Why should the wavelength scale of the spectroscope be calibrated

Quantitative Spectroscope and Visible Light Hands-On Labs Version 42-0305-00-01

Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

Experiment Summary:

In this experiment, you will learn about light and how each light source creates its own unique spectra. You will learn about spectroscopes and how they are used to create and view emission spectra. You will build a diffraction grating spectroscope and will use it to view and draw the spectra of numerous light sources.

EXPERIMENT

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Learning Objectives Upon completion of this laboratory, you will be able to:

● Define the electromagnetic spectrum and explain its relationship to visible light.

● Describe how matter interacts with electromagnetic radiation.

● Draw emission lines for spectra.

● Compare and contrast diffraction grating spectroscopes and prism spectroscopes.

● Build a diffraction grating spectroscope.

● Use a spectroscope to view spectra of various light sources.

● Compare and contrast continuous and line spectra.

● Describe the relationship between wavelength and frequency.

● Calculate frequency from wavelength and wavelength from frequency.

Time Allocation: 2 hours

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Experiment Quantitative Spectroscope and Visible Light

Materials Student Supplied Materials

Quantity Item Description 1 Access to printer 1 Access to street light 1 Fluorescent light 1 Incandescent light 1 Pair of scissors 1 Pencil 1 Ruler 1 Sharp knife or box cutter 1 Tape, clear (such as Scotch® tape) 1 Tape light-blocking (such as duct tape or electrician’s tape)

HOL Supplied Materials

Quantity Item Description 1 Diffraction grating 1 Small cardboard box (8”L x 4”W x 3”D) 1 Spectroscope Grid Template (Included with Manual)

Note: To fully and accurately complete all lab exercises, you will need access to:

1. A computer to upload digital camera images.

2. Basic photo editing software, such as Microsoft Word® or PowerPoint®, to add labels, leader lines, or text to digital photos.

3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.

Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your LabPaq kit.

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Experiment Quantitative Spectroscope and Visible Light

Background The light coming from the window, hall light, or desk lamp appears as white light, however; what is perceived as white light is actually composed of a combination of seven colors: red, orange, yellow, green, blue, indigo, and violet. These seven colors compose the spectrum of visible light. Visible light is the portion of the electromagnetic spectrum that is visible to the human eye. The electromagnetic spectrum is the entire range of all possible frequencies of electromagnetic radiation, ranging from radio waves to gamma rays. See Figure 1.

Figure 1. Electromagnetic spectrum. Visible light is located approximately at the center of the spectrum. Radio waves contain the longest wavelength and lowest frequency; gamma rays contain the shortest

wavelengths and highest frequency. © Milagli

The electromagnetic spectrum is arranged by wavelength and frequency, with radio waves containing the longest wavelengths and low frequency and gamma rays containing the shortest wavelengths and highest frequency. Visible light spans the electromagnetic spectrum from wavelengths of approximately 390 nm to 750 nm, and is further defined by the seven individual colors (purple, indigo, blue, green, yellow, orange, and red) in the visible light region of the electromagnetic spectrum. This is a continuous spectrum, and colors blend into each other with no empty or dark spaces between them (the spectrum consists of light of all wavelengths). See Figure 2.

Figure 2. Visible light region of the electromagnetic spectrum.

There are two important relationships involving the properties of electromagnetic radiation: the relationship between frequency and wavelength and the relationship between frequency and energy. Frequency refers to the number of wave peaks that pass a stationary point per unit time and is measured in units of s-1 (sometimes called reciprocal seconds). Since all light travels at the same speed, wavelength and frequency are related by the equation:

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Experiment Quantitative Spectroscope and Visible Light

Wavelength is often given in nm and will need to be converted to m.

