Wave Nature of Light

Wave Nature of Light

Objective:

The purpose of this lab is to investigate interference, otherwise known as the diffraction of light. A beam of light acts a wave, and we are able to use equations so calculate the wavelength of the light used. The diffraction of a straight edge demonstrates that light waves bend around straight edges, allowing light to enter an area of shadow. When waves are superposed, they reinforce each other when crests are in phase and cancel out when they are not in phase. An image projected by light waves shows this by producing concentric circles of light. When slits are used, the laser superposes itself and creates constructive interference. The resulting spots are measured to give data that allows one to find the wavelength using the equation λm = dsin θ. These diffraction and interference with such slits was successful because of low percent errors in the wavelengths calculated. Additionally, a diffraction gradient creates better-defined spots, and more accurate measurements, yielding the lowest uncertainties.

A. Diffraction by a Straight Edge

Using the slide marked "straight edge/opaque disks," a laser is positioned so that it falls on the edge. The light does go into the shadow region, it bends around the edge and spreads out into the shadow region. The picture below shows what happens when the laser beam falls on a straight edge.

B. Superposition of Waves and Diffraction by a Disk

A 48mm focal length lens and opaque disk slide are used to observe

superposition and the diffraction patterns. Due to symmetry, all waves that arrive from the edge of the disk at the same point on the axis will be in phase, causing a bright spot on the center of the pattern. The first disk used created concentric circles of bright light. The next disk produced a dark circle in the center. The pictures below show the patterns of diffraction.

C. Diffraction by a Narrow Slit

The slide with several narrow slits is placed in the holder, and this causes

diffraction fringes. These fringes are caused by the interference of wavelets from the various areas of the slit. An equation relating the first dark band to the wavelength and angular position is given by asinθ = λ.