More fringes can be shown with a laser, but it is interesting to show that ordinary white light does interfere. Lloyd's mirror shows fewer "extraneous" diffraction effects than other wave-front splitting interferometers. An important point here is to show that a phase change occurs on reflection from the glass surface of the mirror. This is hard to show because the center (plane of mirror) is difficult to determine. However, it is shown indirectly by the sequence of colors in the white light fringes: the first easily visible color separation is the first order dark fringe, red-black-blue going away from the center.

White light fringes are shown in the photograph at the left below and the close-up in the center. Placing a red fileter in the light path allows you to see more fringes by only using a narrow band of red light. The actual pattern seen with the eye is considerably better than that displayed by the video, increasing visibility to several lines.

Projection on a ground glass screen is bright and pretty for individual viewing. Projection on large screen is dim and hard to see from distance. The color minicam with TV projector can be used for large groups.

This is sort of a dumb experiment, but it explains one of the nuisance effects which is often seen in optics demonstrations. Sometimes funny and undesired circles and patterns show up in your laser interference and diffraction experiments whenever you use a lens. This may be caused by interference of light reflecting off the two surfaces of the lens.

The lens is simply placed in the laser beam, creating the interference pattern on a distant screen (off in the distance, not shown).

A laser beam illuminates a fine mesh, with the resultant light passing through a slit onto a beam-splitting mirror. This beam can be viewed directly or it can be focussed by a 20 cm focal length convex lens through a 10X microscope objective lens which displays the result on a screen. Two demonstrations are performed with this setup:

Demo 1: Place the mesh in the laser beam and project its image on the screen at about ten feet with the slit wide open. Flip in the mirror to see all Fourier components. Close the slit until only the central line of dots are seen, then flip in the mirror again. The image is a set of horizontal lines; the vertical lines have been filtered out. See how the images change when the slit is slowly opened and closed.

Demo 2: Rotate the mesh 45 degrees with the slit open, so the image rotates 45 degrees. No horizontal or vertical lines are observed. Flip in the mirror to show that the Fourier components have also rotated. Close the slit until only the central line of dots from Demo 1 are passing through, then flip in the mirror. Observe that the image is a set of horizontal lines. Observe changes in the image when different portions of the Fourier components are passing through.

Cautions: (1) The location of the slit must be far enough from the mesh that far field diffraction has occurred. (2) The position of the microscope objective with respect to the lens and the screen is very critical. Adjust this position carefully until a reasonably sharp image of the mesh is projected onto the screen.

The beam from a laser, expanded to 2-3 cm in size, illuminates a slide containing several points with a line fit to them on a crossed-line grid. The spatial filter is used to remove the grid, leaving only the points and line, as shown in the photograph above.

For this experiment a 10 cm focal length projection lens is used to focus the image of the slide onto a ground glass screen at the right in the photograph which is viewed by a TV camera to display for the class. A crosshair positioned on an x-y adjustment gizzit is moved into position (further from the projection lens than shown in the picture) to filter the grid.

The expanded laser beam illuminates a cloth mesh. The laser beam is focused onto a slit by a 12 cm focal length projection lens. The 5 cm focal length lens images the slit onto a screen.

The image of the of the mesh appears on the screen in the absence of L2, as seen in the photograph at the left below, but its Fourier transform (the Fraunhofer diffraction pattern) appears at the focal plane of L1. By inserting L2 it is possible to image the slit and therefore the Fraunhofer diffraction pattern on the screen. Closing the slit blocks successively lower Fourier components, removing the vertical lines of the mesh, as seen in the photographs at the center and right.

This demonstration uses the technique of dark field imaging to see a fingerprint on a microscope slide.

The laser beam is defocused using an 8 mm convex lens so that a uniform spot falls on a screen (at the right in the picture, not shown). A fine wire mesh placed to the left of the 12.7 cm focal length projection lens is focused on the distant screen. The laser beam is focused by the projection lens at a point where a vertical razor edge is positioned on a horizontal cross carriage. The mesh is removed and replaced by a microscope slide with a fingerprint on it; to make the fingerprint touch your finger into vaseline, then wipe most of it off so that you get a nice clear fingerprint on the slide. Adjust the razor edge so that the direct light from the laser is cut off, leaving only the light scattered by the fingerprint, which forms an image of the fingerprint on the screen.

This can be seen nicely using a ground glass screen and TV camera.

First focus the image of the slide on the screen as shown above. By slight defocussing, you get regions of reversed contrast (light and dark reversed) and rings where there is no image, corresponding to a representation of the optical transfer function's dependence on spatial frequency (Fourier space). This is seen in the photograph at the right as slight bending of the black lines toward the center of the figure.

Light from a bright point source with condenser and iris passes through a yellow filter and two crossed polaroids onto a distant screen. The quarter-wave plate is then positioned between the two crossed polaroids and rotated to any angle except 45 degrees with respect to the crossed polaroids. When the analyzer is rotated periodic minima and maxima of illumination appear on the screen, because the beam is elliptically polarized, and no position of the analyzer will produce darkness. When the polarizer and analyzer are crossed and the quarter-wave plate is set at 45 degrees, rotating the analyzer does not produce any changes in the intensity of the beam. This is the effect of circular polarization.

Note that the yellow filter is used because the thickness of the quarter wave plate is based on the wavelength of yellow light.

Light from a bright point source with condenser and iris passes through a yellow filter and two crossed polaroids onto a distant screen. The half-wave plate, consisting of two identical quarter-wave plates, is then positioned between the two crossed polaroids at the angle which produces the greatest illumination on the screen. Remove the plate and cross the polaroids. The half-wave plate is then inserted set at the angle for darkness. The plate is rotated somewhat less than 45 degrees, producing some light. The analyzer must be rotated by twice the angle of the plate rotation to again produce darkness.

The quarter wave plates have thickness chosen for yellow light.