Questions and Answers about Optics

1. I was wondering if it is possible to access a telescope from our classroom. He has heard that seeing how it is light during the day here that we could access a telescope say in Australia being dark there and then look at the stars. Have you heard of such a thing or is it even possible?

There are a number robotic telescope facilities that allow school students to make observations and get images.  I suggest looking into the program run jointly by Mount Wilson Observatory in California and Jet Propulsion Labs.  The web site is

Marc Pound

2. It is true that an immense amount of space dust is around us that blocks are view. Is it possible that behind that veil of dust are planets and stars that are hidden from our view?

Dust, especially from our own galaxy, does often obscure our view of distant objects - the distribution of dust however is spotty and not at all uniform. Star forming regions are one example where a lot of interesting astrophysics is hidden from us by enormous dense clouds of gas and dust. However, we can get information about what goes on inside the dense clouds from observations of what objects and processes are near the star forming clouds. The Eagle Nebula is a particularly good example of a star formation region because it has several phases of the star forming process going on at the same time. There are large cold dense clouds of gas which appear dark because they are too dense to see through or even into. Inside the dense clouds are smaller denser clouds which are in the middle of forming new stars. The smaller clouds become visible as the larger clouds surrounding them are evaporated away by bright energetic young stars nearby. The evaporation process causes the edges of the large clouds to release a large number of photons which creates a bright outline that contrasts strongly with the dark interior of the gas and dust cloud.  There is a good picture of the Eagle Nebula on the web at - for the detailed explanation go to the complete caption link.

Zoe Malka Leinhardt

3. Do you think that there are more colors in the color spectrum than those that we can see?   If so how many different colors do you think are out there.  Is it possible to find them or are we just stuck with the small group of colors we have.

There are indeed many "colors" of light which the human eye can not detect. There are a number of these "colors". Scientists detect these other "colors" everyday with detectors in labs and on space telescopes. To find out why I keep putting the word "colors" in quotation, keep reading. My answer is long, because I am not sure precisely what you mean by "color spectrum" as this phrase could be used for several things. I apologize in advance if this answer is a bit long.

To understand how many different colors there are, it is very useful to understand the concept of a "wave-length". So let me talk about waves for awhile and it will make my answer easier to understand.

A helpful analogy is that of waves traveling on the surface of water. Imagine a series of waves on the ocean; a "train" of wave crests, each separated from the next crest by the same distance. They move along the surface, say from England to Maryland, but if you were a passenger on a boat who had fallen into the water and were swimming in the Atlantic ocean waiting to be rescued, and one of these wave crests were to pass by, you would bob up and down like a cork or a duck. The oscillations are "transverse" (up and down) to the direction that the crest is moving (east to west). As you bobbed up, you could look out in the distance and estimate that the next crest was perhaps 100 feet away. Then you have made rough measurement of the wave-length of a wave. This is a simplified picture, but it's OK for now.

Here's an experiment you can do: take a glass of water and barely touch the surface of the water. Lift your finger and let the water drops drip off of your now wet finger tip onto the water surface. There will be tiny wave crests (hills) and troughs (valleys) expanding out as concentric circles from the point where the drop hits. The distance between each successive "hump" or "crest" is the so-called "wave-length".

The smaller the wave-length of a wave, the more energy it carries. This is difficult to understand, but a simple analogy that might help convince you that this is a plausible statement. Imagine that a ship may not be damaged by those ocean waves crests mentioned before if the distance between each crest is 100 feet, and it could rise up slowly and fall back down slowly, gently as the wave passed beneath it. But if the distance between each crest was only 20 feet, the boat gets bobbed up and down more times every minute, so it's going up and down much faster, even though the height of the wave crests has not changed. This is not precisely how light wave energy works, but it's the best analogy I could think up.

Light can be understand in terms of waves, (1) with different colors being associated with different wave-lengths, and (2) waves of smaller waves-lengths having more energy. But light is not an oscillation of a water surface. Light is an oscillation electric and magnetic energy, so scientists refer to light as an "electromagnetic" wave. It's not immediately important to understand the details of the "electromagnetic" qualities of light, other than to know that they are a little more complicated than water surface waves. But it is good to know that light has something to do with electricity and magnetism so you can have a little hint as to how our eyes can see colors.

