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Physics Photo of the Week

April 1, 2005

Laser Study of Refraction

This is a photo made by student Alicia Safdie showing laser light entering a tank of water.  An aquarium is filled partway with water - the water level is about 1/4 from the top of the image.  Laser light enters the air above the water from the upper left and strikes the water.  Most of the laser light then enters the water, but the light is bent as it enters the water.  Normally, the laser beam is not visible, but some milk was dispersed in the water making the water slightly cloudy, which makes the laser beam visible.  Smoke from a smoldering match, blown into the air above the water level, makes the laser beam visible in the air.  To obtain the picture without much light for the camera, auto exposure was used with the camera well-supported on the table.  The exposure time was 1 second.

Notice that not all the light enters the water.  A substantial fraction of the light is reflected at the top surface of the water.  The reflection of light from the water surface is responsible for the attractive reflecting pools in landscape architecture. The reflected light from the water is also responsible for the increased chance of sunburn for people on the water in daylight.

Note also that most of the underwater light is internally reflected by the bottom surface of the aquarium.  This does not happen in lakes and ponds because the bottom is not optically flat.  When the bottom reflected ray reaches the top surface of the water, it is again divided into an internally reflected ray (barely visible) and an exiting ray. 

The mottled appearance of the photo is due to myriads of air bubbles that collect on the walls of the aquarium.

The Law of Refraction is diagrammed in the drawing below.  All the angles of the rays are measured from the dotted lines that are perpendicular (normal) to the water interface.  Aa is the angle in air, Aw is the angle in water.  Snell's Law of Refraction predicts the relationship between the two angles as

where n is the index of refraction between water and air.  One may examine the law of refraction by playing the excellent applet by Walter Fendt on refraction of light (requires Java enabled browser).

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. 

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