03 Lloyds Mirror

03 Lloyds Mirror#

Aim#

To show the interference of two coherent beams of light.

Subjects#

  • 6D10 (Interference From Two Sources)

Diagram#

Diagram

Equipment#

  • Laser

  • Simple lens (we use \(+10 \mathrm{~mm}\))

  • Surface mirror

  • White screen/wall

Presentation#

The room is darkened and the laser is switched on. By means of the \(+10 \mathrm{~mm}\)-lens an illuminated disk is projected on the white screen. The surface mirror is placed parallel to the diverging light beam (see Figure 623A)

../../../../../_images/figure_0246.png

Fig. 623 .#

and then turned just a little, so that the outer rays of the beam are reflected (see Figure 623B). In the light spot on the wall the fringes are visible now.

Explanation#

A portion of the wavefront is reflected from S (see Figure 624).

../../../../../_images/figure_1199.png

Fig. 624 .#

The other portion proceeds directly to the screen. Interference occurs in the region where the two portions are superimposed \(S\) and its mirrorimage \(S_{1}\) can be considered as separate coherent sources, placed a distance \(a\) apart. Then the separation ( \(\Delta y\) ) between the fringes is given by \(\Delta y \approx \frac{S}{a} \lambda\) ( \(s\) being the distance between the plane of the two sources and the screen).

Remarks#

In the demonstration the distance between the fringes can be enlarged by placing the screen not perpendicular but more parallel to the beam.

Sources#

  • Hecht, Eugene, Optics, pag. 391-392

  • Leybold-Heraeus, Physikalische Handblätter, pag. DK 535.412;b

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