Measuring Wavelengths of Light Lab (Guided)
PSI Physics: Electromagnetic Waves
Name:___________________________
Measuring the Wavelengths of Light Waves
Problem: How can we apply the diffraction grating to determine the wavelength of light waves?
Theory: When light is incident upon a diffracting grating, it is diffracted by the slits in the grating. An
interference pattern is a result of diffraction. As you look through the grating, you will see the spectrum
of source to the right and the left of position of the source. In the figure below, the light incident upon
the grating is diffracted by the slits in the grating. The lines of colored light you observe are due to the
reinforcement of the light waves from adjacent slits are in phase. In between these lines, the light
waves are out of phase and cancel out.
Constructive interference pattern is given by the equation:
𝑥=
𝑥𝑥𝑥
𝑥
Solving for the wavelength:
𝑥=
𝑥𝑥
𝑥𝑥
Where L is the distance from the diffraction grating to the source, x is the distance from the source to
the maximum, m is the order maximum, and d is the distance between the lines on the diffraction
grating.
Procedure:
1. The distance between the lines on the diffraction grating can be found by taking the number of
lines/cm, taking the inverse and then multiplying by 10-2.
The diffraction grating has ______ __ lines/cm.
𝑥=
1
× 10−2 = _________________𝑥
____________ 𝑥𝑥𝑥𝑥𝑥/𝑥𝑥
2. Arrange the light bulb and the diffraction grating on the optical ramp. The distance along the
meter stick from the grating is L. Set L at value between 60 and 80 cm.
3. To measure x arrange another meter stick in from of light bulb. While one partner is looking
through the grating another partner moves his or her finger along the meter stick until it
reaches where the observer sees the interference pattern.
4. Record x for the first order maxima for red, green, and violet light.
5. Fill in the following chart.
Color
Red
Green
Violet
Distance from
source to first
order
maximum
x (m)
Distance
between
the grating
lines
d (m)
Distance from
source to
grating
L (m)
Wavelength
λ (m)
Frequency
f = c/λ (Hz)
6. Repeat for the second order maximum.
Color
Distance from
source to
second order
maximum
x (m)
Distance
between
the grating
lines
d (m)
Distance from
source to
grating
L (m)
Wavelength
λ (m)
Frequency
f = c/λ (Hz)
Red
Green
Violet
Conclusion:
1. Which color has the longest wavelength?
2. Which color has the shortest wavelength?
3. If energy increases with frequency, what color has the highest energy?
4. Summarize Young’s Double Slit Experiment for light. If light acted only as a particle, what
would he have observed?
5. What causes the maxima and minima in the Double Slit Experiment?
6. A diffraction grating is etched with 7100 lines/cm. The distance between the grating and
the observation screen is 0.65 m. What is the distance from the midpoint of the screen to
the 2nd order maxima for light with a wavelength of 470 nm?