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
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
The mottled appearance of the photo is due to myriads of air bubbles
that collect on the walls of the aquarium.
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
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
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 firstname.lastname@example.org.