Contrived C1

John O’Neill
Teacher, Swifts Creek Secondary College

LASER on Sunglass Lens

Ideally lenses, such as those in eyeglasses, should be clean and smooth in order to allow an unimpeded view of the world. With wear, glasses typically pick up small scuffs and scratches, leading to diminished clarity. This photo shows one reason for this.
A monochromatic red laser at 633nm produces a coherent light beam, which can be seen here as the bright central spot on the relatively unscathed sunglass lens. With a pristine lens the beam essentially passes straight through and appears as a red spot on the screen the same size as the spot observed on the lens.
However, in this case it hits a small scratch on the lens surface, resulting in a characteristic diffraction pattern on the screen, with a maximally bright central fringe where the single spot would otherwise appear, and spreading alternating dark and bright fringes resulting from destructive and constructive interference of the light waves.
Random scratches on the lens surface are not as precise as the slits used to produce diffraction patterns in ideal physics laboratory situations, thus the pattern is not as pure as those produced by standard demonstrations. In this ‘real world’ example we see other less prominent diffraction patterns, and also tiny red speckles all over the screen caused by scattering of the laser light.
An eye viewing the world through this lens would have a less than perfect view due to the combined effects of this scratch and many other small imperfections on the lens.