LED Emission Spectra
I. OBJECTIVES.
In this lab you will learn how to use a spectrometer in order to analyze the light emitted
by a variety of sources. In particular you will complete the following tasks:
a. Examine the continuous spectrum emitted by an incandescent lamp.
b. Determine the wavelengths emitted by a hydrogen lamp.
c. Determine the range of wavelengths emitted by four LEDs.
d. Determine the cutoff wavelength of two optical filters.
II. EQUIPMENT.
Grating spectrometer, LEDs and associated power supply, voltmeter and ammeter,
Hydrogen lamp, incandescent lamp, optical filters, optical jack.
III. INTRODUCTION.
The nature of light can be described in two different ways using either classical physics
or modern (or quantum) physics.
Classical physics describes light as a wave that can propagate through vacuum as well as
through matter. In vacuum light propagates with speed c  3 108 m/s . Associated with
every light wave is an oscillating electric and magnetic field vector (symbols E and B ,
respectively). The resulting wave is called electromagnetic (EM). Such a wave is shown
in fig.1. The electric field E is perpendicular to the magnetic field B , and both vectors
are perpendicular to the propagation direction of the wave. The electromagnetic wave
shown in fig.1 is periodic in space i.e. it consists of a repeating pattern (sinusoidal). The
distance between two adjacent wave crests (or wave troughs) is called the wavelength
(symbol  ). If we observe the EM wave at a particular point along the x-axis the electric
and magnetic fields E and B oscillate rapidly in time with a frequency f, defined as the
number of oscillations per second (unit: Hz). The wavelength and the frequency of an
EM wave are connected via the equation:
f c
(eqs.1)
EM waves have an enormous range of wavelengths and frequencies. This is illustrated in
fig.2. Depending on the wavelength EM waves encompass radio waves of all types (AM,
FM, TV, radar), infra-red (IR) light, visible light, ultraviolet (UV) light, x-rays, and
gamma-rays. Please note that visible light is only a very narrow section of the entire EMwave spectrum with wavelengths from 400 nm (violet) to 700 nm (red).
Note: 1 nm  109 m . This picture was proposed by James Clerk Maxwell.
Quantum physics describes light as a stream of particles called photons. These have
zero mass and travel in vacuum with speed c  3 108 m/s . Each photon carries an
energy given by the expression:
hc
E  hf 
(eqs.2)

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Here h is a constant known as Planck’s constant whose value is equal to:
h  6.63 1034 J  s . When a photon is absorbed by an atom the photon disappears (the
formal term is annihilated) and all its energy is transferred to the atom. This model for
the description of light has been proposed by Albert Einstein.
The two pictures given above (classic and quantum) are complimentary to each
other, something like the two different faces of the same coin. Some effects such light
interference and light polarization are explained by the classic picture. Others, such as
the emission spectrum of Hydrogen and the photoelectric effect can only be explained
using the quantum picture.
IV. EXPERIMENTAL METHOD.
In this experiment you will determine the wavelength range for the light emitted by four
LEDs. For this task you will use the spectrometer shown in fig.3. This instrument uses a
diffraction grating to separate the various wavelengths of light. We will study this
optical component in the next experiment. The light to by analyzed by the spectrometer
enters through a slit at point A (see fig.3). The user observes the various wavelengths
through eyepiece B. A reflector C is used to illuminate the instrument’s internal
wavelength scale. The spectrometer is mounted on post D and is supported by tripod E.
The spectrometer wavelength scale reads in nanometers ( 1 nm  109 m )
V. PROCEDURE.
V1. Continuous emission spectrum of an incandescent lamp. Place the incandescent
lamp on an optical jack. Adjust the jack height so that the lamp is at the same height as
the entrance slit (point A in fig.3) of the spectrometer. The spectrum is continuous with
colors varying in color from the violet to the red. View the spectrum through the
eyepiece (point B in fig.3)
Record on the data sheet the shortest wavelength violet
Record on the data sheet the longest wavelength red
V2. Place the hydrogen lamp at t he entrance slit. The spectrum now is discrete and
consists of 4 separate sharp lines. These are known collectively as the Hydrogen Balmer
lines. Record the data sheet the wavelength as well as the color for each line.
