Category: telescopes and microscopes

How does a parabolic sound collecting dish work? – C

A parabolic dish microphone is essentially a mirror telescope for sound. A parabolic surface has the interesting property that all sound waves that propagate parallel its central axis travel the same distance to get to its focus. That means that when you aim the dish at a distant sound source, all of the sound from that object bounces off the dish and converges toward the focus in phase—with its pressure peaks and troughs synchronized so that they work together to make the loudest possible sound vibrations. The sound is thus enhanced at the focus, but only if it originated from the source you’re aiming at. Sound from other sources misses the focus. If you put a sensitive microphone in the parabolic dish’s focus, you’ll hear the sound from the distant object loud and clear.

I wear glasses for distance vision, but my near vision is good. Why is it that when I use a nearby mirror to view distant objects, I must wear my glasses to see them clearly? I should be able to see the nearby mirror well without glasses. — JFJ

When you view something in a flat mirror, you are looking at a virtual image of the object and this virtual image isn’t located on the surface of the mirror. Instead, it’s located on the far side of the mirror at a distance exactly equal to the distance from the mirror to the actual object. In effect, you are looking through a window into a “looking glass world” and seeing a distant object on the other side of that window. The reflected light reaching your eyes has all the optical characteristics of having come the full distance from that virtual image, through the mirror, to your eyes. The total distance between what you are seeing and your eyes is the sum of the distance from your eyes to the mirror plus the distance from the mirror to the object. That’s why you must use your distance glasses to see most reflected objects clearly. Even when you observe your own face, you are seeing it as though it were located twice as far from you as the distance from your face to the mirror.

How does a parabolic microphone work? — KL, Regina, Saskatchewan

A parabolic microphone is effectively a mirror telescope for sound. When sound waves strike the dense, rigid surface of the parabolic dish, they partially reflect. This reflection occurs because sound travels much faster in a rigid solid than in the air and changes in the speed of a wave cause part of it to reflect. In this case, the reflection redirects the sound waves inward because the reflecting surface is curved and the sound waves form a real image of the distant source that produced them. While you can’t see this real image with your eyes, you can hear it with your ears. If you were to mount a large parabolic dish so that it faced horizontally and then moved your ear around in the focal plane of the dish, you would hear sounds coming from various objects far away from the dish. The same effect occurs for light when it bounces off a curved mirror—a real mirror telescope. A TV satellite dish is the same thing, but this time for microwaves! In all three cases, the real images that form are upside down. To make a parabolic microphone, you normally put a conventional microphone in the central focus of a parabolic surface so that the microphone receives all the sound coming from objects directly in front of the parabola. To listen to different objects, you simply steer the parabola from one to the other. This is exactly what a TV satellite dish does when it wants to “listen” to a different satellite—it steers from one to the other.

Why is the Hubble telescope in space rather than on earth? — L

The earth’s atmosphere has poor optical properties that seriously diminish the resolving powers of even the finest earth-based telescopes. You can see these optical problems by watching the warm air rise above a radiator or hot pavement on a summer day. The little swirls and eddies of heated air distort the scenery beyond them. Earth-based telescopes have to look at the stars through several miles of swirling, inhomogeneous atmosphere and they struggle to compensate for the imaging problems this air causes. Most world-class telescopes are located on mountaintops, far from lighted urban centers and away from humidity and clouds. But even the sky above these mountaintop observatories causes problems. By putting Hubble in space, they got rid of all atmospheric problems—air turbulence, clouds, and nearby lighting. They also made it possible for Hubble to operate around the clock by eliminating the blue sky that blinds telescopes during the day.

If a microscope’s resolution is limited by the wavelength of the light it uses, why don’t we use very short wavelength light instead of electrons or other particles for studying very small objects? — SJ, Philadelphia, PA

Ultraviolet light isused in microscopy to achieve higher resolution than can be obtained with visible microscopes. But beyond ultraviolet light comes X-rays and it’s difficult to build imaging optics for X-rays. There are some X-ray microscopes, but they aren’t nearly as common and practical as electron microscopes. The electrons in electron microscopes have very short wavelengths (atomic and subatomic length scales) and yet electron optics are easy to build. So while very short wavelength electromagnetic waves can be made, they’re just not practical for microscopy.

Why can’t light resolve details smaller than about half its wavelength? — SJ, Philadelphia, PA

Suppose that you have a white card with what appears to be a black line on it. That line might actually be two very closely spaced lines; you’re not sure. To find out, you focus a beam of light to the smallest possible spot and then move this tiny spot of light across the line. You realize that if there are two separate lines on the card, then the spot of light should cross first one line and then the other, and you should see two changes in the reflected light rather than just one.

