Interference of Plane Waves
Using a Fresnel mirror, we let two plane waves interfere on a screen. These waves propagate at a small angle relative to each other.
Experimental setup: He-Ne laser, lens f = +5 mm, Fresnel mirror, screen (not shown in the photo).
Detail of the Fresnel mirror. A laser beam expanded by the converging lens strikes the boundary between two mirrors, the right one being adjustable.
Top view of the Fresnel mirror. On the right is a screw used to adjust the angle between the mirrors.
Interference pattern on the screen. Two plane waves interfering at a small angle create vertical interference fringes. Other patterns are due to diffraction on mirror edges, dust on the lens, etc.
Image on the screen when the reflected waves do not overlap. Vertical fringes arise from diffraction at the edges of the mirrors. These do not change with the angle and are not a demonstration of interference.
As the angle between the mirrors increases, the reflected waves begin to overlap, producing interference fringes. Their spacing is inversely proportional to the angle.
Increasing the angle between mirrors results in denser interference fringes.
Increasing the angle between mirrors results in denser interference fringes.
Increasing the angle between mirrors results in denser interference fringes.
Increasing the angle between mirrors results in denser interference fringes.
Increasing the angle between mirrors results in denser interference fringes.
Increasing the angle between mirrors results in denser interference fringes.
Increasing the angle between mirrors results in denser interference fringes.