The energy of a single photon, or smallest unit, of light in any portion of the electromagnetic spectrum can be calculated by the equation:

Visible light is a form of energy, released from an object (matter) upon exposure to heat or radiation. Emission (release of energy in the form of light) of energy from matter occurs when its electrons are excited and move to a higher energy level, and then subsequently return to a lower energy level. This difference in energy, when moving from the higher “excited” energy level to the lower energy level, is released in the form of visible light. See Figure 3. Every object that releases energy creates emission lines that are unique to that object. Emission lines, also referred to as spectral lines, are a series of bright lines at a specific wavelength in the visible region of the electromagnetic spectrum that are specific to a type of emitted energy. For example, light from the Sun, light from a fluorescent light bulb, and light from a neon light bulb all have their own unique set of spectral lines. See Figure 4.

Figure 3. Formation of emission lines and excitation and emission of energy.

λ ν

λ ν

×

8 -1

-1

c = where:

c = speed of light (3.00 x 10 ms ) = wavelength

= frequency (s )

-34

-1

E = h where:

h = Planck's constant (6.626 x 10 Js) = frequency (s )

ν

ν

×

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Experiment Quantitative Spectroscope and Visible Light

Figure 4. Unique spectral lines. Top to Bottom: Sulfur, Neon, and Iron. © Teravolt

Spectral lines are unique to their source; they can be used to identify an unknown source of light. However, as spectral lines are not visible to the naked eye, a spectroscope must be used. A spectroscope is an optical device which visualizes and spreads out the spectral lines from a source of light, allowing the spectrum to be seen with the human eye. There are two types of spectroscopes. The first type is a diffraction grating spectroscope which is based on the principle of diffraction, where light enters the device and is then diffracted (bent) by a grating material. See Figure 5. The other type of spectroscope is a prism spectroscope which is based on the principle of dispersion, where light enters through a narrow slit in the device and is dispersed through a series of prisms. Diffraction grating bends the light that enters the spectroscope and separates the light by wavelength, as different wavelengths (colors) of light bend at different degrees. See Figure 6.

We often see a continuous spectrum

from sunlight in the form of rainbows. Rainbows are caused by the reflection

and refraction of sunlight in drops of rain.

© Tony Pasma

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Experiment Quantitative Spectroscope and Visible Light

Figure 5. Diagram of diffraction grating spectroscope.

Figure 6. Diffraction grating bends light, by wavelength, to display a spectrum.

The spectrum produced by a spectrometer is either a continuous or line spectrum. A continuous spectrum looks similar to a rainbow, where all the diffracted colors appear to blend together. A line spectrum separates colors into distinct lines, as shown in Figure 4. In today’s experiment, you will build a diffraction grating spectroscope and will align (or calibrate) it to the emission spectrum of mercury, which is in fluorescent lighting. On a perfectly calibrated, professional quality spectroscope, the wavelengths for mercury’s atomic emission lines are: Violet at 436 nm, Green at 538 nm, and Yellow at 580 nm.

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Experiment Quantitative Spectroscope and Visible Light

Exercise 1: Building and calibrating a spectroscope In this experiment, you will build and calibrate a spectroscope.

Note: Please read the experiment section completely prior to designing and building your spectroscope.

Tape: Use heavy, light-blocking tape to seal out stray light. Transparent tape and off-white masking tapes have poor light-blocking abilities. Duct tape or electrician’s tape is suggested. Cracks, seams, and accidental holes can all be covered by this heavy tape that blocks outside light.

Diffraction Grating: IMPORTANT: Avoid making fingerprints on the diffraction grating. Avoid bending, cutting, tearing, or otherwise damaging the grating.

Examine the diffraction grating. Hold the grating 6 to 12 inches away from your eye while viewing the surface at an angle. Look through the diffraction grating at a light source.

CAUTION: Do not look directly at the Sun or other extremely bright light source when using the diffraction grating.

Building a Spectroscope

1. Answer questions A and B in the Questions section.

2. If necessary, assemble the small cardboard box included in the LabPaq. The box provided has the dimensions of 8”L x 4”W x 3”D, and may be pre-assembled.

3. Cut a 0.5 cm-wide slit in the box near one of its corners. Make the cut only along the long, narrow side of the box, approximately 3 cm from the edge of the box corner. This slit will serve as a light inlet. See Figure 7.

