How Are Color and Energy Related?



We know that exposure to X-rays can damage living cells. Also ultra-violet (uv) light can be harmful since extensive exposure to the Sun's uv light can cause sunburn. On the other hand, visible light and radio waves are harmless to living cells. How are the frequencies of different kinds of electromagnetic waves related to the energy they carry? The energy delivered by an electric circuit to a normal incandescent light bulb can be varied with a "dimmer switch". As more current is applied to the bulb, the color of the filament changes and the bulb brightens. Yet we cannot connect a dimmer switch to a fluorescent bulb to vary its brightness. What causes the difference in how light is generated by incandescent and fluorescent light bulbs? And how is the color or frequency of light related to its energy?


1) To investigate various kinds of spectra, and the relation between the color and the frequency or wavelength of light.

2) To investigate the relation between the frequency and the energy of light emitted by an LED.


1. Visit the two stations, collect data and discuss with your group the phenomena observed. (60 min.)

2. Select one of the stations for your White Board topic. Prepare a five minute White Board presentation which addresses the answer to the question posed at the station. (15 min)

Station 1: What factors cause different kinds of spectra? (30 min)


"Star" spectrometer with wavelength scale

incandescent light bulb, 15 watt, ac, 110volt source

glow discharge tubes filled with hydrogen, neon or mercury gas

Poster containing sample spectra

1. Examine the blue spectrometer, and identify the diffraction grating at one end. This is the end through which you look. Note that there is a square-shaped opening at the other end of the spectrometer, on the right side. On the left side is a scale which shows the wavelength of the light you observe when you look through the spectrometer at a light source.

2. Four light sources are available: Three glow discharge bulbs each containing a different kind of gas: 1) Hydrogen, 2) Neon and 3) Mercury, and 4) a normal incandescent bulb attached to a "dimmer switch". Predict what you expect to see as you view each of the fourth light sources through the grating of the spectrometer.

3. Test your predictions by viewing each of the light sources. Note that wavelengths (in nanometers) of the light can be read from the scale inside the spectrometer (to the left as you look through the grating). Make notes on your observations.

Station 2: Is the energy required to light an LED related to the color of light an LED emits?

battery, 9 volt

Light Emitting Diodes (LED's), assorted colors ( blue, green, yellow, red)

circuit board

resistors, 10 kiloOhm (10k )

digital voltmeter

battery clip with leads

Graphical Analysis or graph paper

"Star" spectrometer with wavelength scale (4)

digital voltmeter

spectrum chart


1. Examine the Light Emitting Diodes (LED's). In a previous experiment you found that electric current flows in only one direction through a diode. Note that there are two wires extending from the LED's. The negative side is indicated in two ways: 1) by the flat side of the bulb, and 2) by the shorter of the two wires extending from the LED.

2. Assemble the LED's in a parallel circuit as shown in the circuit diagram. The resistors are to protect the LED from too much current,

and to minimize the amount of current and voltage available to the LED. Does this minimize or maximize the energy available to power the LED's?

3) Turn on the digital voltmeter. Connect the probes of the voltmeter across leads of the LED's. Record the potential difference in volts across each of the LED's. Construct a data table.

4) Observe each of the LED's with the spectrometer to estimate the wavelength corresponding to the maximum intensity of the light emitted. You should calculate the corresponding frequency of the light emitted by each LED. A sample is shown in the table, and in the accompanying sheet.

7) Plot voltage against the frequency for the LED's. Remember that voltage is proportional to energy, so that you are essentially plotting energy required to light the LED vs. the frequency of light an LED emits. Does any simple mathematical function fit your graphed data? How could your results be used to predict the energy of light at different colors or frequencies?

White Board Report

Select a station for your five minute White Board presentation. Your White Board should include: 1) a title in the form of a question, 2) a summary or demonstration of the phenomenon, 3) your proposed explanation or hypothesis for the phenomenon. White Board "grading criteria" have been summarized on a separate sheet.

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