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This java applet generates Fresnel diffraction patterns, which are patterns similar to the shadows cast by small apertures when using a point source of monochromatic light as a light source.

When you start up the applet, a circular aperture will be used. This is the pattern generated when a plane source of monochromatic light is shined on a small circular hole in an otherwise opaque object. The red line is the outline of the geometrical shadow (the shadow which would be cast if there were no diffraction).

You can select a different aperture shape (such as a half plane, double slit, triple slit, rectangle, etc.) from the Aperture popup. Or you can change the size of the circle by dragging the red line with the mouse.

The diffraction pattern generated depends on the size of the aperture compared to the wavelength of the light source. To change the size of the aperture, you can drag the red line; or, you can change the Aperture Scale slider, which will modify the scale of the aperture without changing the scale of the image. If you move the slider to the right, this will make the aperture larger relative to the wavelength of the light; this reduces the effects of diffraction. If you move it to the left, it will make the aperture smaller, which increases the amount of diffraction.

The Zoom slider will modify the scale of the image, without changing the scale of the aperture compared to the wavelength.

The Brightness slider can be used just like the brightness on a TV, to brighten dim areas on the screen. Often times you may see large, boring areas of black or white on the screen which change into interesting patterns when the brightness is turned to just the right setting.

The Image Resolution slider can be used to generate a higher quality image. If you move it to the right, it will increase the resolution of the image, but it will also slow things WAY down, so be careful.

The Set to Defaults button will reset all the settings to the default, which can be a quick way to recover if you set the Image Resolution slider too high and the applet has slowed to a crawl.

The Tri-Chromatic checkbox will cause the light source to emit red, green, and blue light. These three colors have different wavelengths, which means they generate different diffraction patterns. The applet will generate the three patterns and combine them to create a color image. This checkbox makes the images a lot more interesting but it also slows things down a bit.

The Reversed checkbox will reverse the aperture so that transparent areas become opaque and vice versa. So if you have the default setting (a circular hole in an opaque object) and check this checkbox, you will wind up with a circular obstacle in the path of the light source. This is not the same as reversing the colors on the image.

The Show Dimensions checkbox will tell you the width of the aperture (and its geometrical shadow). The point of observation is four meters away from the aperture. The wavelength of the light is 510 nm for a monochrome image, and for a color image the wavelengths are 650 nm (red), 510 nm (green), and 475 nm (blue).

It may be unclear what dimension "width" refers to for some apertures. The width of the half plane or corner is the distance from the left edge of the aperture to the right side of the screen. The width of the double slit is the distance from the left end of the left slit to the right end of the right slit. The width of the cross is simply the width of the screen.

This algorithm used by the circular apertures in this applet is from a paper by Dean Dauger called "Simulation and Study of Fresnel Diffraction for Arbitrary Two-Dimensional Apertures", published in Computers In Physics, Nov/Dec 1996. More info about Fresnel Diffraction, including a Mac application to generate diffraction patterns, can be found on his page.

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