Interference of Light Waves - SS2 Physics Lesson Note

Interference is a phenomenon that occurs when two or more light waves overlap and combine to form a resultant wave. It is a fundamental concept in the field of optics and plays a significant role in understanding the behaviour of light.

Wave Nature of Light:

Light is an electromagnetic wave that exhibits wave-like properties, including wavelength, frequency, amplitude, and phase. When two or more light waves meet, their electric and magnetic fields superpose, leading to interference effects.

Coherent Sources:

For interference to occur, the light waves must have the same wavelength, frequency, and a constant phase relationship. Such sources are called coherent sources. Coherent sources can be obtained from a single source split into two or more paths (e.g., using a beam splitter) or from separate sources that are carefully synchronised.

Types of Interference:

a.    Constructive Interference: This occurs when two waves superpose in phase, resulting in an increased amplitude of the resultant wave. In constructive interference, the peaks of one wave coincide with the peaks of the other wave, leading to reinforcement and a brighter region.

b.    Destructive Interference: Occurs when two waves superpose out of phase, resulting in a decreased amplitude or cancellation of the resultant wave. In destructive interference, the peaks of one wave coincide with the troughs of the other wave, leading to partial or complete cancellation and a darker region.

Young's Double-Slit Experiment:

-       Young's double-slit experiment is a classic experiment that demonstrates interference. It involves a coherent light source, a barrier with two narrow slits, and a screen.

-       When light passes through the two slits, it diffracts and forms two coherent sources of light waves. These waves then interfere with each other on the screen.

-       The resulting pattern on the screen shows alternating bright and dark regions known as interference fringes or bands.

Conditions for Interference:

To observe interference, certain conditions must be met:

a.    The light waves must be coherent and have the same frequency and wavelength.

b.    The waves must overlap in space and time.

c.    The waves must have a constant phase relationship.

Applications of Interference:

Interference finds applications in various fields, including:

a.    Thin Film Interference: Interference in thin films produces colourful patterns seen in soap bubbles, oil slicks, and anti-reflective coatings.

b.    Interferometry: Interferometers are devices that use interference to measure small displacements, wavelengths, and refractive indices.

c.    Holography: Holography is a technique that utilises interference patterns to create three-dimensional images.

d.    Optical Coherence Tomography (OCT): OCT is a medical imaging technique that uses interference to create high-resolution cross-sectional images of tissues.

Interference of light waves provides valuable insights into the wave nature of light and has practical applications in various fields. It allows us to understand and manipulate light, leading to advancements in technology and scientific research.