DoAssignment.ca

E2.1 · Use terminology for diffraction, interference, polarization, and radiation

Learn to use terminology for diffraction, interference, polarization, and radiation through clear examples and targeted practice.

Ontario Grade 12 Physics

The Wave Nature of Light

Diffraction, interference, polarization, and radiation

Waves can spread after passing an opening, overlap with other waves, or be described by the orientation of their oscillations. These behaviours have specific names. Wave radiation is energy transferred away from a source by waves. Electromagnetic radiation is one kind of wave radiation. This lesson focuses on using these terms clearly, not on calculating wave patterns. As a brief prerequisite, a wave is a disturbance that transfers energy. A wave can be described by its direction of travel and, for a transverse wave, the direction in which it oscillates. The physical system in the examples is a wave and the opening, surface, filter, or source it encounters. We describe each situation from the viewpoint of a stationary observer. Where a direction is needed, take the wave’s direction of travel as positive. Direction of travel is a vector quantity, which has magnitude and direction. Wavelength and frequency are scalar quantities, which have magnitude but no direction.

What you will learn

Define and use the terms diffraction, interference, polarization, and radiation.
Distinguish constructive interference from destructive interference.
Describe what happens when waves spread, overlap, or have a selected orientation.
Use the terms accurately when describing familiar wave situations.

1. Diffraction: waves spread

Diffraction is the spreading of a wave as it passes through an opening or around an obstacle. The wave does not simply continue as a narrow, unchanged beam. After the opening, it can spread into a wider region. Water waves passing through a gap are a familiar example. Sound can also diffract around an obstacle, which helps explain why a person may hear a sound without seeing its source.

The amount of spreading depends on the opening’s size compared with the wavelength. Wavelength is the distance between matching points on successive cycles of a wave. When the opening is similar in size to the wavelength, the spreading is noticeable. If the opening is much larger than the wavelength, the wave generally spreads less. This is a qualitative comparison, not a calculation.

A wave diagram can show parallel wavefronts approaching a gap and curved wavefronts leaving it. A wavefront is a line joining points that are at the same stage of a wave’s cycle. In a diagram, the curvature after the gap represents spreading. It does not mean that the wave has stopped travelling forward.

Diffraction means wave spreading at an opening or obstacle.
Spreading is more noticeable when the opening is similar in size to the wavelength.
A wavefront represents points at the same stage of a wave cycle.

2. Interference: waves overlap

Interference occurs when two or more waves overlap in the same region. The combined displacement depends on the displacements of the waves at each point. Displacement means how far, and in which direction, a point on a medium is from its undisturbed position. For light and other electromagnetic waves, the relevant wave quantities are not displacements of a material medium. At this level, use interference to describe the resulting pattern when the waves overlap.

Constructive interference occurs where overlapping waves reinforce one another. For example, a crest meeting a crest gives a larger displacement than either crest alone. Destructive interference occurs where waves reduce one another’s displacement. A crest meeting a trough gives a smaller displacement; equal and opposite displacements can cancel at that point.

The result can vary from place to place. A pattern with regions of stronger and weaker intensity is an interference pattern. Intensity describes how strong the wave’s effect is at a location. Do not say that destructive interference means the waves disappear everywhere. It describes their combined effect where they overlap; other positions can have constructive interference.

Interference is the result of waves overlapping.
Constructive interference reinforces; destructive interference reduces or cancels the combined displacement at a location.
An interference pattern can contain both stronger and weaker regions.

3. Polarization: an orientation is selected

Polarization describes the orientation of oscillations in a transverse wave. A transverse wave oscillates at right angles to its direction of travel. A rope wave provides a simple model: the disturbance can move up and down while the wave travels forward. The up-and-down direction describes the oscillation; the forward direction describes propagation, meaning the wave’s travel.

Unpolarized light has oscillations in many orientations perpendicular to its direction of travel. A polarizing filter can transmit light with one selected orientation and reduce light with other orientations. The transmitted light is polarized. This description concerns orientation; polarization does not mean that the light has changed direction of travel.

Polarization is useful terminology for transverse waves. Do not apply the same description to every kind of wave. In particular, polarization refers to the orientation of transverse oscillations, not to a wave’s colour, brightness, or direction of travel.

Polarization identifies an orientation of transverse oscillations.
A polarizing filter selects an orientation of light oscillation.
The direction of oscillation and the direction of travel are different directions.

4. Radiation: energy carried away by waves

Wave radiation is energy transferred away from a source by waves. Electromagnetic radiation includes visible light, radio waves, and other electromagnetic waves. These examples differ in properties such as wavelength and frequency, but each is electromagnetic radiation. In this lesson, the central idea is energy transfer from a source, not a detailed classification of the electromagnetic spectrum.

A source emits radiation when it sends wave energy away. The word radiation does not by itself mean that the radiation is dangerous. Visible light is radiation, and so are radio waves. Whether a particular kind of radiation creates a risk depends on the situation; the term alone does not state a health effect.

These four terms describe different ideas. Diffraction describes spreading. Interference describes the result of overlap. Polarization describes an orientation of transverse oscillations. Wave radiation describes energy carried away from a source by waves. Keeping those meanings separate makes explanations clearer.

