Your eyes are amazing visible light detectors. The dark-adapted eye is so sensitive it can detect a single photon! There are instruments that can also detect a single photon, used in telescopes to detect and measure the visible spectra from stars and other luminous bodies in space. These detectors convert light energy into digital information that can be transmitted from orbiting spacecraft back to earth. The incredible photographs obtained by the Hubble Space Telescope are examples of digital images transmitted to earth and processed by computers.

Things To Do:
Learn how to make your very own spectroscope, similar to the instrument used on the Hubble Telescope to measure the light coming from distant stars!


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  • When you have finished building your spectroscope, you may look at the continuous spectra of light bulbs and fluorescent lights. Is there any difference in their spectra?
  • You may also use your spectroscope to look at neon signs, sodium vapor lights, and mercury vapor lights. Is there any difference between these spectra and those from light bulbs?
    • If your school has gas vapor lamps containing hydrogen, helium, neon, or argon, you may use your spectroscope to observe the emission line spectra of these gases. Why are the spectra of these gases not continuous like the spectra of a light bulb or the sun?

    • You may be able to use your spectroscope to observe the dark lines in the solar spectrum. BE CAREFUL! Do not look directly at the sun! The dark lines in the solar spectrum result from the absorption of some of the sun's visible light by the gases of the earth's atmosphere.

    • A down-to-earth example of a Charge Coupled Device (CCD) that is used to gather light in digital form is a digital camera. This type of camera oprerates in the same way as an ordinary camera except that is has no film. Light is gathered on screen of a CCD and transferred to a digital memory in the camera. When the digital memory is downloaded into a computer, the image can be seen and printed. If you have access to one these cameras, you can see how images are collected by modern telescopes.

    If possible, enlarge an image from a digital camera on a computer screen. As the image enlarges, you will begin to see little squares called pixels. Pixels are the units of digital images and they are usually too small to be noticed, but the images of distant objects recorded by the Hubble Space Telescope often show the pixels. Neptune
    This close-up view of a portion of Neptune clearly shows the pixels that make up the digital image.


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