It turns out that, however, that no matter how hard you try you can’t focus the light to a spot much smaller than the wavelength of the light. An equivalent problem would occur if you tried to use water waves to create a narrow spike of water above the surface—no matter how you worked with the water waves, you would be unable to make them to merge together into a spike that’s much narrower than the wavelength of the water waves. Because of his limitation, your spot of light can’t be much smaller than the wavelength of light and you can’t distinguish between one line or two if those lines are much closer than a wavelength of the light you’re using. Since visible light has a wavelength of 400 nanometers or more, you can’t use it to resolve details much smaller than 400 nanometers wide.

Actually, there is an exception to this general rule—near-field scanning optical microscopy or NSOM uses light emerging from the tiny tip of a glass fiber to resolve details far smaller than the light’s wavelength. In NSOM, the resolution is determined by the tip size and not the light’s wavelength.

How does a sound dish work? I know that it’s a parabola, but I can only find drawings not explanations. — DW, Omaha, NE

A sound dish is actually a mirror telescope for sound. When sound waves from a distant source encounter a rigid parabolic surface, they reflect in such a way that they focus to a point. If you put a microphone at that point, it will detect the sound waves from the distant source. You can see this focusing effect by drawing a parabola on a sheet of paper and directing parallel lines—the sound waves from the distant source—toward the parabola. If you reflect each line in a mirror-like fashion from the surface it hits, you’ll find that all the reflected lines pass through a single point as they move away from the parabola.

Why is a satellite dish necessary to receive satellite broadcasts? Why doesn’t a conventional radio antenna work? — AW, Karachi, Pakistan

Normal television broadcasts use electromagnetic waves with relatively low frequencies and long wavelengths while satellite broadcasts use waves with relatively high frequencies and short wavelengths. The short wavelength waves from a satellite are known as microwaves while the longer wavelength waves from a normal broadcast station are generally known as radio waves. Since the optimal antenna size for receiving a particular electromagnetic wave is proportional to the wavelength of the wave, you need a smaller antenna to receive the microwaves from a satellite than you do the radio waves from a normal television station. However, the microwaves from a satellite are much weaker than the radio waves from a nearby television station and a small microwave antenna isn’t likely to absorb enough of them to produce a useable signal.

The solution to this dilemma is to concentrate the microwaves from a satellite with the help of an optical imaging system. Although it may not look like one, a satellite dish is really a carefully shaped mirror telescope. Just as the curved mirror of the Hubble space telescope can bring light from a distant star to a focus on an optical image sensor, so the curved wire mesh of a satellite dish can bring microwaves from a distant satellite to focus on a small microwave antenna. This microwave antenna sits at the focus of the satellite dish and absorbs the microwaves that the dish collects. The dish’s imaging behavior also ensures that microwaves from only one satellite are brought to a focus on the microwave antenna. You must redirect the dish or move the antenna in order to switch from one satellite to another.

A friend was telling me of a guy who created a TV satellite dish out of chicken wire in his attic — how would you do it, adjust it, and what kind of home-brew receiver would be required to use it? – BP

Since the microwaves used in satellite transmissions have wavelengths of several centimeters or more, they can’t pass through holes in a conducting material if those holes are less than about a centimeter in diameter. As a result, chicken wire reflects microwaves as though it were a sheet of solid metal. You can form a dish antenna by bending chicken wire into a parabola. When the microwaves from the satellite strike this parabolic reflecting surface, they are brought together to a focus at a particular point above the center of the parabola. If you then place a microwave receiving device at this focal point, you’ll be able to watch satellite TV.

If you want to do this, you should make a cardboard template for the parabolic shape and bend the chicken wire carefully to match this template. The more highly curved the parabola, the closer the focus will be to the dish’s surface. You should aim this dish directly at the satellite and put the receiving unit at the focus of the parabola, above its center. However, you’ll have difficulty building the receiving device yourself, although there are probably kits you can buy. The receiver should have a tiny antenna, a microwave amplifier, and a frequency down-converter, all together on a single circuit board. Working with microwave-frequency electronics is difficult because the wave character of the electric signals is painfully obvious in those circuits. Designing microwave circuits is a job for experts. In short, you can build the dish, but you should buy the receiver that sits at the center of the dish.

What happened to the Hubble mirror?

The mirror of the Hubble space telescope was ground with the aid of a flawed measuring device. Although the mirror was perfectly ground, it was given the wrong curvature and thus did not form a clear image at its focus. Light from one star that hit different points on the mirror did not converge to a single point on the imaging chip. To correct for this problem, the astronauts inserted a corrective optic into the path of the light. This refractive lens compensates for the incorrect convergence of the light so that it reaches a single point on the imaging chip. However, because it is a refractive optic, it cannot pass all wavelengths of light. Any light that is absorbed by the refractive optic is no longer measurable with the telescope.