Download and ShareUp and down the EM spectrum, the various energies of light – radio waves to gamma rays – share some key properties. All light can be reflected, which occurs when light hits a boundary it can’t penetrate. It gets reflected back based on the Law of Reflection: “The angle of incidence equals the angle of reflection.” A common visual model for this (Classically understood) physical phenomenon is a billiard ball hitting the side of the table and reflecting off at an angle that’s symmetric with respect to a line orthogonal to the boundary. For people who create and use optical instruments like microscopes, telescopes, lens systems, and lasers, designing and shaping their reflective properties are crucial factors that define the resolution and accuracy of those instruments. For example, one goal of telescope design – as this video explores – is to ensure that as much light as possible makes it to the detector, minimizing the light that gets scattered. For a telescope, scattered light is lost information. Photography is an everyday example of another system that uses light to gather, capture, and interpret information. Cameras that are very efficient at collecting light can create higher-precision images. The same is true for space-based telescopes.
Another way light can interact is through refraction, which occurs when light passes between two different materials and changes direction and speed. You can observe refraction by looking at the side of a glass half-filled with water with a spoon in it. The spoon appears to have a “break” at the water line. That’s because light is being refracted at that boundary. Light can pass through objects, too. Depending on the light’s frequency, it could pass through as if the object weren’t even there. An object can also be transparent to some wavelengths but not others. When you get an x-ray, you’re seeing an example of this. X-rays pass through soft tissue – but not through bones. That’s why you can see your bones on an x-ray image of your body. Just as we use light of different energies to “see” different parts of our bodies, so, too, we use telescopes with different types of optics to “see” the universe at different energies along the EM spectrum.
If light isn’t reflected by an object, and if it also doesn’t pass through, it will be absorbed. When an object absorbs light, the light’s energy is transformed into heat (or thermal) energy within the object or material. A good example of light absorption is photosynthesis. Plants take in light - absorbing the energies in the red and blue parts of the visible spectrum – and they reflect green. Plants are green because they DON’T absorb green light. The color of an object tells you the light it DOESN’T absorb. What you see is the light that’s NOT absorbed.
This video describes how NASA is using carbon nanotubes to help telescopes capture and analyze even more of the light they collect. Nanotube comes from the same root as nanotechnology and nanometer. A nanometer is one BILLIONTH of a meter, or 1 x 10-9 m. This is about the scale of a single atom, and carbon nanotubes are tiny tubes made out of carbon atoms. So, why is this useful? Carbon nanotubes are able to absorb light much more effectively than even the blackest of black paint NASA has available. The result is an experimental initiative to test whether a sheet of carbon nanotubes can enhance the accuracy and seeing of space-based telescopes.
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Classroom Activity: Color Scenes
Students work in groups to discuss and respond to the prompts below. Depending on the time available, the instructor can offer these color thought-experiments in sequence to all groups or can offer groups different scenarios.
Discussion Questions
STEPHANIE GETTY: My name is Stephanie Getty. I use micro and nano technology to make better scientific instruments for spaceflight.
OHN HAGOPIAN: My name is John Hagopian. I am an optical physicist at the NASA Goddard Space Flight Center. The exciting part about this work is; it’s kind of pushing new boundaries in what we do with nano technology in terms of optics.
STEPHANIE GETTY: It is a hollow tube that’s made entirely out of carbon and the diameter is a nonometer. If this was the size of an actual nanotube and you were to scale me up proportionately, then I would be tall enough to reach the moon. Because the nanotubes are so small, we can only use a scanning electron microscope to be able to see them. The method that we use is called catalyst assisted chemical vapor deposition and that grows carbon nanotubes on a substrate.
JOHN HAGOPIAN: You put the substrate in this tube, you heat the tube up to about 750C and you flow a gas and the gas has carbon in it. Because of the catalyst layer you start to assemble these tubes; carbon takes a very specific form as it grows.
STEPHANIE GETTY: So one example where carbon nanotubes can enhance the performance of a scientific instrument in space is through their ability to absorb light.
JOHN HAGOPIAN: The Z306 paint is the blackest thing that we put on instruments right now. The fact that we are blacker than that I guess makes us blacker than black in terms of performance. When light from the Earth or a star hits an instrument or structures inside of the instrument, it gets scattered over all angles. A lot of the data gets contaminated. So, it turns out up to 40 percent of the data could be unusable.
STEPHANIE GETTY: So the current telescopes use black paint to reduce the reflection but the black paint isn’t perfect; it still shows a reflection.
JOHN HAGOPIAN: Over the course of our work, we were able to optimize the carbon nanotubes to make them 10 times darker than the paint. You could get a better observational efficiency, you are not throwing away 40 percent of your data. The Goddard samples were grown multi-walled so they are not just single walled nanotubes and they are also oriented straight up and down. The reason that the oriented samples are darker is because they are low density, light can go in, it gets rattled around in there and it gets absorbed. Over a long period of time after all these experiments, we discovered that aluminum is really the trick to getting the nanotubes to stick so now you have to scratch them off, they are very robust.
STEPHANIE GETTY: So, we are interested in vibration testing for these carbon nanotubes to determine how well they adhere to the substrate and whether they will be liberated during launch.The other thing that we do test is thermal conditions. When your spacecraft is flying through space, it gets very cold and actually it gets exposed to radiation and so those are two of the other tests that we expose our technologies to before we fly them.
JOHN HAGOPIAN: So, the first instrument that we are using them on right now is actually ORCA. That’s an Earth science instruments. Another thing that we’ve looked at is using them on LISA, which is a gravity wave experiment.
STEPHANIE GETTY: One area where carbon nanotubes have made it into the market place is in sporting goods; to make stronger more robust, lighter weight, bicycle frames, tennis rackets. Those are some examples where you can go out and buy carbon nanotube composites.
JOHN HAGOPIAN: At this point, we feel like we have nanotubes that are robust, we can grow them on different materials, they are very dark, so we are very close now to getting to a point where we are going to qualify these for spaceflight use.
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