Lloyd's mirror

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Lloyd's mirror is a classic optics experiment and was first described in 1834 and again in 1837 by Humphrey Lloyd in the proceedings of the Royal Irish Academy of Science. In the experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come from a virtual source as a result. The reflected light interferes with the direct light from the source, forming interference fringes.[1][2]

Setup

Figure 1. Lloyd's mirror
Figure 2. Young's two-slit experiment displays a single-slit diffraction pattern on top of the two-slit interference fringes.

Lloyd’s Mirror is used to produce two-source interference patterns that have important differences from the interference patterns seen in Young's experiment.

In a modern implementation of Lloyd's mirror, a diverging laser beam strikes a front-surface mirror at a grazing angle, so that some of the light travels directly to the screen (blue lines in Fig. 1), and some of the light reflects off the mirror to the screen (red lines). The reflected light forms a virtual second source that interferes with the direct light.

In Young's experiment, the individual slits display a diffraction pattern on top of which is overlaid interference fringes from the two slits (Fig. 2). In contrast, the Lloyd's mirror experiment does not use slits and displays two-source interference without the complications of an overlaid single-slit diffraction pattern.

In Young's experiment, the central fringe representing equal path length is bright because of constructive interference. In contrast, in Lloyd's mirror, the fringe nearest the mirror representing equal path length is dark rather than bright. This is because the light reflecting off the mirror undergoes a 180° phase shift, and so causes destructive interference when the path lengths are equal or when they differ by an integer number of wavelengths.

Underwater acoustics

An acoustic source just below the water surface generates constructive and destructive interference between the direct path and reflected paths. This can have a major impact on sonar operations.[3]

See also

References

  1. ↑ Fresnel's and Lloyd's Mirrors
  2. ↑ "Interference by the Division of the Wavefront". University of Arkansas. Retrieved 20 May 2012. 
  3. ↑ Carey, W. M. (2009). "Lloyd's Mirror—Image Interference Effects". Acoustics Today 5 (2): 14. doi:10.1121/1.3182842. 

Further reading

External links

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