Why Are There Two Different Equations for Light Diffraction?

General Diffraction Explained

• Light is made up of waves. Diffraction of these waves occur when light deviates from its straight path, either by going through an opening, around an obstacles, or past a sharp corner. Diffraction grating occurs when waves are sent in multiple directions. One example of this is rotating a CD. The regular grooves on the CD diffract the light into multiple wavelengths of color.

Two Equations

• The best way to consider how waves have different characteristics at different distances from their source is to consider a simple illustration. Drop a ball in a pool and observe how the waves spread away from it. Draw that on paper, with a circle for the ball and concentric rings around it representing the waves. Draw two parallel lines through the waves. Notice how much more curve the wave segments between the lines have near the ball compared to father away from it, where the segments are nearly straight.

Fresnel Diffraction

• The Fresnel Diffraction Equation measure the diffraction of waves that are near their source, or near-field diffraction. Specifically, Fresnel Diffraction is equal to the squared size of the aperture --the object causing the diffraction -- divided by the distance from the observation point to the aperture times the wavelength of the wave. From this basic equation many variations are derived to measure certain aspects of near-field diffraction under different conditions.

Fraunhofer Diffraction

• The Fraunhofer Diffraction Equation measures the diffraction of waves that are planar, which means that they are far enough from their source that their curvature is able to be negated. The waves must be close to parallel by the time they reach the aperture. The equation measures the intensity of the diffraction at any given angle using the sinc function. It also has many variations, depending on the different characteristics of the aperture, such as its shape.