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Name: Brittney H.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Thursday, April 18, 2002 8:36:18 PM

When it comes to diffraction, the greater the wavelength, the greater the angle of diffraction. Why is that?

Brittney, One way to "imagine" diffraction through a single slit is by thinking about a single slit as two slits, side-by-side, 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 one-half 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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