Abstract

Objective

The goal of this project is to investigate whether different frequencies of light contain different amounts of energy.

Introduction

Light is an example of an electromagnetic wave. Electromagnetic waves can travel through the vacuum of interstellar space. They do not depend on an external medium—unlike a mechanical wave such as a sound wave which must travel through air, water, or some solid medium. Electromagnetic waves cover a huge range of frequencies, from high-frequency gamma rays and x-rays, to ultraviolet light, visible light, and infrared light, and on into microwaves and radio waves. As the frequency decreases, so does the energy. The wavelength of an electromagnetic wave is inversely proportional to its frequency. So waves with high frequency have short wavelengths, and waves with low frequency have long wavelengths.

Electromagnetic waves interact with materials in different ways, depending on the nature of the material and the frequency of the electromagnetic wave. Light is the range of electromagnetic waves that are visible (Figure 1). For humans, the range of visible wavelengths is from 400 to 700 nm (1 nm = 1 ×10⁻⁹ m).

Figure 1. The visible spectrum. X-rays, light, and radio waves are examples of electromagnetic waves. Light is the part of the electromagnetic spectrum that we can detect with our eyes. At the blue end of the visible spectrum, the wavelength of light is shorter (about 400 nm). At the red end of the spectrum, the wavelength of light is longer (about 700 nm) (Illustration from Abrisa Glass & Coatings, 2005).

This range of wavelengths is called the visible spectrum of light. When you see a rainbow in the sky, or white light that has been refracted through a prism, or diffracted by the regular surface of a CD, you are seeing a spectrum of colors. The different colors are related to the different wavelengths of light. Violet light is at the short-wavelength end of the visible spectrum (400 nm), and red light is at the long-wavelength end of the visible spectrum (700 nm), with the rainbow of colors in between.

We perceive different colors because our visual system has evolved to make use of the spectral information in reflected light. When light interacts with an object, the light can be absorbed by the object, reflected by the object, or transmitted by the object.

For example, when you look at yourself in the mirror, the light that you are seeing has been relected by the mirror, transmitted through the air, through your cornea, through the lens of your eye, and through two layers of cells in your retina before it is absorbed by light-sensitive pigments in your photoreceptor cells. The energy from the absorbed light starts a cascade of chemical reactions in your photoreceptors that ultimately leads to your percpeption: seeing yourself in the mirror.

Light that is absorbed by an object is usually converted into heat energy. The goal of this project is to measure how much heat is produced by the absorption of light of different colors. You'll use LEDs of different colors as your light sources. You'll measure the time it takes for a drop of isopropyl alcohol (rubbing alcohol) to evaporate when it is illuminated by the different LEDs. By comparing the evaporation times under different illumination conditions, you will be able to draw some conclusions about the relative energy of light of different frequencies.

Before getting started on the experiment, it will be helpful to understand how LEDs work. An LED (light-emitting diode) is a special kind of diode that produces light (see Figure 2).

Figure 2. A red LED (top). The longer lead is the anode (+) and the shorter lead is the cathode (−). In the schematic symbol for an LED (bottom), the anode is on the left and the cathode is on the right (Hewes, 2006).

As the current through the LED increases, the brightness also increases. Typically, the recommended current for an LED is 20 mA or less. Above this value, the lifetime of the LED will be decreased significantly. Far above this value, the LED will fail catastrophically, like a flashbulb.

To keep the LED current at a reasonable level, LEDs are typically connected in series with a current-limiting resistor, as shown in Figure 3.

Figure 3. Schematic diagram of an LED in series with a 1kΩ resistor (Hewes, 2006).

The voltage drop across an LED is about 2 V (except for blue or white LEDs, where the voltage drop is about 4 V). In the circuit in Figure 2, the voltage drop across the resistor will be 9 − 2 = 7 V. Using Ohm's law, the current, I, through the resistor will be V/R = 7 V/1kΩ = 7 mA.

Figure 4 (below) shows you how to use Ohm's Law to calculate what size resistor you need to limit the current through the LED to the desired value. The voltage drop across the resistor will equal the supply voltage minus the voltage drop across the LED (or, V_S − V_L). You can then use Ohm's Law to calculate the resistance, R, needed to produce a desired current, I:

So, if the supply voltage is +7 V, what resistor would you need for a 15 mA current? R = (7 − 2)/0.015 A = 333Ω. The closest standard resistor value would be 330 Ω. For more details, and a set of online calculators, see the LED references in the Bibliography section (Hewes, 2006; Ngineering, 2003).

Figure 4. Schematic diagram showing how to use Ohm's Law to calculate the correct value for the current-limiting resistor (Hewes, 2006).

In this project, you'll use what you've learned about LEDs and the electromagnetic nature of light to investigate how light energy changes with frequency. You'll use illumination from different colored LEDs to evaporate a set volume of isopropyl alcohol. You'll measure how long the evaporation takes with each different light source. Which color of light will produce the fastest evaporation time?

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Electromagnetic spectrum
• Visible light
• Ultraviolet light
• Infrared light
• Frequency
• Wavelength
• Ohm's Law
• LED (light emitting diode)

Questions

• What is the relationship between wavelength, frequency, and the speed of light?

