Optics Lessons: Part 7 - Physical Optics

Thin-Film Interference

You may have asked yourself once, how do bubbles and thin films of oil have rainbow coloring in them?

The answer lies in constructive and destructive interference--more specifically in the reflected light waves from the top area interfering with reflected light waves from the bottom area of the film.

To determine what type of interference has occurred to result in the distinctive coloring of the film, we have to first look at the index of refraction. If a light wave reflects from a medium with a greater index of refraction, then the wave is shifted 180 degrees. If the medium it is reflected from has a smaller index of refraction, no phase shift occurs.

This explains why there is a phase shift when light reflects from an oil film--Oil film possesses a greater index of refraction than air and thus, the reflected light experiences a 180-degree phase change--

Now, based on this phase change, we can differentiate if certain reflected waves result in constructive interference or destructive interference:

If the wave's path length within the film were an integral amount of wavelengths, then after the 180-degree phase change, the reflected waves from the two mediums would not be in sync, and hence interfere destructively. (Destructive interference is used for non-reflective coatings, which are usually on camera or binocular lenses.)

If the wave's path length within the film were an odd amount of halved wavelengths, the reflected waves would be in sync and hence result in constructive interference.

To determine the wavelength for constructive interference, the following equation is used:

Optical Flats and Newton's Rings

Mirrors and lenses are very important in the study of Optics. Often, error in optical experiments is caused by irregularities and lack of uniformity in such optical instruments. Thin-film interference can be used to check the quality and smoothness of lenses and mirrors by use of optical flats, which are glass plates (not lenses) that are grinded until they are very smooth and flat. First two optical flats are placed together and thin-film interference is used to determine their reliability. The optical flats are then placed under the mirror and if the latter is smooth, a regular interference pattern will be observed. If the latter has any irregularity, the interference pattern will not be regular.

When the same thin-film interference testing is applied to lenses and a lens is placed upon a smooth optical flat, there is space between the area of the lens not touching the optical flat and the flat. This gives rise to concentric dark and light fringes, called Newton's rings. They were named after the scientist because he was the first to discover this strange fringe pattern.

Polarization

Pole. This word is most commonly used in reference to the two extreme ends of our world, the North and South poles. It has also been used to describe the opposite poles of a magnet. The polarization of light does not really focus on two extremes, but does mean the orientation of light waves towards a specific direction, traveling on a certain plane.

Light is produced from a source: the vibration of atoms, which forms electromagnetic waves. Since each atom moves a different way, the waves are emitted in all sorts of directions. This medly of waves is called unpolarized light. Paritally polarized light is when the waves are oriented towards a general direction, but not traveling exactly in one plane. If the light is traveling in only one plane, it is defined as linearly polarized.

Polarization proves that light travels in transverse waves, not longitudinal ones. Transverse waves are waves that travel up and down. The actual components of the wave do not move, but carry energy by ondulating. Longitudinal waves are waves that shift back and forth. For these waves as well, the components do not travel along with the motion, but carry energy by oscillating.

Polarization by Reflection

When light strikes a material such as glass, part of the light gets reflected and part of it gets refracted. It is this effect that causes the light to become either unpolarized, partially polarized, or completely polarized, depending upon the angle at which the light strikes the surface, or the angle of incidence.

The light is unpolarized only if the angle of incidence is 0 degrees, which does not occur often. Therefore, it is more common that polarization does occur. The greater the angle of incidence, the greater the polarization. When the light strikes at an angle of 90 degrees or perpedicular to the surface, the light is completely polarized. This angle is known as the polarizing angle, or Brewster angle, since it was discovered by David Brewster.

Polarization by refraction is when light is bounced off of numerous surfaces, instead of only one. Often, a stack of glass plates is used, although in contemporary times, for practical purposes, thin films of transparent material are preferred. The beam of light that is refracted is continuously polarized, since refraction causes polarization. Because of this polarization, the beam of light that is refracted is shown to increase in intensity.

Polarization occurs frequently. Even the light from a rainbow is polarized, because the angle of incidence of light off a water beam is similar to a Brewster angle.

Polarization by Double Refraction (Birefringence and Dichroism)

Materials such as glass are described to be isotropic, because it does not matter how light passes through them. The characteristics of the glass are such so that it has no different effects on the light even though the direction of the beam varies. Other materials such as ice and certain crystals are said to be anisotropic, because the direction of the light as it passes through does matter. The characteristics of these materials affect the speed of the light passing through. They are described as birefrigent.

These anisotropic materials tend to polarize light into two different beams. One, the ordinary ray, passes through the material generally without being deflected. The other, the extraordinary ray, is refracted as it passes through. In cases where the material does not absorb either of the rays, the two beams emerge separate from one another.

Sometimes the property of dichroism will arise in a crystal, where the material itself actually absorbs one beam or another. If the beam is absorbed completely, then only one polarized plane of light emerges. Dichroic crystals were manipulated by Edwin Land in 1930, who invented a polarizing material that would be called Polaroid and give a huge benefit to the camera industry.

The property of dichroism can also be seen with electrons, in furthering the quest for discovering good camera film. Polymers were used, whose long chains and electrons formed a chain. When light passed through this chain, the rays parallel to this chain were absorbed, but the ones perpendicular were left alone. The perpendicular direction, or direction that the emitted rays followed, is called the transmission axis or the polarization direction. It is thus that the polymer sheet polarizes light, which is necessary in developing film.

Since our eyes cannot tell the difference between polarized and unpolarized light, an analyzer must be used, which is usually a sheet of polarizing film. If the transmission axis is parallel to the plane of the polarized light, a great amount of light will be transmitted. If the transmission axis is perpendicular to the plane of the polarized light, very little light will be transmitted.

To apply physics to our daily lives, polarization is used in sunglasses to block out some of the light. When the sunrays are very intense, the polarizing glasses are oriented so that their transmission axes are perpendicular to much of the light and thus blocks it from passing through. In addition, polarization is used in 3-D glasses, to change the direction of light that enters them and thus create a 3-Dimensional effect.

Certain transparent materials have the property of optical activity, where they can change the direction of polarization, depending on circumstances. This characteristic can be found among certain proteins, amino acids, and sugars. It is also in glass that has been put under a lot of stress.

Atmospheric Scattering

Why is the sky blue? Why are clouds white? Well, as you can guess, the answer is in the topic of this section, scattering. Scattering is a procedure that spreads energy-It involves particles of a medium of a varying index of refraction spread parts of oncoming light in every direction. The basic principle of scattering is illustrated below in the diagram:

There are three kinds of scattering: Mie scattering, Rayleigh scattering, and Nonselective scattering.

Now, let’s quickly check your comprehension of Rayleigh scattering vs. Mie scattering.
Examine the following diagram.

Pretend you are where that green line is. Which kind of scattering will dominate and what effects with it have on the atmosphere in each situation? Refer above to the brief descriptions of Rayleigh and Mie scattering for help.

In the first situation, located higher in the atmosphere, where the particles are smaller, Rayleigh scattering is favored. Because this type of scattering takes placed with shorter wavelengths, blue light is mostly scattered, making the sky appear blue.

In the second situation, located lower in atmosphere where the particles are larger, Mie scattering is favored. Because this type of scattering is more independent of wavelengths, a white blur appears.

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