The terminology can get confusing. Sometimes when people write about the electromagnetic waves that we can see, they use the word "light". When they talk about electromagnetic waves that we can not see, they might call it by other names (see below). All wave-lengths of light are collectively referred to as "the electromagnetic spectrum". But it is easier to say "light", so I use this word more freely.

The waves that light are made of can come in almost any wave-length. Unfortunately, our eyes are very limited as to which wave-lengths they can detect. Our eyes can detect light waves that have a wavelength somewhere between 400 and 700 nanometers long. A nanometer is about the size of a large atom or small molecule. 400 nanometers is about the size of some protein molecules and 700 is about the size of a blood cell or some single-celled organisms. Imagine if you had a yard stick on which you could draw tick marks not for inches [36 of them] but for nanometers [1,000,000,000 (billion) of them]. To get an idea of how small this is, imagine that the number of atoms which can fit inside a bowl the size of and orange is roughly the same number of oranges than could fit inside a bowl the size of the planet Earth.

It turns out that the beautiful biology of our eyes and the cells that make up our retinas, send signals to our brain that allows us to perceive wave-lengths close to 400 nanometers as "blue" and those close to 700 nanometers as "red". Sometimes, we see many wave-lengths as being the same color. For example, your eyes and brain can't tell the difference between 400 nanometers and 401 and 402 nanometers. So there is a "band" of wave-lengths (400 to 402), with all of the individual wave-lengths within this "band" being perceived by our brains as just one color.

The concept of "color" involves biology and perception. I'll talk a little about the biology and physics, but not the psychology or philosophy arguments that some people make. There is probably some variability from person to person as to which wave-lengths make up what they perceive as "blue" or the other colors.

The two types of photoreceptor cells you have likely heard of in your biology classes are the rod cells (used in low light, incapable of sensing color) and cone cells (sensitive to color). There have been experiments on the rod-shaped photoreceptor cells of the human eye that demonstrated that these cells can detect a single incoming packet of light (photon). But it is the cone cells which operate in bright light and sense colors. Defects in such cone cells can cause problems for people to detect greens and reds and distinguish the two ("color blindness"). There are in fact three types of cone cells, one type for red, one for green and one for blue. Each type of cone has a similar protein which uses a large molecule called retinal, similar to vitamin A, in different ways to absorb light of the three different colors. This is where my knowledge as a physicist is limited. A biologist could tell you more, and I stop here so as not to tell you anything wrong. The best source of information on color perception, with focus on the biology which underlies it, would be books on the human eye. Particularly, ones that concentrate on the photoreceptors of the retina.

Back to the physics of the light waves themselves... Often people refer to the entire "range" of what we can see with our eyes, from 400 (blue) to 700 (red) nanometers, as "visible light" or the "visible spectrum". Sometimes scientists will refer to wave-lengths outside of this range as "colors", even though humans can not see them. Since there are so many other wavelengths, the scientists don't invent new names for very narrow bands of color (like "violet" and "red"), but simply refer to these other "colors" in terms of their wave-length or in terms of very broad bands of wave-lengths, giving a name only to a large range of them (analogous to naming the range we called "visible light").

A good book for scientists called "The CRC Handbook of Chemistry and Physics" lists the following ranges of wave-lengths (in nanometers) for each visible color which are the conventional ones used by scientists:


Color    Range(nm)


Violet   400-424

Blue     424-491

Green    491-575

Yellow   575-585

Orange   585-647

Red      647-700


Light waves that are just a little shorter than "blue" are called "ultra-violet", as they start with wave-lengths just beyond that of the shortest wave-length color our eyes can see (violet). While human eyes can not detect wave-lengths shorter than about 400 nanometers, honey bees and other insects can see some of them.  Many species of flowers have amazing patterns that can only be seen when you see the UV light which is reflected off their petals. Some flowers care more about impressing insects than people. The same UV light contains more energy, per light packet, than visible light, and can cause sun-burn by damaging your skin cells.

