization is not perfect. Some diffuse reflection and some unpolarized direct reflection are mixed with the glare. Glossy subjects produce a greater amount of polarized reflection, but even matte surfaces produce a certain amount.

Polarized direct reflection is more visible if the subject is black or transparent. Black and transparent subjects do not necessarily produce stronger direct reflections than white ones. Instead, they produce weaker diffuse reflection, making it easier to see the direct reflection. This is why you saw the change in apparent brightness of the black objects, but not of the white ones, when you walked around your room a while ago.

Glossy black plastic can show us enough polarized reflection to make a good example. The scene in Figure 3.13 includes a black plastic mask and a feather on a sheet of glossy black plastic. We used the same camera and light position as in the pictures of the newspaper and the makeup mirror. You can tell by the size of the reflections that we used a large light source.

Both the mask and the plastic sheet produce nearly perfect polarized reflection. From this angle, glossy plastic produces almost no unpolarized direct reflection; black things never produce much diffuse reflection. However, the feather behaves quite differently. It produces almost nothing but diffuse reflection.

The light source was large enough to fill the family of angles defined by the plastic sheet, creating direct reflection over the entire surface. The same light was large enough to fill only part of the family of angles defined by the mask. We know this because of the highlights we see only on the front of the mask.

Now look at Figure 3.14. We made it with the same arrangement used in the previous picture, but now we've placed a polarizing filter over the camera lens. Because polarized reflection was almost the only reflection from the black plastic in Figure 3.14, and because the polarizing filter blocks glare, little of the light reflected from them reached the camera. As a result, the plastic now looks black.

We did have to open our aperture by about two stops to compensate for the neutral density of the polarizing filter. How do you know that we did not accidentally miscalculate the exposure? (Maybe we did so deliberately, just to get the image dark enough to prove our point.) The feather proves that we did not. The polarizer did not block the diffuse reflection from the feather. So, with accurate exposure compensation, the feather is about the same light gray in both pictures.

3.13 The glossy black plastic sheet and mask produce almost nothing but polarized direct reflection. The feather gives off almost nothing but diffuse reflection.

Polarized Direct Reflection

3.14 A polarizer over the camera lens blocks the polarized direct reflection. Only the feather, which gives off diffuse reflection, is easily visible.

3.13 The glossy black plastic sheet and mask produce almost nothing but polarized direct reflection. The feather gives off almost nothing but diffuse reflection.

3.14 A polarizer over the camera lens blocks the polarized direct reflection. Only the feather, which gives off diffuse reflection, is easily visible.

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  • tuukka
    Can a mirror polarize the electomagnetic waves?
    8 years ago

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