Diffraction Lab – PHY 142/162 – Short Form
Name/Signature: _______________________________
Name/Signature: _______________________________
Name/Signature: _______________________________
Introduction & Background. In all cases, 𝜃 is the measured angle to the 𝑚th
maximum or minimum of light intensity, and  is the light wavelength.
1) Use a diffraction grating to measure the wavelength of laser light. The light
is scattered at angles given by 𝑑 sin 𝜃 = 𝑚, 𝑚 = 0, ±1, ±2, …, where 𝑑 is
the spacing between the slits in the grating.
2) Use the laser pen to analyze various structures: single slit, double slit,
random and regular dot patterns. In each case, calculate some distance
parameter(s) from the data.
a. Single slit: Find the slit width 𝑤. The light minima (dark fringes) are
given by 𝑤 sin 𝜃 = 𝑚, 𝑚 = 0, ±1, ±2, …
b. Double slit:
i. Find the slit separation 𝑑. The light maxima (bright fringes) are
given by 𝑑 sin 𝜃 = 𝑚, 𝑚 = 0, ±1, ±2, …
ii. Find the slit width 𝑤. There is also a single slit pattern acting as
an envelope for the double slit pattern. The light minima (dark
fringes) are given by 𝑤 sin 𝜃 = 𝑚, 𝑚 = 0, ±1, ±2, …
c. Other structures: In all other cases, describe the diffraction pattern
with words and a sketch, and calculate some structural distance scale
from the data.
Data & Analysis. In each case, measure the distance 𝐿 from the diffraction
structure to the screen, and the distance 𝑦 from the center of the pattern to the
relevant maxima or minima. The angle 𝜃 is given by tan 𝜃 = 𝑦/𝐿. For small
angles, this can be approximated as sin 𝜃 = 𝑦/𝐿; this will work well in all cases
except for the diffraction grating results. Print and submit all graphs and tables for
each case with this report.
1) Diffraction grating: Use Excel tab “grating.”  = __________________
2) Single slit: Structures A, B, and C. Use Excel tab “single slit.”
A. 𝑤 = ______________.
B. 𝑤 = ______________.
C. 𝑤 = ______________.
3) Double slit: Structures D, E, and F. Use Excel tab “double slit.”
D. 𝑤 = ______________, 𝑑 = ______________.
E. 𝑤 = ______________, 𝑑 = ______________.
F. 𝑤 = ______________, 𝑑 = ______________.
4) Other: Structures I and J. Provide a verbal description, a sketch, and a
(rough) measure 𝑥 of structure size.
I. 𝑥 = ______________.
J. 𝑥 = ______________.