Wave radiation is energy transferred away from a source by waves.
Visible light and radio waves are examples of electromagnetic radiation.
The word radiation alone does not mean harmful radiation.

Four wave terms at a glance

Term	What it describes	Clue in a situation
Diffraction	Wave spreading	A wave passes an opening or obstacle
Interference	The combined result when waves overlap	Waves reinforce or reduce one another
Polarization	Orientation of transverse oscillations	A filter selects an oscillation orientation
Wave radiation	Energy carried away from a source by waves	A source emits electromagnetic waves

Worked example

Naming spreading at a gap

A student describes water waves passing through a narrow gap. The waves spread into the region beyond the gap. Which term describes this behaviour, and what feature of the setup helps explain why it is noticeable?

Identify the behaviour
The system is the water wave and the gap. The observer is stationary beside the tank, and positive is the wave’s direction of travel through the gap. The description says that the waves spread after passing through an opening.
Use the matching term
Spreading at an opening is diffraction. The spreading is noticeable when the gap is similar in size to the wavelength. The description gives no measurements, so the comparison is qualitative.

Answer: The behaviour is diffraction. A gap similar in size to the wavelength makes the spreading noticeable.

Check: The term describes the stated change in wave spread, not wave overlap or orientation.

Worked example

Describing overlapping waves

At one location, a crest on one wave meets a trough on another. The two displacements are equal in size and opposite in direction. Name the type of interference at that location and describe the combined displacement.

Identify the overlap
The system is the two waves where they meet. Use a stationary observer and take upward displacement as positive. A crest has positive displacement and a trough has negative displacement.
Compare the displacements
Equal displacements with opposite signs add to zero at this location. Because the waves reduce one another’s displacement, this is destructive interference.

Answer: The waves undergo destructive interference at that location, and their combined displacement is zero.

Check: This conclusion applies at the specified location. It does not mean the waves cancel everywhere.

Worked example

Separating polarization from travel direction

Light travels horizontally through a polarizing filter. The filter transmits light with vertical oscillations. State what is polarized and distinguish the oscillation direction from the travel direction.

Identify the wave directions
The system is the light and filter, viewed by a stationary observer. Take horizontal travel through the filter as positive. The stated vertical direction is the orientation of the light’s oscillations.
Name the result
The transmitted light is polarized because its oscillations have a selected orientation. The light travels horizontally while its oscillations are vertical; these are not the same direction.

Answer: The transmitted light is polarized. It travels horizontally and oscillates vertically.

Check: The answer identifies oscillation orientation without confusing it with the direction of travel.

Common mistakes and how to avoid them

Calling every change in a wave’s path diffraction.

Correction: Use diffraction when the wave spreads at an opening or around an obstacle.

Saying destructive interference removes waves everywhere.

Correction: Destructive interference describes a reduced or cancelled combined displacement at a location where waves overlap.

Treating polarization as the direction in which a wave travels.

Correction: Polarization describes the orientation of transverse oscillations. State that orientation separately from the direction of travel.

Assuming radiation always means something dangerous.

Correction: Wave radiation means energy transferred away from a source by waves. The word alone does not describe a hazard.

Lesson summary

Diffraction is wave spreading at an opening or obstacle.
Interference is the combined result when waves overlap. Constructive interference reinforces; destructive interference reduces or cancels displacement at a location.
Polarization describes the selected orientation of oscillations in a transverse wave.
Wave radiation is energy carried away from a source by waves. Electromagnetic radiation is one kind of wave radiation.

Check your understanding

Question 1

Which term describes sound spreading around the edge of an obstacle?

Diffraction
Polarization
Constructive interference
Radiation

Show answer and explanation

Diffraction

Diffraction is the spreading of a wave around an obstacle or through an opening.

Question 2

Two equal, opposite displacements meet at one point. What is the combined displacement there?

Twice either displacement
Zero
Unchanged from the crest
The wavelength

Show answer and explanation

Zero

The equal and opposite displacements cancel at that point, which is destructive interference.

Question 3

A filter transmits light oscillating in one selected orientation. What term describes the transmitted light?

Diffracted
Destructively interfered
Polarized
Radiated only if it is harmful

Show answer and explanation

Polarized

Polarization means that a transverse wave’s oscillations have a selected orientation.

Question 4

Which statement best describes wave radiation in this lesson?

Energy transferred away from a source by waves
The spreading of waves at an opening
The cancellation of every wave in a region
The direction of oscillation in a transverse wave

Show answer and explanation

Energy transferred away from a source by waves

Wave radiation is energy carried away from a source by waves.

Key terms

Diffraction: The spreading of a wave as it passes through an opening or around an obstacle.
Interference: The combined result when two or more waves overlap.
Constructive interference: Interference in which overlapping waves reinforce one another.
Destructive interference: Interference in which overlapping waves reduce or cancel displacement at a location.
Polarization: The orientation of oscillations in a transverse wave, or the selection of an oscillation orientation.
Wave radiation: Energy transferred away from a source by waves.
Electromagnetic radiation: Energy carried away from a source by electromagnetic waves.
Transverse wave: A wave whose oscillations are at right angles to its direction of travel.

Continue through SPH4U

View the complete SPH4U Ontario Grade 12 Physics curriculum and lessons

About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 12 Physics (SPH4U), expectation E2.1. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question