Phys 1020 Lab 3: Exploring
Light, Color, and Atoms
NOTE: PRE-LAB ASSINGMENT is at the end of the lab description.
Lab Logistics:
You and your group will work together
to complete the lab and write up the group lab report. Remember, everyone
will need to assume a new job. For instance, if you were the manager for
Lab 1, then you should either be the recorder or the skeptic for this
lab. Again, everyone should be helping with the hands-on stuff.
o The manager:
This person is responsible for making sure that the group follows the lab
procedure and completes everything that is asked for in the lab.
o The recorder: This
person is responsible for keeping the lab notebook for the day, recording the
observations observed by the group and the group’s answers to the questions
asked in the lab.
o The skeptic:
This person is there to question the results of the lab. Is everything
making sense? Are we taking the data correctly? Are the results and
conclusions reasonable? Did we skip a step?
Begin each lab report by titling the lab, listing your lab partners who are present, and listing the jobs that each lab partner has assumed for the lab. Remember, your lab report should give an explanation of all of your observations and measurements. Also, you need to think of and try one additional experiment for either Part 1, 2, or 3 of this lab that will further test your explanation of the results that you found.
Lab Description:
In this lab, you will use a spectrometer to investigate the colors of light emitted from three different sources: a hydrogen discharge lamp, a mercury discharge lamp, and an incandescent light bulb. The spectrometer that you will use to study these light sources is a grating spectrometer. The grating spectrometer works by using a diffraction grating to separate out the colors of an incoming light source according to their wavelength. A diffraction grating is made of many very closely spaced grooves or slits. The interference between the light that scatters off the different grooves makes each color come out at a particular angle depending on the wavelength of that color. For instance, blue light will diffract off the grating less than red light, so you would expect the blue light to come out of the grating at a smaller angle than the red light. This makes it possible to measure the precise colors of light emitted from a particular light source. Figure 1 below shows the grating spectrometer that you will use in this lab.
Figure 1: The Grating Spectrometer.
As seen from Figure 1, the light source that you will study is placed in front of the collimating tube. Light that enters the collimator will then go through the diffraction grating, and can be observed on the other side using the telescope. The telescope is connected to a turntable so that it can be rotated around the grating. This allows you to observe different colors of light that are diffracted at different angles.
In addition to using the spectrometer to study the light, you will be using color filters to explore how light is transmitted through certain types of materials and to study some of the properties of the various light sources that you will investigate.
Part 1: Hydrogen Spectrum:
1) Turn on the hydrogen gas lamp source. What color is the hydrogen lamp? The gas tube that is emitting the light is placed between two electrodes (pieces of metal) that are held at different voltages. Explain how the voltage difference between the electrodes cause the lamp to light up… draw a picture showing the electrodes, the gas atoms, and how the light is generated. (You should think back to how a fluorescent light works and remember there is no wire connecting the two electrodes to each other) Is there any current flowing through the gas between the two electrodes?
2) Set up the spectrometer to look at the hydrogen lamp. Position the lamp close to the collimator. The best spectrum will be obtained if you line up the center of the gas tube with the collimator (you may have to place the lamp on a block to get it high enough to align with the collimator). Once the lamp is placed in front of the collimator, align the lamp with the collimating tube by looking down the length of the collimating tube through the diffraction grating. Adjust the horizontal position of the lamp until the light coming out of the collimating tube is brightest. Once the lamp is in place, cover the diffraction grating and the ends of the collimating and viewing tubes with the black felt cloth, making sure that none of the cloth falls between the tubes and the diffraction grating. The black cloth will prevent other light sources from going through the grating and will also help you to see the spectrum better.
3) Observe the spectrum. Look through the viewing tube and describe what you see. You should see at least three distinct color lines in the spectrum (there is a fourth, but it is hard to see). Draw the spectrum in your lab notebook and estimate the wavelengths of each of the types of photons that you see (pg. 386 in the text should be helpful). Compare the color of these types of photons to the color you see looking directly at the lamp.
4) The color spectrum that you observe is coming from the hydrogen atoms in the glass tube. When a hydrogen atom inside the tube collides with an energetic electron, the atom absorbs some of the electron’s energy and the atom becomes excited. After being promoted to this excited state, the atom will then will then decay into a lower energy state and emit a photon of light. The color or wavelength of that photon is determined by the energy difference between the excited state and the lower energy state.
