Vocabulary. The Wavelength of He-Ne Laser Light

The Wavelength of He-Ne Laser Light

(See Adams and Allday, p 256)

IMPORTANT: Before commencing this experiment, read the following safety notes.

Laser safety notes

1. Do not look straight into the laser beam. Permanent eye damage can result.

2. Do not look straight at the beam after it has been reflected from a metal surface or after it has passed through an optical system without losing its intensity.

3. Never look straight into the beam though a focussing element such as a lens.

4. Avoid prolonged looking at the beam directly scattered from a matt (non-shiny) surface.

5. The lasers used in this experiment cause no damage if the direct beam falls on your skin or clothing. However, it is good practice to avoid this situation as other lasers do cause damage.

6. Turn off the laser when not being used.

7. If you are unsure about any aspect of the safe use of a laser, speak to your teacher or technical staff.

Introduction

When waves from coherent sources overlap a phenomenon called interference occurs. In the region of overlap waves add together algebraically. Thus if a peak from one wave coincides with a peak of another, the waves reinforce each other and constructive interference is said to take place. However, if the peak of one wave exactly coincides with a trough of another wave the waves can cancel each other out and destructive interference is said to occur. If the waves correspond to visible light, we observe constructive interference as a region of brightness and destructive interference as a region of darkness.

A diffraction grating consists of many closely spaced slits usually obtained by ruling closely spaced lines on a small piece of glass. When light falls on such a grating each slit behaves as a secondary source of light – and an interference pattern of bright and dark lines (often called fringes) appears on a screen placed on the other side of the grating. The same occurs for reflection of light from lines ruled on a reflecting surface.

In this experiment we shine pure coherent intense red light from a Diode laser on to the closely spaced divisions of a metal ruler. As a result, light is not just reflected but forms an interference pattern consisting of a number of bright spots of light which will be displayed on a screen. (Although we use the word diffraction grating, it is really interference rather than diffraction that we are observing).

Vocabulary

Interference, constructive, destructive, slit, diode laser, diffraction grating, reflection grating, beam

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