This article describes the physics project I was involved in.

Interferometer page 2

Interferometry: Measuring Displacement on a Molecular Level

Our project is to very precisely measure static frictional forces between various materials. The high precision comes in using a Michelson-type interferometer.

An interferometer overlaps two beams of same-frequency laser light. This creates an interference pattern due to the wave nature of light (Fig. 2). A laser pointer is shown above (Fig. 1), which is replaced by a Helium-Neon laser for the actual experiment, in order to gain more significant digits for the wavelength. As shown, the beam is first passed through two lenses. The only purpose for this is to make the laser beam diverge and ultimately create a larger beam for viewing. After the lenses, the beam is split into two partial beams using a half-silvered mirror (a beam splitter). Each partial beam is bounced from a mirror back onto the beam splitter so that they overlap and project as a spot of light on a viewing surface (in the direction of the sheet of paper in Fig. 1). One mirror holds a fixed position and creates a reference beam. The other test mirror is attached to a movable block. The surface under the block can be changed, and a measured force can be applied to it. Static frictional forces act to hold the block in place, but not without allowing some movement. When the force is let off, the block returns to its original position as if held in place by springs.

The interference pattern formed has the appearance of tree rings, with alternately bright and dark circles. What we are interested in is the central ‘dot’ which may be initially bright or dark. When the test mirror is shifted forwards or backwards, the central dot will grow or shrink. Assuming the central dot was initially bright, as it grows it will turn into a ring and a dark dot will appear at its center. As this dark spot grows, it also turns into a ring with a bright spot at its center, this pattern alternating. As the test mirror is shifted back to its original position, the rings will converge into alternately colored central dots. The appearance of each alternate-color dot corresponds to the test mirror having shifted its position by a quarter-wavelength of light – that is, 1/4 of 632.8 nanometers for the Helium-Neon laser ultimately used. This is roughly just over 1,100 times the diameter of a carbon atom!

Interferometer page 2