؟What is Inside an LED

LED's are special diodes that emit light when connected in a circuit. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not. A a clear (or often colored) epoxy case enclosed the heart of an LED, the semi-conductor chip.

LED leads

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side lead on flat
side of bulb = negative


The two wires extending below the LED epoxy enclosure, or the "bulb" indicate how the LED should be connected into a circuit. The negative side of an LED lead 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. The negative lead should be connected to the negative terminal of a battery. LED's operate at relative low voltages between about 1 and 4 volts, and draw currents between about 10 and 40 milliamperes. Voltages and currents substantially above these values can melt a LED chip.

The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow, and the electrons cross the junction into the p region.

In the absence of a large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to the flow of electrons.



 

What Causes the LED to Emit Light and What Determines the Color of the Light?



 

When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions. In the p region there are many more positive than negative charges. In the n region the electrons are more numerous than the positive electric charges. When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region. Once in the p region the electrons are immediately attracted to the positive charges due to the mutual Coulomb forces of attraction between opposite electric charges. When an electron moves sufficiently close to a positive charge in the p region, the two charges "re-combine".

Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus). Only photons in a very narrow frequency range can be emitted by any material. LED's that emit different colors are made of different semi-conductor materials, and require different energies to light them.



 

How Much Energy Does an LED Emit?



 

The electric energy is proportional to the voltage needed to cause electrons to flow across the p-n junction. The different colored LED's emit predominantly light of a single color. The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qV Joules. This expression simply says that the voltage is proportional to the electric energy, and is a general statement which applies to any circuit, as well as to LED's. The constant q is the electric charge of a single electron, -1.6 x 10-19 Coulomb.



 

Finding the Energy from the Voltage



 

Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is E = qV or E = -1.6 x 10-19 (1.71) Joule, since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives E = 2.74 x 10-19 Joule.



 

Finding the Frequency from the Wavelength of Light



 

The frequency of light is related to the wavelength of light in a very simple way. The spectrometer can be used to examine the light from the LED, and to estimate the peak wavelength of the light emitted by the LED. But we prefer to have the frequency of the peak intensity of the light emitted by the LED. The wavelength is related to the frequency of light by , where c is the speed of light (3 x 108 m/s) and is the wavelength of light read from the spectrometer (in units of nanometers or 10-9 meters). Suppose you observed the red LED through the spectrometer, and found that the LED emits a range in colors with maximum intensity corresponding to a wavelength as read from the spectrometer of = 660 nm or 660 x 10-9 m. The corresponding frequency at which the red LED emits most of its light is or 4.55 x 1014 Hertz. The unit for one cycle of a wave each second (cycle per second) is a Hertz.

 

 

PHS 110: Experiment 13
How Are Color and Energy Related?

Background

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?

Purpose

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.

Procedure

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)

Materials

"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.