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University students’ ideas about the role of the aperture and laser beam dimensions in formation of diffraction patterns

Successful application of the Huygens–Fresnel principle often requires reasoning about the interplay of aperture and light beam dimensions for purposes of identifying the unobstructed part of the light beam which is the source of secondary waves. Therefore we decided to identify university students’ ideas about the role of this interplay in the formation of diffraction patterns. We conducted a survey research with 191 first-year students from the Faculty of Chemical Engineering and Technology at the University of Zagreb, Croatia. They were administered six constructed-response questions in which aperture or laser beam dimensions were varied and students were expected to verbally and pictorially describe how these changes would affect the diffraction pattern. It has been shown that 63% of students think that a change in the length of the vertical slit necessarily results in a change of the diffraction pattern, even when the illuminated portion of the slit remains the same. In addition, it has been found that nearly 40% of students believe that in optical grating diffraction an increase of beam diameter leads to bigger diffraction fringes. A possible way to overcome some of these difficulties would be to insist on consistent application of the Huygens–Fresnel principle.