Circular fringes can be produced from a gently defocused laser beam showing very nice spherical two-beam interference. Align the mirrors first without the collimator and lens, so that the beams from the two mirrors overlap on the beam splitter and screen. Then add the collimator and the lens. The photographs at the center and right above were taken with (center) and without (right) a neutral density filter.

Block either path and the fringes disappear. Room vibrations cause interesting fringe motion. A finger on the mount can move the pattern by a single fringe. A finger on the edge of one path shows heating of the air.

The gas cell is positioned in one arm of the interferometer between the beam splitter and the moveable mirror such that the surfaces of the cell are perpendicular to the laser beam. After obtaining a clear set of fringes, pump some of the air out of the cell. Measure the pressure before and after and the number of fringes passed while pumping. The refractive index of the gas varies directly with its density, and the index of refraction of a vacuum is one. Thus a graph of refractive index as a function of pressure must go through the point x=0, y=1. Determine the slope of the line by calculating the change in the refractive index: if n1 is the index at the initial pressure and n2 is the index at the final pressure, then n1-n2=mL/2d, where m is the number of fringes counted, L is the wavelength, and d is the length of the gas cell (only the gas region, not the end plates).

The final photograph above is a detail of the fringe pattern. In the video linked below, you can see the fringes shift as air is pumped from the gas cell. You can hear the clicks from the hand pump as it removes air from the cell.

A wire cookie cooler is used as the beam splitter in a microwave Michelson interferometer. Two aluminum plates function as the mirrors on the two legs. As either of the plates is moved along the optic axis the intensity of the beam at the location of the receiving antenna varies, with the intensity displayed on an overhead projector meter. The wavelength of the microwaves is about 12 cm, so a change of about three cm is required for the two beams to change in or out of phase.

The sequence of pictures above show the reflector at the right being withdrawn by intervals of one-quarter wavelength, resulting in alternating minima and maxima of the recombined microwave beam at the location of the receiving antenna.

Light reflecting from the rear surface of the front plate (left) and the front surface of the rear plate (right) interfere. When the reflected waves are in phase that color will be focused on a screen to the left of the picture by the large lens. When the reflecting waves are out of phase no reflected light is seen. The light baffle at the left prevents direct light from the source from getting into students' eyes. Flex the two glass plates to vary their spacing. A thin piece of paper or a hair can be squeezed between the plates at one end to produce a wedge filter.

The photograph above shows the interference pattern.

This is a dielectric coating with an index of refraction between that of air and glass, that covers one-half of the glass plate. If the coating is one-quarter wavelength thick for yellow light it prevents reflection of yellow light because the reflections from the two surfaces are exactly out of phase.

Light from a bright point source with a condenser lens and iris is focused by a 20 cm focal length convex lens through a glass plate onto a distant screen. In the photos above the glass plate reflects some light which is reflected a second time by a front surface mirror to form a spot to the right of the direct beam from the bright point source.

With no anti-reflective coating (left above) the direct beam is less intense because of the reflected beam. When the anti-reflective coating is raised into the beam (right above) the direct beam is more intense and the reflected beam is less intense. When the yellow filter is used the beam reflected by the anti-reflective coating is slightly magenta colored, because the thickness of the coating is not quite one-quarter wavelength for the extreme colors of the spectrum.

These filters are glass plates that contain a series of equally spaced high refractive index dielectric layers. The layers reflect light of a particular wavelength, leading to destructive interference, so the reflected and transmitted light are complementary colors. The filter is labeled by the transmitted color, so a red filter transmits red and reflects cyan, a yellow filter reflects blue, a green filter reflects magenta, and a blue filter reflects yellow. To facilitate re-mixing and display, the spacing of the reflective layers in these filters are designed for light incident at 45 degrees.

In the photographs above the direct beam is at the left and the reflected beam is at the right. The reflected beam from the red filter (left) is a bit saturated, so the cyan coloring is not readily visible.

A Bright point source of light at the right illuminates a soap film in the wire ring. The transmitted light is seen at the left and the reflected light at the rear between the light source and the wire ring. Note that the reflected and transmitted colors are complements, although the transmitted light is desaturated by the white light background, as seen above.

After a while the top section of the film becomes less than one-quarter wavelength thick, so the waves reflected from the front and rear surfaces of the film are out of phase, resulting in no reflected light.

The rotation of the soap film produces a radial thickness gradient, which leads to rings of color around the soap film. The center becomes black when it is less than one-quarter wavelength thick.

Also see the close-up photographs of the soap film geometrical figures.

Soap film interference is created in a large rectangular area, shown above for sodium light and for white light, using the apparatus photographed below. The light sources are inside the large wooden box, and reflect off the screen to yield a very uniform, broad source. The interference patterns created are very large and possess extraordinarily beautiful saturated colors.

NOTE: The audience views this demonstration from the left in the photograph of the equipment to see the patterns in the photographs at the top above.