Figure 7. Spectroscope box with light inlet slit.

4. On the opposite side of the box, cut a 2.5 cm x 2.5 cm wide hole. This hole will serve as a place to mount the diffraction grating. Make the cut approximately 1.5 cm from the edge of the box corner so that the light inlet hole and diffraction grating hole are aligned. See Figures 8 and 9.

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Experiment Quantitative Spectroscope and Visible Light

Figure 8. Spectroscope box with diffraction grating hole cut through the box wall and box closure flap. The light inlet slit is located on the opposite side of the box directly across from the grating.

Figure 9. Top view of spectroscope box showing placement of light inlet hole and diffraction grating hole. Note: The diffraction grating hole is centered on the light inlet slit.

5. Place the diffraction grating over the diffraction grating hole from the inside of the box. Secure the diffraction grating to the inside of the box using small pieces of clear tape.

Note: The tape should only cover the paper edges of the diffraction grating. Do not cover the transparent part of the diffraction grating with clear tape.

6. Close the box. Hold the spectroscope up to one of your eyes as you point the inlet slit on the other side at a light source. Look into your spectroscope through the diffraction grating. You should see a spectral pattern to the left and to the right of the slit inside the spectroscope.

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Experiment Quantitative Spectroscope and Visible Light

If the pattern appears to be at the top and bottom, remove the grating, rotate it by 90o, and re-secure it. Make certain that the spectral patterns now display to the left and to the right of the inlet slit.

SAFETY WARNING: Do not use the Sun as the light source.

7. Cut a 1 cm wide horizontal slit adjacent to the light inlet slit. This slit will serve as a place to mount the spectroscope grid template. It will also provide background lighting so that you can easily see the spectroscope grid. See Figure 10.

Figure 10. Horizontal slit adjacent to the light inlet slit.

8. Print the spectroscope grid template provided with your manual. Use scissors to cut out the grid along the black box outlining the grid. See Figure 11.

Note: Use the grid template provided with your kit, it has been specifically sized for use in the spectroscope.

Figure 11. Spectroscope grid template.

9. In this step, you will determine where to place the spectroscope grid template within the spectroscope chamber. While looking through the grating to see where the spectrum falls across the horizontal slit, use a pencil to mark the left and right boundaries of the spectrum on the outside of the box.

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Experiment Quantitative Spectroscope and Visible Light

10. Apply the light-blocking tape onto the horizontal slit on both sides outside of the marked area where the spectrum appears. See Figure 12. Blocking the unused part of the horizontal slit will keep too much light from entering the spectroscope, thus making it difficult later to see and measure light spectra.

Figure 12. Light-blocking tape applied to both sides outside of the marked area where the spectrum appears.

11. Use a small piece of clear tape to apply the spectroscope grid to the opening where the spectrum appears. Make certain that the spectrometer grid can be easily removed so that you may perform the calibration procedure later. See Figure 13.

Figure 13. The spectroscope grid applied to the opening where the spectrum appears.

12. Use the light-blocking tape to narrow the 0.5 cm light inlet slit. See Figure 14. Ideally, the slit should be less than 1 mm wide. This will make the spectral lines appear narrower and better defined. Narrower spectral lines are also easier to measure. Test the spectroscope using fluorescent light as a light source. You should see discrete bands of color, with darkness between distinct colors. If you have spectral lines that overlap, try to further narrow the slit. If necessary, use 2 pieces of light-blocking tape to create a narrower light inlet slit.

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Experiment Quantitative Spectroscope and Visible Light

Figure 14. Use light-blocking tape to narrow the 0.5 cm light inlet slit. Ideally the slit should be less than 1 mm wide.

13. Apply light-blocking tape over any cracks or openings that may allow outside light into the spectroscope. Without the interference of outside light, it is easier to see the spectrum.

Calibrate the Spectroscope Grid

In this procedure, you will observe spectral patterns using a fluorescent light source. You will then position the spectroscope grid so that it may be ready for calibration later.