· What part of the atom is gaining and loosing energy when an electron in the gas collides with the atom and then the atom emits a photon of light?
· Why do you only see a few color lines in the spectrum? Which color photons have the most amount of energy? Which have the least amount of energy?
· What does the spectrum of the hydrogen gas lamp tell you about the energy levels of electron in the hydrogen atom?
Using the spectrum that you sketched in part 3), make a sketch of what you think the energy levels of the hydrogen atom might look like and how this is consistent you’re your spectrum (Hint: when a photon of any of the colors that you see has been emitted, the hydrogen atom will decay to the same lowest energy level). Check your energy diagram with your TA.
5) Explore how the filters affect the spectrum. Place each of the filters in front of the
collimator, and describe how it affects the spectrum that you see. Why can you see some of the color lines with
some of the filters, but not with the others?
Do the filters change the wavelength of the photons passing through
them? Explain.
Part 2: Mercury Spectrum:
6) Setup the spectrometer for the mercury lamp using the steps outlined in part 2). What is the color of the mercury lamp?
7) Observe the spectrum. Look through the viewing tube and describe what you see. You should see four distinct color lines in the spectrum. Draw the spectrum in your lab notebook and estimate the wavelengths of each of the types of photons you see. How does the color of these photons compare to the color that you see looking at the lamp?
8) From the spectrum, make a sketch of what you think the electronic energy levels of mercury might be. How do your observations for mercury compare to what you saw for hydrogen? What are the similarities? The differences? How do you account for this?
9) Explore how the filters affect the spectrum. Which filters block some of the colors in the spectrum? Which ones have no effect? Can you explain this?
10) The light from the mercury lamp is coming from a gas of mercury atoms inside the glass tube. If the pressure of this gas is increased, then the atoms will begin to collide more and more. These collisions will cause shifts in the electronic energy levels in the mercury atoms. If the pressure of the mercury is drastically increased, what effect do you think this will have on the color spectrum? Why?
Part 3: Spectrum of White Light:
11) Setup the spectrometer to look at the spectrum of the incandescent light bulb. What is the color of the light bulb when you look at it directly?
12) Look through the viewer to observe the spectrum. Make a sketch of the spectrum in you lab book. How does this spectrum compare to the spectrums for both hydrogen and mercury? Explain why the spectrum for white light appears the way it does and why the hydrogen and mercury spectrums look so different.
13) Experiment with the filters by placing them in front of the collimator. How does this change the spectrum? For one of the filters, make a sketch of the spectrum in your lab notebook. For the filter that you have chosen, explain what it is doing to the incoming light.
14) How do the colors of the hydrogen lamp, the mercury lamp, and the light bulb compare? From what you have seen from the spectrums, what can you say about types of photons (color/wavelength) that make up white light? What about for the other colors we see?
Additional Experiment:
15) As usual you are to think up an additional experiment from this lab and record your procedure and results.
Final Thoughts:
16) From what you’ve learned in this lab, how would you go about using a spectrometer to determine which atoms are in a gas containing a mixture of unknown atoms?
17) Why do you look sickly green when standing under a mercury street lamp?
18) In you own words, describe how atoms and light interact with each other.
19) Do you think that your understanding of light, color, and their relationship with atoms has improved? Give an example of something you learned or were able to explore more deeply.
PRE-LAB ASSIGNMENT: To be completed and turned in before lab. You should come to lab having read through the description of the lab and thinking about how the lab applies to what we are learning in class and fits into our everyday lives.
1) In this lab, we will study visible light, a form of electromagnetic radiation. Sketch the shape of the spectrum (intensity (# of photons) vs the wavelength) for an incandescent light bulb, making sure to indicate the wavelength for the important colors (red, orange, yellow, green, blue, indigo, violet). You should also indicate the type of electromagnetic radiation that borders the red light and violet light. (See 1010 Lecture notes and the 1010 homepage blackbody simulation if you are at all sketchy on the spectrum for an incandescent bulb).
2) The color of a photon is determined by its energy. Which photon has more energy, a blue photon or a red photon? Does photon energy increase or decrease as the wavelength of the light increases?