Types of Spectra

We will shortly be doing a demonstration, involving a group of students. One student will be a hot, dense object, one will be a cool, thin cloud, and the rest will be observers from various angles. The cool cloud will stand in the center, and the others will surround the "cloud" in a circle.

  1. Let's relate this "model" to atoms and light. Which color candies represent photons with the highest energy?

    The violet candies represent violet photons, which have the highest energy of all visible light photons.

  2. Which color candies represent light with the longest wavelength?

    The red candies represent red light photons, which have the longest wavelength of all visible light.

  3. PREDICT: What color candies will the observer who sees the hot, dense object through the cloud have?

    This observer will have all of the red, yellow, green, and purple (violet) candies (photons) which were tossed (emitted) by the hot, dense object. In addition, this observer will have a few blue and orange candies (photons) which were intercepted (absorbed) by the cool cloud, and then tossed (re-emitted) to them.

  4. PREDICT: What color candies will the other observers have?

    The cool cloud intercepts (absorbs) blue and orange candies, then tosses (re-emits) them in random directions. Since these observers are looking only at the cool cloud (they are receiving photons/candies only from the cool cloud), they will have only orange and blue candies.

Now pause while we carry out the demonstration.
  1. Were your predictions correct for both categories of observers?

    In my case, both predictions were correct. In your case, one or the other or both may have been incorrect (you should specify such which was correct/incorrect in a question like this).

  2. If the candies were really photons, what kind of spectrum would be seen by the TWO categories of observers (emission, absorption, or continuous)?

    The observers looking only at the cool cloud see only orange and blue photons, and so see an EMISSION line spectrum.

    The observer with the bucket (who is looking at the hot, dense object through the cool cloud) sees an ABSORPTION line spectrum, since they see far fewer orange and blue photons than other photons.

  3. What kind of spectrum is being emitted by the hot, dense object (In other words, what kind of spectrum would you see if you looked at the hot, dense object with no intervening cool cloud?)

    The hot, dense object is emitting all colours of photons, and so is emitting a CONTINUOUS spectrum.