1. Hold the spectroscope up to one of your eyes as you point the inlet slit on the other side at a fluorescent light source.

2. Look through the diffraction grating to view the spectra. You will see spectral lines spread across the spectroscope grid. See Figure 15. Spectral line placements will vary.

Note: If calibrating a professional grade spectroscope, the violet line would position at 436 nm. For our exercise, we will calibrate violet to 450 nm.

3. Notice the position of the violet line relative to the 450 nm mark. If the violet line does not lie over the 450 nm mark, reposition the spectrometer grid on the outside of the box. After repositioning, the violet line should lie across the 450 nm mark. See Figure 15.

Figure 15. Violet line placed correctly over the 450 nm mark. Notice the displacement of the other lines.

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Experiment Quantitative Spectroscope and Visible Light

4. Tape the spectroscope grid in place so that it cannot move.

Important Note: The spectroscope will be used in the second exercise of this experiment, so please do not discard it until you have finished the experiment.

Questions Please complete the questions as you work on the exercise.

A. Hold the grating several inches from your face, at an angle. Look at the grating that you will be using. Record what details you see at the grating surface.

B. Hold the diffraction grating up to your eye and look through it. Record what you see. Be specific.

C. Before mounting the diffraction grating, look through the opening that you made for your grating. Record what you see across the back of your spectroscope.

D. After mounting the diffraction grating, look through your spectroscope and record what you see across the back of your spectroscope. Be specific.

E. Starting at the light inlet slit and going outward, what colors do you see in the spectrum? List them all.

F. When you view the spectrum, you should be able to see a spectral image to the right and left of the light inlet slit. How are the spectral images the same? How are they different? Record your findings.

G. Try narrowing and widening the light inlet slit. How does this affect the spectra that appear? Compare the shape, thickness, and resolution of the spectral lines before and after narrowing the slit. Record your findings.

H. Write up to five sentences describing how a spectroscope works. Make certain to mention things like the light inlet slit, diffraction grating, light, spectrum, etc.

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Experiment Quantitative Spectroscope and Visible Light

Exercise 2: Using the Spectroscope In this experiment, you will use the spectroscope to observe the spectra of various light sources.

1. Check the calibration of the spectroscope. If necessary, re-calibrate the spectroscope as described in Exercise 1.

2. With the spectroscope, view the spectra of fluorescent light.

3. Determine if the spectra is continuous or line, and record in Data Table 1.

4. Draw the spectra, as viewed in the spectroscope, along the scale in Data Table 1 of your Lab Report Assistant. (An example is Figure 15).

Note: Use colored lines in Microsoft® Word or PowerPoint®, or draw the spectrum on paper.

5. Insert the spectra into Data Table 1. Either scan or photograph the image and insert the images into Data Table 1, or simply copy and paste from Microscoft®Word or PowerPoint® into Data Table 1.

6. Repeat steps 1 through 5 for the incandescent and street lights.

7. Repeat steps 1 through 5 for the car headlight. It is VERY important that when viewing the car headlight the car is turned off, and only the headlights are turned on.

SAFETY WARNING: Carry out the exercise with the car headlight only when the car engine is turned off, NOT when the engine is running.

8. Repeat steps 1 through 5 for an additional light source of your choice. It is VERY important that the additional light source is not a laser beam, the Sun, or a halogen lamp.

Questions A. Describe the similarities and differences between the spectra of incandescent light and

fluorescent light. Use your results in Data Table 1 to explain your answer.

B. The wavelength (λ) and frequency (ν) of light are related through the equation:

λ ν

λ ν

×

8 -1

-1

c = where:

c = speed of light (3.00 x 10 ms ) = wavelength

= frequency (s )

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Experiment Quantitative Spectroscope and Visible Light

Using the following emission spectra:

Calculate the frequency for the each of 8 emission lines:

a. Violet (450 nm)

b. Indigo (470 nm)

c. Blue (490 nm)

d. Green (520 nm)

e. Yellow (620 nm)

f. Orange (630 nm)

g. Red (690 nm)