Eyeglasses in the Classroom

摘要

Optical phenomena can be divided into two categories:ray optics and wave optics. The former is also knownas geometrical optics, and examples are reflection andrefraction, while the latter is also known as physical opticsand includes interference and diffraction. In most textbooks,these two topics are presented in separate chapters.' To helpstudents grasp corresponding principles, instructors oftenemphasize the contrast between these two categories. Forexample, the length scale of objects encountered by lightbeams is taken as an indicator to differentiate these two.When the scale is microscopic (down to the wavelength at550 nm for green light), the light shall be treated as a wave,while at a macroscopic scale (start from a few mm for a tinylens), the light shall be taken as a ray. Diffraction effectsare almost always negligible in the formation of images bylens, and refraction effects can also be neglected safely inthe discussions of interference. As a result, some studentsget the wrong impression that these two phenomena mustmanifest in quite different regimes. In this paper, we showthat eyeglasses can be a very handy tool to demonstrate thesetwo phenomena at the same time, working as a lens for lightrays and also showing interference of light waves from reflec-tions off the lens coating. According to a recent survey, morethan 80% of college students in Taiwan are nearsighted, andeyeglasses are easily accessible in the classroom. Studentsare guided to observe, describe, and then provide qualitativeexplanations. And some of the students might take the chal-lenge to work out quantitative calculations as exercises afterclass.