Why is it much more difficult to see interference fringes for light reflected from a thick piece of glass than from a thin film? Would it be easier if monochromatic light were used?

Imagine looking at a soap bubble and noticing the swirling colors glinting across its surface. This beautiful effect is an example of thin film interference, an optical phenomenon that occurs when light waves reflect off the very close, parallel surfaces of a thin film and subsequently overlap.

In thin films, such as oil on water or bubbles, the separation between the reflective surfaces is typically less than a few hundred nanometers. When light hits the film, some of it reflects off the top surface while some transmits through, reflects off the bottom surface, and finally emerges out of the top surface again. These two sets of light waves then combine, or interfere, to produce alternating bright and dark bands known as interference fringes.

The brightness or darkness of these fringes depends on whether the light waves from the two reflections are in phase (constructive interference) or out of phase (destructive interference). Since the path length difference in a thin film is relatively small, these effects are more easily observed, leading to the visible patterns that we can see. However, if the film's thickness varies, the colors and intensity of the interference pattern can change, due to the changes in the phase relationships between the light waves.

For interference patterns to be observable, the overlapping light waves must maintain a steady phase relationship, a quality known as coherence. Coherent light waves are synchronized; they crest and trough together over time. This coherence is essential for producing sharp, stable interference fringes.

Light sources like lasers produce highly coherent light, making them excellent for experiments involving interference. In contrast, most common light sources, such as the sun or a light bulb, emit incoherent light. The waves from these sources are not in phase with one another and are constantly shifting in phase relative to one another.

To visualize coherence, picture two rowers rowing in perfect unison across a lake—their oars hitting the water together and emerging at the same rate, their rowboats moving smoothly as one. This is analogous to coherent light waves traveling in the same phase. If the rowers are out of sync, similar to incoherent light, the interference effects are lost, and no clear pattern develops on the water.

Monochromatic light is the simplest form of light in terms of wavelength consistency—it consists of light of a single wavelength and color. The word 'monochromatic' breaks down to 'mono-' meaning single and '-chromatic' meaning color.

This uniformity in wavelength means that monochromatic light waves are inherently more coherent than polychromatic light (light containing multiple wavelengths), as they do not have different wavelengths interfering with each other's phase relationships.

Using monochromatic light in interference experiments, such as observing fringes from a thin film, minimizes complexity and enables clearer, more pronounced fringe patterns. This is because any variations in the path length that the light travels through the film affects all wavelengths in the same manner, leading to a consistent phase difference across the entire light wavefront.