# Measuring Visible Light

## How can we measure visible light?

The electromagnetic spectrum includes all forms of light energy, known as electromagnetic radiation. (1) Different types of electromagnetic radiation such as radio waves, microwaves, visible light, and x-rays are associated with specific wavelengths. A spectrophotometer is an instrument that performs measurements within the wavelength range of 325 to 1100 nanometers. (2) This range of wavelengths includes visible light, which is the form of electromagnetic radiation that our eyes can detect.

## Lesson Overview:

Students will use a spectrophotometer to learn about the wavelengths that correspond with visible light.

## Learning Objectives:

• Students will investigate the visible light band of the electromagnetic spectrum.
• Students will measure wavelengths that correspond with different colors.
• Students will compare observations to develop wavelength ranges for different colors.

## Key Vocabulary:

• spectrophotometer
• electromagnetic radiation
• wavelength
• visible light

## Procedure:

• Check that the sample compartment is empty before turning on the spectrophotometer.
• Power ON the spectrophotometer. The power-up sequence will take about 2 minutes to complete. Allow the instrument to warm up for 30 minutes before using it.3
• Press A/T/C to select absorbance mode.
• Set the spectrophotometer to a wavelength of 325 nm. (This is the lowest wavelength available for the Genesys 20 spectrophotometer.)
• Press 0 ABS/100%T to set the blank to 0 Absorbance.
• Place a thin strip of white paper in the sample compartment of the spectrophotometer.
• Leave the sample compartment open. Beginning with the wavelength of 325 nm, increase the wavelength until you can first see a color. Record your observation in the chart under the Results section.
• Continue increasing the wavelength and watching the color displayed on the paper strip in the sample compartment. Record the last wavelength that you are able to still observe the first color.
• When you notice a color change, record the new color and the first wavelength of which you observe this new color.
• Continue increasing the wavelength on the spectrophotometer. Record each new color with both initial and final wavelengths observed for each color.

## Results:

 Observed color Initial wavelength (nm) Final wavelength (nm) Red Orange Yellow Green Blue Violet Indigo

If you do not have access to a visible spectrometer, you can use the video above for data collection. We recorded the sample compartment while we ran the lab.

## Data Analysis:

• Enter the values that you observed in the Google Sheet: Data Analysis: Measuring Visible Light
• Observe the graph that is generated as initial and final wavelength values are entered by all groups/participants for each observed color.

## Assessment:

• How do your observed wavelength values for each color compare to the wavelength values displayed on the graph?
• Why might your observations of colors at different wavelengths be different from the observations of a classmate?
• Why do we show individual colors as a range of different wavelengths instead of one single wavelength?

## Supplementary Activity: Superheroes of Science Content Expert Video

• Listen or view this Superheroes of Science podcast: Astrophysicists use the electromagnetic spectrum to study stars, pulsars, black holes and more. Complete the guided listening worksheet as you listen or view the podcast. Audio only  YouTube video

## References:

1. NASA Goddard Space Flight Center - Imagine the Universe! The Electromagnetic Spectrum https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html
2. Thermo Spectronic Service Manual: Genesys 20 Spectrophotometer https://archive-resources.coleparmer.com/Manual_pdfs/genesys%2020%20service%20manual.pdf
3. Thermo Spectronic Operator’s Manual: 20 Genesys Spectrophotometer https://archive-resources.coleparmer.com/Manual_pdfs/Genesys%2020%20Op%20Manual%20(2).pdf

Purdue University College of Science, 150 N. University St, West Lafayette, IN 47907 • Phone: (765) 494-1729, Fax: (765) 494-1736

Student Advising Office: (765) 494-1771, Fax: (765) 496-3015 • Science IT, (765) 494-4488

Trouble with this page? Disability-related accessibility issue? Please contact the College of Science Webmaster.