BACKGROUND INFORMATION:
Visible light is a form of energy composed of the colors of the "rainbow": red, orange, yellow, green, blue and violet. This energy is called the electromagnetic spectrum, and it comes to us from the sun. It is a continuous spectrum, that is to say, a full rainbow of colors. When an element becomes excited (gains extra energy) it must lose energy, which it emits as light energy. Each element exhibits its own distinct spectra called Emission Spectra. Diffraction gratings allow us to see an element’s spectra.
In this lab, we are going to determine these wavelengths by the use of lab materials and trigonometric functions, then later compare our results to accepted values.
CONCEPTS:
Individual elements, when excited, emit distinct wavelengths of light.
GOALS:
Students will demonstrate the use of an Emission tube power supply,
a diffraction grating and a scientific calculator to determine the different
wavelengths of light.
EQUIPMENT: (per group of 3-6 students)
PROCEDURE:
Students will gather needed materials and set up lab table as indicated by Illustration I or teacher set-up.
Students will elect one person to view through the grating (student 1), one person to measure distance (student 2) and one person to record the data (student 3).
Student 1 will stand 1 meter from the spectroscope at eye level. He will place the diffraction grating in front of one eye and shut the other to view the light that appears to be focusing on various bands of color. He will direct Student 2 to move the flat ruler along the meter stick lying parallel to the emission tube power supply until the outermost color comes into view. Student 3 will list the color and the distance in the provided data table. Continue moving the flat ruler and collecting measurements until data is recorded for all colors in tube. Ask the teacher to change the emission tube and repeat the experiment until data has been collected for three emission tubes.
Once all data has been collected, the students are ready to calculate the angle of each color band. Calculations are to be done by taking the inverse tangent of the ratio of the recorded distance divided by 100cm/meter.
q = Tan-1 (d/100) where q equals the unknown angle,
d = (distance from spectroscope to diffraction grating)
Record the angle in the datasheet. Students will find the wavelength by taking the sine of the angle multiplied by 2.54cm/inch divided by 13,400 lines per inch (of the diffraction grating).
l
= d sin q q = found
angle
l
(Lambda) = wavelength
d = diffraction grating
per cm. (2.54cm/13,400 gratings)
Multiply the found value by 1,000,000 to get the wavelength in nanometers. Record this value in the data sheet. (This can be done by the movement of decimal places.) Wavelength in nanometers is equal to l times 1,000,000. To calculate the experimental error find the absolute value of the difference between your experimental wavelength and the accepted wavelength, divide by the accepted value and multiply by 100%.
The instructor should be prepared to provide the accepted wavelengths
after the students have calculated their experimental wavelength.
Caution the students that the emission tubes are very expensive, get
very hot and should not be handled by anyone but the teacher.
Lab has been modified for elementary and can be used for Physics. It
has been used for a Chemistry Lab in conjunction with the Flame test exercise.
The following questions can be either completed in groups or used by
the teacher as an oral summary:
1.When you are burning something, are the colors given off in the flame dependent on the material being burned?
2. Could this concept have applications in determining the presence of specific elements in a mixture? If so, how and where?
3. What is one demonstration of this concept that the majority of the class has witnessed?
ADAPTATIONS FOR SPECIAL NEEDS STUDENTS
A student with mild mental retardation could hold the diffraction grating and direct the measurer. They could be the measurer and tell the measurements as long as someone was double-checking their numbers. They could also help with lab set up and tear down.
A student who speaks English as a second language could be given a diagram and help with lab set up and tear down. They could be the measurer and call out the measurements to the recorder. (Note: We permit electronic translators. They seem to be very helpful.)
A student with ADHD could be assigned lab set up and equipment alignment during the trials.
RUBRIC:
Two instruments, Lab Participation and the Lab Report will grade this lesson. Lab Participation will be worth 5 points and the Lab Report will be worth 15 points. The grade will be the sum of the two.
Lab Participation: To earn 5 points the individual must be on task at all times and obey proper lab safety procedures. Points will be deducted for the following: 2 for lab safety violations, 1 for not being on task, 1 for trying to interfere with others on task and 1 for non-participation.
Lab Report: 15 points are earned by the following:
Introduction: 3 points. Two or three sentences in a general statement including applications mentioned in class.
Purpose statement: 3 points. The correct concept must be stated in clear concise language that is grammatically correct.
Summarized Procedure: 3 points for a complete summary. 2 points for missing one critical concept. 1 point for missing a concept or concepts and grammatical and/or spelling errors.
Calculations and Observations: 3 points for showing all work and listing all observations. 2 points for showing partial work, final answer and some observations. 1 point for final answer with no work and some observations. 0 points for final answer with no work and no observations.
Conclusion: 3 points for verifying if the data supports the tested theory,
how to improve accuracy and precision in the experiment and explaining
where errors were made in their calculations or procedure. 2 points for
leaving out one of the above. 1 point for leaving out two of above. Zero
points for leaving out all of the above.
MEASURING THE WAVELENGTHS OF VISIBLE LIGHT
BACKGROUND INFORMATION:
Visible light is a form of energy composed of the colors of the "rainbow": red, orange, yellow, green, blue and violet. This energy is called the electromagnetic spectrum, and it comes to us from the sun. It is a continuous spectrum, that is to say, a full rainbow of colors. When an element becomes excited (gains extra energy) it must lose energy, which it emits as light energy. Each element exhibits its own distinct spectra called Emission Spectra. Diffraction gratings allow us to see an element’s spectra.
PURPOSE:
The purpose of this lab is to determine the wavelengths of different
elements by the use of lab materials and trigonometric functions, then
later compare our results to accepted values. You will use an Emission
Tube Power Supply, a diffraction grating and a scientific calculator to
determine the different wavelengths of light.
EQUIPMENT: (per group of 3-6 students)
PROCEDURE:
q = Tan –1(d/100) where q
equals the unknown angle,
d = (distance from spectroscope to diffraction grating)
l = d sin qq = found angle
l (Lambda) = wavelength
d = diffraction grating per cm. (2.54cm/13,400 gratings)
To calculate the experimental error find the absolute value of the difference between your experimental wavelength and the accepted wavelength, divide by the accepted value and multiply by 100%.
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Data Table 2 (note: you may not use all the wavelength columns).
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Questions:
1.When you are burning something, are the colors given off in the flame dependent on the material being burned?
2. Could this concept have applications in determining the presence of specific elements in a mixture? If so, how and where?
3. What is one demonstration of this concept that the majority of the
class has witnessed?
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