Rayleigh's Criterion

What it shows:

The criterion for the resolution of two sources is that the central maximum of the single slit interference pattern of one source falls on the first minimum of the pattern of the second source.

How it works:

Each laser beam passing through the slit will form a diffraction pattern on the screen. With the aperture closed down, the pattern will be spread out and the central maxima of both sources will overlap giving a blurry image. Opening up the aperture and the diffraction patterns will get narrower, until the point where the Rayleigh criterion is met and two images are clearly seen.

figure 1. Configuration of laser resolution (looking down).

rayleighs criterion

rayleighs criterion

Setting it up:

The two 5mW HeNe laserstot are mounted at the same level and side by side on an optics bench. To do this, one mount will have to be laterally adjustable or both be offset from the rail axis (Alternatively, use one 12mW HeNe in conjunction with a 50% beam splitter and a mirror). The beams should overlap each other at the slit face. The slit aperture is on the other side of the hall on a second optics bench, and a little beyond it the projection screen. Open up the slit so that the two spots are clearly resolved on the screen, then close it down leaving only a soft red blur. Always start the alignment from the laser end. A camera with zoom lens will be needed to see the images on the screen.


Rayleigh's Criterion is usually applied to circular apertures, where the criterion for resolution is when the center of one Airy disk pattern falls on the first minimum of the Airy disk pattern of the second source. The angular separation θ is then

θ = ( 1.22 λ ) / D

where λ is the wavelength of light, and D the aperture diameter. This experiment uses a linear aperture, which changes the constant in the above equation. Used only qualitatively.