A video camera is optionally available to make this more visible in large lecture halls.

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.

(1) Depth of field: Open the lens aperture with the Fresnel spotlight off, set the camera to full zoom (100), and place the camera at the closest distance at which it is possible to focus on the nearest can. Focus on the middle can, and observe the focusing of the extreme cans (photograph at right). Then turn on the spotlight so that it illuminates all of the cans and close the f-stop so that the lighting level is the same as before. Observe that the depth of field increases, because the increase in light level allows use of a smaller aperture (photograph at left).

Field of view: Vary the zoom and observe the change in the size of the field of view.

A lens at the left (out of picture) produces parallel light, as from a star. The second lens (at left in photograph) is the telescope objective, and the lens at the right is the eyepiece. The parallel beam of rays entering the objective lens is wider than that emerging from the eyepiece, indicating the ability of the telescope to "gather" light. When the position of the "star" is changed (photograph at right) by moving the source up (rays entering the telescope at a downward angle), the image is moved down and the angle at which it is viewed (inverted) is magnified relative to the incoming light, indicating the ability of the telescope to increase the angular separation of the stars it is viewing.

Choice of slit baffle and distance of baffle from source determine the number of rays and their spacing.

The TV camera functions as the eye, so what the eye sees can be viewed on a monitor or the rear projection screen. At the left below is a direct view of the object scale by the video camera as displayed on the monitor. A 10 cm or a 5 cm convex lens acts as the eyepiece and a 20 cm convex lens is the objective lens for the telescope, with the TV camera focused at infinity (relaxed eye). The rule is positioned across the lecture hall (at infinity) from the telescope. The image is seen to be inverted and to have a magnification M=f(ob)/f(eye). The magnification can be checked by viewing a two-meter stick with the TV camera directly (left) and then with the telescope. The image with a 10 cm eyepiece has a magnification of 20/10=2, seen in the center picture. The image with a 5 cm eyepiece has a magnification of 20/5=4, seen in the picture at the right.

To set up in practice, set the camera focus and lens positions and move the objective lens until the image is seen by the camera.

The TV camera functions as the eye, so what the eye sees can be viewed on a monitor or the rear projection screen. A 10 cm or 5 cm convex lens acts as the eyepiece, a large 20 cm convex lens is the objective lens, and a 10 cm convex inverting lens between the objective and the eyepiece, with the TV camera focused at infinity (relaxed eye). The rule is positioned across the lecture hall (at infinity) from the telescope. The image is seen to be erect and to have a magnification M=f(ob)/f(eye). The magnification can be checked by viewing a two-meter stick with the TV camera directly and then with the telescope.

The photographs below show the image with no telescope (left), using a 20cm focal length eypiece (M=2), and using a 10cm focal length eyepiece (M=4). Vignetting is seen in both telescopic images, and is very obvious in the center image.

To set up in practice, set the camera focus and lens positions and move the objective lens until the image is seen by the camera. The inverting lens is used with equal object and image distances (twice the focal length), and provides no magnification.

The TV camera functions as the eye, so what the eye sees can be viewed on a monitor or the rear projection screen. A 10 cm or a 5 cm concave lens acts as the eyepiece and a large 20 cm convex lens is the objective lens, with the TV camera focused at infinity (relaxed eye). In this case the image of the objective lens is on the left side of the concave eyepiece - behind the eyepiece. The rule is positioned across the lecture hall (at infinity) from the telescope. The image is seen to be erect and to have a magnification M=f(ob)/f(eye). The magnification can be checked by viewing a two-meter stick with the TV camera directly and then with the telescope. Compared with the astronomical telescope, the image is more faint and has a smaller high-quality field of view.

The images below show the object rule viewed directly by the camera "eye," the telescope image with a 10cm fl eyepiece (M=2) and the telescope image with a 5cm fl eyepiece (M=4).

To set up in practice, set the camera focus and lens positions and move the objective lens until the image is seen by the camera.

Background

Collimated light waves come from the laser and pass through a pair of narrow slits in the slide; the light passes through and then projects on the distant screen. But light travels as an electromagnetic wave, so when the light comes out of the two slits, it forms two wavefronts, just like ripples from two stones dropped in a pond. These two wavefronts can interfere with each other, as we can model with this pair of overlapping concentric circles. Where two peaks or two valleys of the wave pattern line up, they add together, interfering constructively; when a peak and a valley overlap, they cancel out, interfering destructively. The same happens with light waves; the light from the two slits overlaps, and creates a pattern of bright spots (constructive interference) and dark spots (destructive interference). The spacing between the bright and dark fringes ultimately depends on three things: the distance between the slits and the screen, the wavelength of the light, and the spacing between the two slits.