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Physics Photo of the Week

March 31, 2006

Diffraction


Diffraction is the phenomenon that all waves possess that cause the waves to bend around corners and to spread out when going through small apertures or slits.  To make the photo above, the slit (right) was held close to the camera lens while the camera focused and photographed a spot of light on the wall (the spot from a laser).  Normally, a camera, or your eye, has a wide open lens with a fairly large circular aperture.  The pupil in your eye is the corresponding circular aperture, and ranges in size from about 2 mm in bright light to about 10 mm in the darkness.  If you hold the small slit at right up to your eye, or make a small slit with your fingers, and look at a small light (such as a LED) across the room, you will see the similar pattern.  It is quite difficult to see the effect in your own eyes because the fringes are very faint.  The above picture was made with a time exposure of about 5 seconds, something your eye cannot do.

A major characteristic of diffraction of light passing through small apertures is that it not only spreads out the light perpendicular to the main long axis of the slit, but it also develops fringes - the regions of light and dark.  The fringes prove that light behaves as a wave.  The wavelength is very small and doesn't become noticed unless the slit width is a few hundred times the wavelength.  The slit above is 0.16 mm wide - about 270 times the wavelength of the red light.  The dark fringes occur when the light from the center of the slit travels a half wavelength - or an odd multiple of a half wavelength - less than light from the edges of the slit.  This path difference arises from the fact that the fringes are set at an angle from the main path so the light rays from the edges have further to travel.  When the path difference between the rays from the center and rays from the edge differ by a half wavelength, or any odd multiple of a half wavelength, the waves cancel and produce a dark fringe.


The image at right was made by the Physics class using a CCD camera (a digital camera) with the lens removed.  Laser light passed through a slit toward the photosensor in the CCD camera.  The sensor was about 100 mm distant from the slit.  The CCD camera permits a quantitative measure of the light intensity.  The diffraction image in this photograph is rather "noisy" due to imperfections in the slit.  The imperfections become highly magnified due to the same phenomenon of diffraction.  To obtain the intensity profile, each column of pixels is added to give an intensity value for the total of that column.  This eliminates the noise of the original image.  The measured intensity profile is an excellent fit to the predicted profile based on the mathematics of diffraction.





Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.




Observers are invited to submit digital photos to: