Home Optics Experiments PE-0500 Interference of light

Keywords:

  • Fresnel's Mirror
  • Newton's Rings
  • Fabry Perot Plate
  • Optical Resonator
  • Spatial / Temporal Coherence
  • Coherence Length
  • Fresnel's Zone Plate
  • Diffraction • Interference

 

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

 

Introduction

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PE-0500 Interference of light

Interference of light, although very common in almost all fields of optics, is not always easily observed, often overlooked or just neglected. When Isaac Newton did his investigations on optics, he described interference observations in detail. But only Thomas Young was on the right way of interpretation when he stated the general law of constructive and destructive interference of light in 1801 and when he gave the experimental prove of interference of light in 1802. His work had influential consequences in considering light as a wave. On one hand interference is an impressive way to demonstrate the wave character of light, on the other hand, many optical instruments are based on interference, e.g. instruments for exact determination of distances or the wavelength of light, so called interferometers. This experiments provides the famous Fresnel mirror dividing one light beam source into two and superimpose their coherent portions. As a must the Newton’s rings module demonstrates interference  caused by thin layers. Further on a Fresnel plate is used to illustrate imaging based on interference and finally, a Fabry Perot plate demonstrates the working principle of an optical cavity.

PE-0500 Interference of light

The laser beam of the green laser is expanded and hits the first mirror in grazing incidence in such a way that one halve of the beam is deflected by the first and the other halve by the second mirror. The second mirror is slightly tilted against the first one. Both beams are imaged via an expanding lens to a white screen. In the overlap area of both beams the famous Fresnel interference becomes visible.

Two flat mirror are aligned parallel to each other by a spacer forming a Fabry Perot cavity with a mirror distance d of 3 mm with a free spectral range of 50 GHz. The green laser is connected to the control unit allowing to precisely tune the temperature. Since the frequency change of the green laser is 4.5 GHz per °C we will observe the frequency change as shift of the interference fringes on the screen.

The classical experiment “Newton’s rings” comes with a combination of a glass plate and a plano-concave lens. It is illuminated by means of a green laser whose beam diameter is enlarged by a beam expander. The interference pattern is imaged onto a white screen. The figure shown on the left shows the setup for observing the fringes in transmission. For observing in reflection an image screen with a small centre hole is used, which is located  behind the  beam expander.

It is amazing to learn that an arrangement of concentric rings shows almost the same property as a lens. Such an arrangement, a Fresnel zone plate is designed in such a way that it will create a lens like image with a focal length of 60 mm. The plate is illuminated by means of a green laser whose beam is enlarged by a beam expander.

PE-0500 Interference of light

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