Wave-lengths that are within a range just longer that about 700 nanometers can also not be detected as "colors" by our eyes. Called "infra-red" (abbreviated as "IR"), this band is most commonly associated with heat. Our bodies naturally emit a lot of IR light waves. Our unaided eyes can not see this IR light directly, but there are devices used by the military called "Night Vision Goggles" which detect the IR light and convert it to visible light and then pass this to our eyes.

Similarly, scientists use a huge collection of machines and tools to take light which is of wave-lengths not detectable by the human eye, and converts it into a form that we can see or measure. Before the 20th century, all astronomy was done using tools to enhance visible light emitted from stars and planets so that people could see things which were otherwise to distant and faint to see with the human eye. Throughout the 1900s, incredible developments in science and technology provided the tools astronomers needed to detect, collect, and transform non-visible light such that scientists could study it and the nature of the stars and other objects which emitted it.

A complete table of light, similar to the one above, might look like the one below. I have purposefully left some entries blank. I leave it to you to fill in the missing entries.

"Color"       Range (nm)    Size of...              Used to study...


Gamma Ray     ?-?            nucleus or smaller       ?

X-ray         ?-?            atoms                    Black holes

UV            ?-400          molecules                ?


Violet        400-424        proteins                 Planets, Stars

Blue          424-491

Green         491-575

Yellow        575-585

Orange        585-647

Red           647-700        large cells


IR            700-?          ?                        ?

Microwave     ?-?            insects                  Cosmology

Radio         ?-?            mice or larger           Galaxies -----------------------------------------------------------------

I hope somewhere in this is a satisfactory answer to your question. I hope you are encouraged to read more about light in astronomy, physics, and biology. Have a nice summer and use some UV-blocking sun-screen and sun-glasses!


4. What is gravitational lensing and how can you tell an object is gravitationally lensed or not?

If I understand correctly, you're asking about gravitational lensing, and how one can tell whether a given object that we see is just there on its own, or if its image has been "lensed" into our line of sight.  I'll start by going over what a gravitational lens is and how it works.  Basically, if enough mass is concentrated into one place it can warp spacetime, not unlike a bowling ball sitting in the middle of a trampoline.  Light travels in the fabric of spacetime like everything else, just at a (constant) very high speed.  So if the spacetime itself is warped in the vicinity of a massive object, light gets its path bent when it approaches the object, not unlike what the path of a marble would be if you rolled it along the trampoline from one end to the other with the bowling ball sitting there in the middle.

So let's say you have a distant galaxy very far away from us along our line of sight, so dim that we wouldn't be able to see it.  Let's also say there is a large, massive galaxy cluster sitting between us and the distant galaxy along the same line of sight, obscuring our view of it. Light from that galaxy is being emitted in all directions, but none would get to us because the cluster is in the way.  However, because the cluster is so massive, it warps the spacetime around it, bending some of the light emitted by the galaxy that would ordinarily be aimed away from us at some angle.  Now instead of shining away from us, this light gets bent until it shines right at us.  One can imagine that if their was a perfectly straight line connecting us, the center of the cluster, and the center of the galaxy, that we would effectively see a "ring" around the cluster, which is just multiple distorted (and also magnified and brightened!) images of the background galaxy.  Real systems like this do exist, but are rarely found on a straight line through their centers, so seeing an "Einstein Ring" like I've described is quite rare.  More often than not, when we detect a gravitational lens we see one or more distorted images, but not a full, symmetrical ring.

So in answer to your question, we can tell if an object has been "lensed" into our view by looking for evidence of shape distortion, or perhaps mirror images of the same object at different points around a central massive source.  Obviously, the more subtle the lens (i.e. the less massive the cluster, or other intervening source), the harder it is to detect by examining other objects within the field of view.

I'll include some helpful links that I think deal with this topic (or slight variations of it) well:

Higher level, but cool:

Laura Brenneman

5. I was told if there was a solar eclipse, you could see the same star more than once because of how the light was bent.  This is very confusing to me and I was hoping that you could explain how it works.

Actually, that's not true.  The Sun is a gravitational lens, but it is not strong enough to produce more than one star image at the distance of Earth, or anywhere in our solar system.