Bibliography

• For more information on how light interacts with matter, see:
  Hoff, K. Mellendorf and V. Calder, date unknown. "Reflection and Absorption," NEWTON Ask a Scientist, Physics Archive, Argonne National Laboratory, Division of Educational Programs [accessed June 25, 2007] http://www.newton.dep.anl.gov/askasci/phy00/phy00232.htm.
• For high-school level references on color and vision, see:
  • Henderson, T., 2004. "Color and Vision," The Physics Classroom, Glennbrook South High School, Glennbrook, IL [accessed June 25, 2007] http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/light/u12l2a.html.
  • Wikipedia contributors, 2007. "Color," Wikipedia, the Free Encyclopedia [accessed June 25, 2007] http://en.wikipedia.org/w/index.php?title=Color&oldid=110725908.
• These webpages have useful information on LEDs:
  
  • Hewes, J., 2006. "Light Emitting Diodes (LEDs)," The Electronics Club, Kelsey Park Sports College [accessed June 25, 2006] http://www.kpsec.freeuk.com/components/led.htm.
  • Ngineering, 2003. "LED Calculators," Ngineering.com [accessed June 25, 2006] http://www.ngineering.com/led_calculators.htm.
• The visible spectrum illustration in the Introduction is from:
  Abrisa Glass & Coatings, 2005. "Understanding Light and Color," Abrisa Glass & Coatings [accessed April 10, 2011] http://abrisatechnologies.com/docs/Guide%20to%20Glass%20Final%20April%202011.pdf.
• Advanced. For background information on the different ways of measuring light, see:
  Newport Corporation, 2007. "Optical Radiation Terminology and Units," Newport Corporation [accessed June 25, 2007] http://www.newport.com/Optical-Radiation-Terminology-and-Units/381842/1033/catalog.aspx.
• Advanced. Here is the data sheet for the RGB LED suggested for this project (LFL550M from Seoul Electronics): Seoul Electronics, . "LFL550M Data Sheet," Seoul Electronics [accessed June 25, 2007] http://www.seoulsemicon.co.kr/_homepage/home_kor/product/spec/LFL550M.pdf.
• Advanced. This website has descriptions and calculators for several statistical tests, including the Student's t-test that you can use in this project:
  Kirkman, T., date unknown. "Student's t-Tests," Department of Physics, College of St. Benedict & St. John's University [accessed February 23, 2006] http://www.physics.csbsju.edu/stats/t-test.html.
• This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award:
  Jacobs, S., 2007. "Do Different Frequencies of Light Contain Different Amounts of Energy?" [accessed June 25, 2007] http://www.usc.edu/CSSF/History/2007/Projects/S1610.pdf.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• RGB LED, for example:
  • part number 604-WP154A4-RGB from Mouser Electronics.
• Resistors
• Wire
• Soldering iron
• Solder
• 9 volt battery
• Battery clip (for 9 volt battery)
• Isopropyl alcohol (rubbing alcohol)
• Eyedropper or plastic transfer pipette
• Small cardboard box
• Small piece of white cardstock
• Flashlight
• Stopwatch (resolution to 0.1 second)

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Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
2. Use the information in the Introduction to calculate the desired current for each color LED that you use.
3. Make a small three-sided enclosure from a cardboard box to minimize air currents (reduces basal evaporation rate). The enclosure will also minimize illumination from light sources other than the LED. The open side allows you to observe the evaporation of the isopropyl alcohol.
4. Use the eyedropper (or transer pipette) to place a single drop of isopropyl alcohol (rubbing alcohol) on an index card.
   1. The alcohol will soak into the paper, that's OK.
   2. Make sure to use the same size drop each time.
   3. When the alcohol has evaporated, there will no longer be a dark spot on the card. The flashlight can help with this observation, but use it sparingly(and the same number of times for each test).
5. Measure evaporation time without LED illumination (basal evaporation rate).
6. Measure evaporation rate with each LED as an illumination source. For each trial:
   • make sure that each LED is the same distance from alcohol drop.
   • make sure that the current through each LED is the same.
7. Repeat each test condition at least 10 times.
8. Calculate the average evaporation time for each condition.
9. Subtract the average basal evaporation time from the evaporation time for each illuminated condition. The result is the evaporation time due to each LED.
10. More advanced students should also calculate the standard deviation for each condition.
11. Make a graph of evaporation time vs. frequency of the LED. Remember that you can calculate the frequency (f) for each LED from the center wavelength (λ) for the LED from the equation c = fλ, where c is the speed of light, 3×10⁸m/s.

Variations

• Advanced. More advanced students can perform t-tests on the results to see if the average evaporation rates with the different LEDs are significantly different from one another (Kirkman, date unknown).
• For a related experiment on the energy of various frequencies of light, see the Science Buddies project How Does Color Affect Heating by Absorption of Light?.
• For a fancier circuit with continuous control of the LED current, see the Science Buddies project Color Mixing with Red, Green, & Blue LEDs.

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award:

Jacobs, S., 2007. "Do Different Frequencies of Light Contain Different Amounts of Energy?" [accessed June 25, 2007] http://www.usc.edu/CSSF/History/2007/Projects/S1610.pdf.

Last edit date: 2009-10-20 15:00:00