Interference, Diffraction, and Polarization

Light can add, cancel, spread around openings, and be filtered by direction. Those behaviors reveal its wave nature.

This lesson supports Chapter 23 of Physics for Beginners. Keep the physical picture in view, write the units, and check the direction of every vector before trusting a calculation.

Superposition creates interference

When waves overlap, their displacements add. Crests meeting crests produce constructive interference. A crest meeting an equal trough produces destructive interference. In a two-slit pattern, bright bands occur where the path difference is a whole number of wavelengths.

Diffraction becomes strong at small scales

Waves spread after passing through an opening or around an edge. The spreading is easiest to notice when the opening is comparable to the wavelength. A narrower slit produces a wider central diffraction maximum, which limits the detail any optical instrument can resolve.

Polarization selects a transverse direction

Light is transverse, so its electric field can oscillate in different directions. A linear polarizer transmits one component. Two ideal polarizers with axes at \(90^\circ\) block the light that passed through the first. Longitudinal sound in air cannot be polarized in this way.

Worked example

Two coherent waves arrive in phase at a point. Their crests and troughs line up, so they interfere constructively and produce a larger amplitude. If one wave were shifted by half a wavelength, equal amplitudes would cancel there.

Watch the idea in action

Khan Academy gives a focused explanation of the chapter’s central idea. Pause before each calculation and predict the next step on paper.

Practice questions

1. What principle governs overlapping waves?
2. When does constructive interference occur?
3. When is diffraction easiest to notice?
4. What happens to diffraction as a slit narrows?
5. Why can light be polarized?
6. What do crossed ideal polarizers do?

Answers

1. Superposition.
2. When waves arrive in phase.
3. When an opening or obstacle is comparable to the wavelength.
4. The spreading increases.
5. Light is a transverse wave.
6. They block the transmitted light.

Keep studying

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