An arrow/circle object is placed in front of a bright light source, and this object is focused onto a distant screen by a 20cm convex lens, as seen in the photograph at the left. Inserting the prism inverts the image IN THE VERTICAL DIRECTION ONLY, as seen in the photograph at the center. Note that a pentaprism is used to invert the image in both directions simultaneously.

The optical arrangement is shown in the photograph at the right.

This technique is sometimes used in optical instruments.

A clear plastic tank with a plugged spout is elevated above a second, shallower tank. The upper tank is filled with water. A laser is aligned so that it passes through the upper tank and is centered on the spout.

When the spout is unplugged, the water streams out into the lower tank. Several internal reflections of the laser beam should be visible in the outgoing stream of water, down to the point where it becomes too turbulent to see clearly.

As the water level in the tank drops, the water flow becomes so slow that the stream bends too sharply and there is no internal reflection, and the laser beam ceases to follow the flow of water.

Background:

The laser here is illustrating internal reflection: depending on the index of refraction of the water and the angle the light hits it at, more light can be reflected back and forth within the stream of water than passes through it. At a certain point, it exhibits total internal reflection, where essentially all of the light is traveling along the stream rather than heading straight out the side.

This demonstration can beneficially be used in combination with demonstrations of fibreoptic technology such as L5-13 or L5-23.

One end of the fiber optics strand is held pointed into a laser beam, and the other end held by a student who is free to move about. The light emerging out the other end can easily be seen over the entire lecture hall, even when some of the cable is coiled up.

Sections of fiber optics cable are used which were left over from the installation of fiber optics cabling in underground conduits all over the University of Maryland campus. These cables are now being used for both data and video transmission. The fiber optics strands below have adapters so that they can be plugged into an optical source directly.

Electromagnetic boundary conditions across a totally reflecting surface lead to a wave that dies off exponentially (within a few wavelengths) beyond the surface, but does not carry energy across it. However, this wave can be transmitted if it is made to interact with dense matter sufficiently close to the surface. The demonstration shows the hypotenuse face of a right angle prism acting like a perfect mirror, except for a spot around the point of contact with the lens. If the angle of incidence (on the hypotenuse) is close to the critical angle (42 degrees), a set of Newton's rings appear around the "spot." These can be used to measure the distance of penetration.

Close one eye. Let the reflected light come from a light area, the transmitted light from a darker area, or vice-versa.

Electromagnetic boundary conditions across a totally reflecting surface lead to a wave that dies off exponentially (within a few wavelengths) beyond the surface, but does not carry energy across it. However, this wave can be transmitted if it is made to interact with dense matter sufficiently close to the surface. The demonstration shows the hypotenuse face of a right angle prism acting like a perfect mirror, except for a spot around the point of contact with a second prism. If the angle of incidence (on the hypotenuse) is close to the critical angle (42 degrees), a bright spot will appear where the two prisms close to within a few wavelengths, allowing the wave to tunnel through.

The spot can easily be seen on the overhead projector.

Electromagnetic boundary conditions across a totally reflecting surface lead to a wave that dies off exponentially (within a few wavelengths) beyond the surface, but does not carry energy across it. However, this wave can be transmitted if it is made to interact with dense matter sufficiently close to the surface. The demonstration shows the hypotenuse face of a right angle prism acting like a perfect mirror (light internally reflected upward onto screen), except for a spot around the point of contact with a second prism. If the angle of incidence (on the hypotenuse) is close to the critical angle (42 degrees), a bright spot will appear where the two prisms close to within a few wavelengths, allowing the wave to tunnel through.

The spot can easily be seen on the white screen.

A beam of 12cm microwaves reflects internally on the hypotenuse of a right angle paraffin prism. Bringing the hypotenuse of a second identical paraffin prism within a few cm of the first prism allows the wave to be transmitted straight through the two prisms.

The microwave sensing antenna can be located at the position of the internally reflected ray to show that this ray disappears when the second prism is moved close. Alternatively, the antenna can be located in the straight through direction to show that waves continue in a straight line when frustrated total internal reflection occurs, as shown in the photograph at the right above.

Note that with a single prism the evanescent wave extends a few wavelengths beyond the surface of the prism. The intensity of the microwaves decreases exponentially, so they are easily observed up to about 50cm from the exit surface of the prism.