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Monochromatic light refers to light that consists of a single wavelength or color. This is an important concept in various experiments involving light interference and diffraction. Light emitted from a monochromatic source does not change its wavelength. This is what makes it particularly useful in experiments like a double-slit experiment. For these exercises, we often use lasers because they generate monochromatic light.
Using monochromatic light ensures that the pattern observed on the screen is due to interference alone, without the complications of multiple wavelengths. In the context of interference of light, having a single wavelength makes it simpler to calculate and observe the resultant phase differences between waves. This uniformity is crucial in obtaining clear and predictable interference patterns. Understanding monochromatic light allows us to isolate and explore how light waves interact without the added variables introduced by multiple wavelengths.

Coherent light is crucial for studying interference patterns in experiments. Coherent light waves maintain a constant phase relationship with one another over time. This consistency means that the light waves are in sync with each other.
In experiments like the double-slit experiment, coherence ensures the light waves will interact in a predictable manner. The result of this interaction can form areas of constructive interference, where the light waves amplify each other, and destructive interference, where they cancel each other out.
The sources of coherent light are usually laser beams. These beams are capable of producing the necessary constant phase differences required for interference patterns. In simple terms, coherent light can be thought of as rhythmic waves, each wave crest and trough perfectly aligned, leading to clear and consistent interference results.

The double-slit experiment is a foundational experiment in understanding wave interference phenomena. It involves passing light through two close, narrow slits onto a screen.
As light passes through these slits, it diffracts and spreads out, causing the waves from each slit to overlap. This overlapping leads to an interference pattern on the screen that can be observed. Through this setup, we can observe bright and dark fringes. Bright fringes occur where constructive interference happens, where two waves add together to make a wave of greater amplitude, while dark fringes occur where destructive interference happens, where two waves cancel each other out.
The double-slit experiment beautifully demonstrates key principles of wave theory and light's wave-particle duality. Originally conducted by Young, this experiment vividly illustrates the interference of light, showing how wave properties play a role in how light behaves.

Phase difference refers to the difference in the phase of two waves meeting at a point. In terms of light, the phase difference is crucial in determining whether two light waves interfere constructively or destructively.
In interference patterns created in experiments like the double-slit experiment, the phase difference between light coming from different slits determines the positions of bright and dark fringes on a screen. For example, if two waves are perfectly in phase (a phase difference of 0°), they will constructively interfere, resulting in a bright fringe. On the other hand, a phase difference of 180° will cause destructive interference, leading to a dark fringe.
Phase difference is often calculated considering the path difference the waves have traveled. When waves from two sources travel different distances to meet at a point on a screen, they may be out of phase if their path difference equates to a fractional wavelength. But if the path difference is a multiple of the wavelength, the two waves will be in phase, leading to constructive interference. This concept helps us calculate and predict the pattern of interference fringes in various optical setups.