

Diffraction, Wavelength, and Angle
Name: Brittney H.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Thursday, April 18, 2002 8:36:18 PM
Question:
When it comes to diffraction, the greater the wavelength,
the greater the angle of diffraction. Why is that?
Replies:
Brittney,
One way to "imagine" diffraction through a single slit is by thinking about
a single slit as two slits, sidebyside, with no distance between them.
When light passes through the slit, it goes out in all directions. At an
angle to the left, light from the left half of the slit travels less
distance than light from the right half. If this difference is onehalf of
a wavelength, light from the left half interferes destructively with light
from the right half. This is the angle of the first dark fringe. For
longer wavelengths, this dark fringe occurs when the difference in path
lengths is greater: at a greater angle. A similar explanation applies for
all bright and dark fringes in the diffraction pattern. Increasing the
wavelength affects the diffraction pattern in the same way as decreasing the
width of the slit.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
Diffraction is an interference effect. The path lengths from the
neighboring diffraction sources (e.g., lines on a grating) must equal
one wavelength. If you consider the geometry of the reflections, the
path lengths are equal for reflections along the normal (i.e., straight
out from the grating). The difference in path lengths increase for
greater angles. Thus, short wavelengths have equal path lengths (and
constructive interference) at smaller angles than long wavelengths.
Greg Bradburn
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Update: June 2012

