Download 7. Spectra II

7. Spectra II - PreLab
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Blackbody Radiation
Every substance emits light. Our bodies glow in infrared colors. This is why you can see human bodies through walls with
IR goggles. The hotter the object, the higher the energy the emitted photons will have. A star like the sun is about 6000K,
which peaks in greenish-blue visible light. Hotter stars peak in the Ultra-Violet, and the most energetic (hottest) objects in
the universe emit X-rays and gamma rays. Objects around the temperatures on Earth emit mostly infrared radiation. Even
though objects tend to peak in a certain color, they still emit light at other colors too. Here is a spectrum of an object with
various temperatures.
The overall shapes of these spectra are called blackbody curves. Any object will produce a spectrum that looks roughly
like the ones shown here. The real spectrum of a substance or object will contain other peaks and valleys, but follow the
same overall trend. This overall trend is called the continuum.
From the previous lab you may remember the law relating the wavelength (remember that’s just like color) of the peak of
the spectrum to the temperature of the object. The law is (Wien's law), it states...
(Brightest Color Light Emitted) = .29/(Temperature in Kelvin)
Note that the answer you will get for “Brightest Color Light Emitted” will be a wavelength, in units of cm.
(3) Question 1: Some stars can have temperatures in excess of 100,000K. Use the law above to calculate the brightest color
of light emitted (your answer will be a wavelength, in cm). What color, or type of light do these stars primarily emit? Search
for electromagnetic spectrum for some plots which may help you decide.
(3) Question 2: Is your writing utensil emitting light? If so, what type of light is it emitting?
Spectra and Line Formation, Revisited
Just connect the dots, again.
When photons pass through some material, that material will tend to absorb certain colors of light. After white light (made
of all colors) passes through this material, an observer will notice that the light is missing that color. This is called
absorption. The lack of light at a certain color is called an absorption line. Absorption happens when light passes through a
thick material, which blocks out certain colors. This is typical in stars which are surrounded by atmospheres filled with H,
He, and other material. Here is a sample spectrum with many absorption lines.
If a material is heated up, the collisions of atoms can knock elections into higher shells. When these electrons come back
down the atom releases photons. As there are only select color photons the atom can emit, and observer will see the atoms
glow at this color. This is called an emission line. Emission lines happen when a substance is heated. Hot, thin gases often
will have emission lines. Here is a spectrum with emission lines.
When atoms absorb light of a certain color, it knocks electrons into higher shells. When these electrons come back down the
atom will re-emit the same light, but in a random direction. This is called scattering.
(2) Question 1: If a star is surrounded by a thick, cool, dense cloud of Helium, will we see Helium emission lines or
absorption lines? Why?
(2) Question 2: If a star is surrounded by a hot, thin layer of Hydrogen will we see Hydrogen emission or absorption lines?
Why?
8. Spectra II – Lab
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Section:
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We are going to look at several different objects with spectrometers in today's lab. You will record what the spectrum looks
like below. Most of these objects are thin hot gases and so we will be seeing emission lines.
For each spectrum, label which object you observed. Make a note of any features you see in the spectrum, and draw a
simple picture for your memory. Here's an example of what a spectrum might look like...
Here is an example of what to record...
Orange Lamp in Middle of Room. Very bright Orange/Yellow line. Two faint red lines and a few blue lines.
Blue
Green/Blue
Orange/Yellow
Red
Leave a few inches of space for some space for additional writing.
Please observe the following objects.
- All of the tube lamps set up in the room. Some are duplicates, you only have to do each type once.
- An incandescent light
- A florescent light
- Go outside with a white sheet of paper. Observe the reflection of the sun. DO NOT LOOK DIRECTLY AT THE SUN.
- The sky, away from the sun.
- A cloud, if possible.
Record your observations and attach them to the lab.
(6) Use the known spectra given to you to identify the substance in each one of the gas tubes. Record your guess and
why in the space below. If you are unsure, explain why as well. You are not being graded on the correct answer, but
rather your rationale.
(2) Question 1: Take a look at your florescent light bulb spectrum. Speculate about what substance is contained within the
florescent bulb.
(2) Question 2: Our atmosphere is rather cold. What type of light do you think clouds emit? When you pointed your
spectrometer at the sky you saw visible light. Why?
Solar Spectrum