Instructors_Guide_Ch22

# Instructors_Guide_Ch22 - 22 Wave Optics Recommended class...

This preview shows pages 1–2. Sign up to view the full content.

22 Wave Optics Recommended class days: 3 Background Information Wave optics—the interference and diffraction of light—is a continuation of Chapter 21, where interference was introduced. Now, while these ideas are still fresh in students’ minds, we want to extend them to the domain of optics. That is one reason for placing wave optics before ray optics. Another is the research finding that students have a difficult time distinguishing the domain of wave optics from the domain of ray optics. Teaching wave optics first will allow us to establish a clear criterion for when each is appropriate. In addition, this ordering allows us to treat Rayleigh’s criterion for the resolution of lenses as a fitting conclusion to the chapter on lenses. The primary research on student understanding of interference and diffraction is from the McDermott group (Ambrose et al., 1999). They found that many students have serious difficulties understanding the basic features of the ray model and the wave model of light. Student explanations of interference and diffraction phenomena tended to be a confused and undifferentiated mixture of features from both models. Even the strongest students in the class had significant conceptual difficulties, and these were found to persist among physics majors in sophomore- and junior-level courses. In particular, their research has found that: • After studying wave optics, many students treated all apertures, regardless of width, as narrow slits. These students drew pictures of Huygen waves spreading out from 1-cm-wide apertures. This is a misapplication of the wave model in the domain of ray optics. • On a post-instruction exam, only 20% of students correctly predicted with correct reasoning that the minima in a single-slit diffraction pattern would move further from the center if the slit were narrowed. Some students misapplied ray-optics reasoning to predict that the minima would move in. Others made a correct prediction, but their reasoning was incorrect and based on incorrect models of light. • One group of students employed a hybrid model in which they interpreted the diffraction maxi- mum as being the geometric image of the slit, and they attributed the fringes to “edge effects” of the slit. One student stated, “Light that strikes the edges will be diffracted off.” • Many students think that no light will pass through a slit if its width a is less than the wavelength λ . They state that the light will not “fit” through the slit in this situation. These students did, however, seem to recognize that wavelength is measured along the direction of propagation, perpendicular to the slit dimensions. • Other students thought that diffraction occurs only if a < . One obtains a geometric image of the slit if a , but the light “has to bend in order to fit through the slit” when the width is less than the wavelength, and this causes diffraction. • Students were shown a two-slit interference pattern and asked to predict what would happen

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## This note was uploaded on 01/14/2011 for the course CD 254 taught by Professor Kant during the Spring '10 term at Central Oregon Community College.

### Page1 / 8

Instructors_Guide_Ch22 - 22 Wave Optics Recommended class...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online