Quantitative Spectroscope and Visible Light lab

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 identify two spectroscopes and how they
are used to create and view emission spectra. You
will build a diffraction grating spectroscope and
use it to view and draw the spectra of a variety of
light sources. You will also calculate frequency from
wavelength.

EXPERIMENT

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

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

● Describe the relationships between frequency and wavelength, and frequency and energy.

● Relate spectral lines to the excitation and emission of energy.

● Define spectroscope and compare diffraction grating spectroscopes and prism spectroscopes.

● Compare and contrast continuous and line spectra.

● Build a diffraction grating spectroscope.

● Use a spectroscope to view and draw the spectra of various light sources to determine if they
have a continuous or line spectra.

● Calculate frequency from wavelength.

Time Allocation: 2 hours

Experiment Quantitative Spectroscope and Visible Light

Materials
Student Supplied Materials

Quantity Item Description
1 Box cutter
1 Computer printer
1 Fluorescent light
1 Incandescent light
1 Pair of scissors
1 Pencil
1 Street light
1 Tape: clear or masking
1 Tape: duct or electrical

HOL Supplied Materials

Quantity Item Description
1 Cardboard box, 8”L x 4”W x 3”D
1 Diffraction grating card
1 Metric ruler
1 Spectroscope Grid Template (Included in 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.

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

The electromagnetic spectrum is arranged by wavelength and frequency, with radio waves
containing the longest wavelengths and lowest 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.

Experiment Quantitative Spectroscope and Visible Light

Frequency and Wavelength

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:

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

Frequency and Energy

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.

Experiment Quantitative Spectroscope and Visible Light

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

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

Experiment Quantitative Spectroscope and Visible Light

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.

Spectroscopes

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.

Figure 5. Diagram of diffraction grating spectroscope.

Experiment Quantitative Spectroscope and Visible Light

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 this 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.

Experiment Quantitative Spectroscope and Visible Light

Exercise 1: Building and Calibrating a Spectroscope
In this exercise, you will build and calibrate a diffraction grating spectroscope.

Note: Please read the experiment section completely prior to designing and building your
spectroscope. Then complete the questions as you work on the exercise.

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.

Procedure

Part 1: Building a Spectroscope

1. Answer questions A and B in the Questions section at the end of this exercise.

2. If necessary, assemble the small cardboard box included in the lab kit. 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.

Experiment Quantitative Spectroscope and Visible Light

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.

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.

Experiment Quantitative Spectroscope and Visible Light

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.
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.

Experiment Quantitative Spectroscope and Visible Light

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.

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 making it easier 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.

Experiment Quantitative Spectroscope and Visible Light

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.

Figure 14. Using light-blocking tape to narrow the 0.5 cm to 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.

Part 2: 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.

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

15. 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.

16. 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.

Experiment Quantitative Spectroscope and Visible Light

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

17. 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.

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Experiment Quantitative Spectroscope and Visible Light

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

Procedure

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 of your Lab Report
Assistant.

4. Draw the spectra, as viewed in the spectroscope, along the scale provided in Data Table 1 (see
Figure 15 for an example).

Note: The best option for drawing the spectra on the template is to insert lines directly into the Word®
document and adjust the formatting accordingly. You may also use other photo editing software such
as PowerPoint®, Paint®, or Adobe Photoshop®. If you choose this method, you will need to save the
image and upload it into the Lab Report Assistant.

5. Repeat steps 1 through 4 for the incandescent and street lights.

6. Repeat steps 1 through 4 for the car headlight.

Safety Warning! Carry out the exercise with the car headlight only when the car engine is turned off,
NOT when the engine is running.

7. Repeat steps 1 through 5 for an additional light source of your choice. Indicate the light source
in Data Table 1.

Safety Warning! It is VERY important that the additional light source is NOT a laser beam, the Sun,
or a halogen lamp.

Cleanup:

8. Return the spectroscope and all other materials to the lab kit for future use.

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.

Experiment Quantitative Spectroscope and Visible Light

Experiment Quantitative Spectroscope and Visible Light

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

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)

Name:__________________

College Chemistry I – Chem. 1806

Lab Report 2assignment

1. Indicate the colored lines you observe in fluorescent light, street lights, car headlight, and an additional light source of your choice by creating an illustration similar to that of figure 15 in your lab. (10 points)

Data Table 1
Fluorescent Light ImageCar Headlight ImageStreet Light ImageAdditional Light Source

2. Calculate the frequency, wavelength, and energy for each wavelength you observed in the fluorescent and street light. (10 points)

3. Try narrowing and widening the light inlet slit. How does this affect the spectra? Compare the thickness, resolution, and shape of the lines. (10 points)

4. Imagine you are writing a lab report. In your own words, provide an introduction for a lab report. Include the purpose of the lab and use the background section of the experiment to help you. Feel free to find outside resources for additional information. (10 points)

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