The Triad Admissions Academics Alumni & Friends Programs Resources Student Life

Physics Photo of the Week

Slit Photograph

April 8, 2005

What shape is this light?

It turns out that this light is round.  See the photo below.  It is the pilot light on my computer monitor - fairly monochromatic.  However, it was photographed through a very narrow slit (about 0.3 mm wide) placed directly in front of the lens of the camera.  The slit is vertical.  Because most of the light is blocked by the slit, the camera had to be set to a long time exposure (5 sec).  The normal photo below was created with no slit with a time setting of 1/8 sec.

The reason the image of the pilot light is spread out is diffraction.  Diffraction is a consequence of light being a wave.  Diffraction is essentially the phenomenon of a wave bending around corners.  If light consisted of pure microscopic particles, the light beam would travel straight through a tiny opening and not spread out perpendicular to the axis of the slit.  Another feature of the diffraction is the interference fringes.  The image of the spread-out lamp has faint fringes - areas of decreased light intensity.  This is the interference of light waves.  Some areas the waves are always out of phase and cancel.

The slit was made simply by taping two pieces of vinyl electrical tape to a flat piece of clear plastic (a CD case), the tape pieces were placed with a narrow gap on one end and overlapping on the other end.  This permits a tapered slit.  The reason for the tapered slit is that it is very difficult to align the tape exactly parallel and keep the tiny space uniform.  The clear plastic with the make-shift slit was held directly in front of the camera lens to make the photo.  It is amazing that a decent image - although distorted by diffraction - can be made simply by looking through such a narrow slit!  Speaking of CD's, the rainbow-like colors that you see on a CD are another consequence of diffraction of light.  That is a photo for another day!

All photos by Donald Collins

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

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.