November 2001 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - November 2001 Issue

The Telescope Nut
by Jeff Baldwin

Reflection, Refraction and Diffraction

What are the three basic ways that light can change direction? What are the three ways that waves in general can change direction?

Light exists as a duality -- as a particle, and as a wave. To explain optics, we let light exist as a wave, and the explanations become possible. The difficult part is explaining what the medium of the wave is. For sound, the medium is air, the molecules bopping off of each other, transferring their energies to one another. Sound requires a medium; sound does not travel in a vacuum.

Light, however, has a weird problem, in that the medium is space itself. Back in the old days they created the idea that the universe was filled with a substance they called "ether". It worked pretty well, except that there was no physical substance in which light was required to travel, only empty space-time itself. (I personally can't explain this topic, but I bet Dr. Mielbrecht or Dr. Lark can. I'm only going to play out the story as it pertains to my narrow field of work, that of what light does as it interacts with materials.)

Let's start with reflection. Light travels as a wave through space in a direction perpendicular to its wave-front.

Imagine at the ocean how the waves come in toward the shore, in long lines (the wave front) parallel to the beach, moving in a direction which is perpendicular to that long line the wave forms.

Light from a star travels toward your telescope as a sheet. The sheet of light from the star will slap the mirror face to face. Following that sheet will be another sheet, one wavelength behind the first one (sort of). As the sheet of light strikes a reflecting surface, the energy of the wave must go somewhere. If it had mass, it would place its energy into the reflecting material, and some of the energy would be converted into mass. Most of the energy would be transmitted back out at an angle since, with a concave mirror, the energy is first responded to on one end, through the wave front toward the other end. This causes two things, one is a reflective angle, the other is diffraction rounding the edges (I'll talk about that later). Since the direction of the wave must be perpendicular to the wave-front, the direction of the wave changes as well.

The rule of reflection is that the angle of incidence (q) is equal to the angle of reflection (f).

Diffraction can be thought of as how light behaves as it deals with corners. If a dog is outside your door and around the corner such that you can't see it, you can still hear it bark. The sound waves must have traveled around the corners of the doorway in order to reach your ears. As light travels past things like secondary mirror spiders, mirror and lens edges, eyepiece field stops, corneas, etc., it tends to alter its pathway in intervals of the wavelength of the light. If you've ever seen stars in high quality telescopes at very high magnifications under good sky conditions, you've no doubt seen the diffraction rings around the star's Airy disk. This is an effect not of the star, but of the edge of your telescope's optic aperture. If you've seen spikes from spiders, this is a diffraction effect.

Refraction is the act of light changing speeds as it passes through other media. The speed of light is about 1.5 times faster through a vacuum than through glass, so glass has an index of refraction of about 1.5. Since light goes more slowly through glass, the wave sheets that hit the glass at an angle start going slower at one end before the other. The end that hits first is now traveling more slowly through the glass, which means that the distance between the waves is now closer. As the rest of the wave enters the glass, the wave-fronts have to remain connected, yet travel more slowly, which means the wave-fronts are now aligned at an angle to the original wave front. Since the direction of the wave is always perpendicular to the wave-front, the direction of the wave has to change. Thus light changes direction as it refracts through glass (and other material).

All three actions generally occur in telescopes. Let's take a refracting telescope, like an 80mm Celestron refractor, for example. As light enters the telescope, a small portion of it reflects back off the objective lens, and is lost to the observer. Some of it passes through the lens and is refracted. Since the lens surface is spherical, refraction is greater toward the outer region than it is at the middle of the lens, where the light strikes perpendicular to the glass. This smooth transition in shape allows the light to refract towards a focus.

Light that passes the edge of the lens is diffracted, and rings are seen around the Airy disk of the star.

When light is refracted through a lens, the more energetic light is more refracted than the less energetic light. This is called dispersion. Blue is more energetic than red, so blue is refracted more than red. This causes a color imbalance at each part of the focus. Using a different glass of a different index of refraction in the shape of a negative lens allows the different colors to spread back out a little, and when they come to a focus, these colors are closer to being focused at the same point. These two-element lens combinations are called achromats. Almost all refracting telescopes sold today are either 2-element or 3-element lens systems, ensuring color correction.

Clear Skies...Jeff Baldwin
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