V3. Place the Red LED on the optical jack so that the diode faces the spectrometer.
Adjust the jack height so that the LED is at the same height as the entrance slit (point A
in fig.3) of the spectrometer. Apply a voltage V between the emission voltage Ve and the
maximum allowed voltage Vmax for the red LED. The emission spectrum of the LED is a
continuous band of red light. Record on the data sheet the shortest and longest
wavelengths   R min and   R max of the emission band.
V4. Repeat the procedure described in section V3 for the yellow LED. Record on the
data sheet the shortest and longest wavelengths   Y min and   Y max of the emission
band.
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V5. Repeat the procedure described in section V3 for the green LED. Record on the
data sheet the shortest and longest wavelengths   G min and   G max of the emission
band.
V6. Repeat the procedure described in section V3 for the blue LED. Record on the data
sheet the shortest and longest wavelengths   Bmin and   Bmax of the emission band.
V7. Place the red filter (#29) between the incandescent lam and the entrance slit A of the
spectrometer as shown in fig.4. This is known as a low pass filter in the sense that it
absorbs the shorter wavelengths and transmits the longer wavelengths of the incident
light. For filter #29 only the red part of the lamp’s spectrum is transmitted through the
filter. Record on the data sheet the shortest wavelength of the transmitted light 29 .
This is called the cut-off wavelength of the filter.
V8. Repeat the procedure described in section V7 for the orange filter (#15). For filter
#15 only the red and yellow part of the lamp’s spectrum is transmitted through the filter.
Record on the data sheet the shortest wavelength of the transmitted light 15 .
VI. FOR THE REPORT.
VI-1. List the wavelengths violet and red measured in section V1.
VI-2. List the four wavelengths of the hydrogen Balmer lines as well as their colors,
determined in section V2.
VI-3. List the minimum and maximum emission wavelengths   R min and   R max for
the red LED measured in section V3. Calculate the average emission wavelength
  R min    R max
  R avg 
for the red LED.
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VI-4. List the minimum and maximum emission wavelengths   Y min and   Y max for
the yellow LED measured in section V4. Calculate the average emission wavelength
  Y min    Y max
  Y avg 
for the yellow LED.
2
VI-5. List the minimum and maximum emission wavelengths   G min and   G max for
the green LED measured in section V5. Calculate the average emission wavelength
  G min    G max
  G avg 
for the green LED.
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VI-6. List the minimum and maximum emission wavelengths   Bmin and   Bmax for
the blue LED measured in section V6. Calculate the average emission wavelength
  B min    B max
  B avg 
for the blue LED.
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VI-7. List the cut-off wavelength 29 for the red filter (#29) measured in section V7.
VI-8. List the cut-off wavelength 15 for the orange filter (#15) measured in section V8.
VII QUESTIONS.
VII-1. Based on what you learned in this lab suggest a method to verify the presence of
hydrogen in the atmosphere of a distant star without sending a spacecraft.
VII-2. Using equation 2 calculate the photon energies E  R  , E  Y  , E  G  , and E  B
which correspond to the average emission wavelengths   R avg ,   Y avg ,   G avg , and
  B  avg calculated in sections VI-3, VI-4, VI-5, and VI-6.
VII-3. Is there any correlation between the photon energies E  R  , E  Y  , E  G  , and
E  B calculated in question VII-2 and the corresponding emission voltages Ve  R  ,
Ve  Y  , Ve  G  , and Ve  B determined in the previous lab for each LED? If such a
correlation exists, describe it.
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Fig. 1: A snapshot of an
electromagnetic wave propagating
along the x-axis (Halliday, Resnick and
Walker, Fundamentals of Physics)
Fig.2: Wavelengths and frequencies of the electromagnetic spectrum. (Halliday, Resnick,
and Walker, “Fundamentals of Physics”
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B
A
C
D
E
Fig.3: Picture of the grating
spectrometer used in this
experiment
Incandescent
light source
A
Filter
Spectrometer
B
Observer
Fig.4: Experimental setup
used in sections V7 and V8
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