The Sun's gravity field IS strong enough to bend starlight, though, enough to make stars look like their positions have changed slightly.  This was first predicted by Albert Einstein and then demonstrated in 1919 by Arthur Eddington, an astronomer who observed the solar eclipse of that year.  He took a picture of the Hyades star cluster, which was very close to the edge of the eclipsed Sun, during totality.  Six months later, he took another picture of the Hyades cluster, without the Sun around (of course) and was able to show that the relative positions of the stars in the cluster appeared to be slightly different in the two photographs.

While it is possible for one star to lens another, the distances involved are so vast that when we see this from Earth (and we do), we can't actually resolve the very tiny angles between the separate images.  Instead, we see a characteristic pattern of brightening and dimming as the lensing star passes by the background star.

It takes something the size and distance of galaxies to produce multiple images (of other galaxies) that we can see from here.  And whether you see multiple images of the same galaxy, a single enlarged image or some other odd distorted light pattern depends on how the Earth, the lensing galaxy, and the farther galaxy all line up and how far away from each other they are.

Albert Einstein predicted the gravitational lens effect of the Sun and calculated how much the stars would seem to shift.  Here's a URL to a page with one of his illustrations:

Here's a link to a page with definitions and a diagram of how a massive galaxy bends the light of a background galaxy:

And finally, someone sent a similar question to the MadSci Network site a few years ago.  Here's a link to the answer, which includes links to some excellent images showing the various lensing effects. Hope this clears up the confusion.

Anne Raugh

6. Are there any new telescopes in production that will look at the Universe in a different way? I know that there is a project currently taking slices of the universe and compiling them. Is there a telescope/camera that allows us to get the same information as the slices in larger portions?

You'll be happy to hear that there are a *ton* of proposed projects that will enhance our knowledge of the universe in a dramatic way. In fact, that's one of the cool things about astronomy: instruments and telescopes are improving so much that each generation gets a *much* better understanding of things.  Surveys such as the project you mentioned are one way, or one can get more sensitive instruments and get the larger portions you described.

There are many examples, but let me give you a few of the more eye-catching proposals.  Most of these are still just proposals, so whether they get funded is up in the air.

OWL: the Overwhelmingly Large Telescope.  This would be a 100 meter diameter telescope to look at optical wavelengths.  For comparison, the largest current telescopes are roughly 10 meters in diameter. With an hour's staring, this telescope could see a 100 Watt light bulb at the distance of Jupiter.

SKA: the Square Kilometer Array.  This would be a huge set of radio telescopes, and would be able to detect every pulsar in our Milky Way galaxy, among other things.

MAXIM: the Microarcsecond X-ray Imager.  This would us to resolve (as in, actually get an image of) the black hole in the center of our galaxy and a few others.

The Auger array: this one actually exists now and is about to take data.  It isn't a telescope that looks for photons (i.e., electromagnetic light).  Instead, this is an array of detectors that is looking for extremely high energy cosmic rays, which are particles.  It is likely to tell us about some of the most energetic events in the universe.

LIGO and LISA: these are detectors that people hope will provide direct detections of gravitational waves, which are ripples in spacetime predicted by Einstein's theory of gravity.  LIGO is on the ground and exists now (with ever-improving sensitivity), and LISA is planned for space.  These would certainly provide new views of the universe, since gravitational waves have never been detected directly!

Cole Miller

7. Images of astronomical nebulae look somewhat like clouds in Earth's atmosphere. Though they are extremely different in size and composition, they have the same shape and appearance.  Why is this?  Are the processes that shape these cosmic structures similar to those that are at work in Earth's atmosphere?

That's an interesting question.  There are some similarities, such as that both are composed of molecules.  This leads to some effects (such as surface tension) that apply to both.  There are also some differences, such as the presence of stars in nebulae, which cause light to shine from the inside.  Clouds in our atmosphere are also affected by the atmosphere as a whole in a way that doesn't have a strong parallel in nebulae.  Therefore, I'd say it's a mixed bag: some similarities, some differences, so that in particular if you wanted to apply a result from one to the other you'd have to be careful.  Still, that's a neat idea!

Cole Miller

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