Destructive Interference and Constructive Interference
Waves
The repeating and periodic disturbances that are practiced through a medium from one end of the location to another is referred to as a wave. The wave is a transverse wave if the wave is moving perpendicular to a medium, in which the particles of the medium are displaced in a direction perpendicular to the direction of energy transport.
What is a Medium?
A medium is a substance or material by which the wave is carried; also called wave medium. In general, a wave transports energy and not matter. The particles of the medium get temporarily displaced from its rest position. The force acting upon the particles will restore them to their original positions. So, waves are said to be transporters of energy. The disturbance in the position of one particle will pass to its nearby particles in a repeated and periodic way. Thus the energy is transported from one end to the other.
Anatomy of Wave:
A wave has many crests that are often followed by other crests; same happens in the case of trough. Every crest is broken by a trough to create an alternating pattern of crests and troughs.
Amplitude: The amplitude of a wave refers to the maximum amount of displacement of a particle from its rest position.
Wavelength: The wavelength of a wave is simply the length of one complete wave cycle, comprising a crest and a trough.
Interference of Waves
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Constructive Interference
This interference occurs at any location along the medium where the two interfering waves have a displacement in the same direction. Here an upward displacement is greater than the displacement of the two interfering pulses. Simply, a constructive interference is observed at any location along a medium where the two interfering waves are displaced upward. It is also observed when both interfering waves are displaced not only upward, but also downward. This is shown in the figure below for two downward displaced pulses.
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Destructive Interference
Destructive interference can take place at any location in the medium where the two interfering waves have a displacement in the opposite direction; that the two waves are travelling in an opposite direction when they get into interference. For instance, when a sine pulse with a maximum displacement of +1 unit meets a sine pulse with a maximum displacement of –1 unit, a destructive interference takes place. This is shown in the figure below.
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There is no need of having same amplitudes when two waves interfere in opposite directions for destructive interference to occur. Say for instance, a pulse with a maximum displacement of +1 unit could meet a pulse with a maximum displacement of –2 units. The resulting displacement of the medium during complete overlap would be –1 unit.
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The meeting of the two waves along a medium might not alter the individual waves or even deviate them from their travelling path. Yet the two waves will meet to produce a net resulting shape of the medium, and then just continue their travel as before the interference took place.
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FAQs on Destructive Interference
1. What is destructive interference in Physics, as per the CBSE 2025-26 syllabus?
Destructive interference occurs when two waves of the same type meet and their displacements are in opposite directions, resulting in a decrease in the overall amplitude. If the waves are exactly out of phase, their amplitudes subtract, potentially cancelling each other out completely.
2. How does destructive interference differ from constructive interference?
In destructive interference, the amplitudes of overlapping waves subtract because they are out of phase, which can reduce or nullify the resultant wave. In contrast, constructive interference adds the amplitudes of in-phase waves, increasing the overall amplitude.
3. Can you give a real-world example of destructive interference?
A real-world example of destructive interference is noise-cancelling headphones. These devices create sound waves that are exactly out of phase with ambient noise, reducing or cancelling the unwanted sound.
4. Why is understanding destructive interference important for CBSE Class 12 board exams?
Understanding destructive interference helps students solve questions on wave phenomena, diffraction, and superposition, which are core areas in the syllabus and frequently appear in board exams for higher-order thinking skills.
5. What are the necessary conditions for destructive interference to occur?
The conditions for destructive interference are:
- The two waves must have the same frequency and amplitude (ideally for perfect cancellation).
- They must be travelling in the same medium.
- The waves meet such that their phase difference is an odd multiple of π (or 180°).
6. How does path difference relate to destructive interference?
Destructive interference occurs when the path difference between two coherent waves is an odd multiple of half-wavelengths [(2n+1)λ/2]. This causes the waves to arrive out of phase, resulting in cancellation.
7. What happens at the nodal points in an interference pattern, and how are they linked to destructive interference?
Nodal points are locations in an interference pattern where destructive interference consistently occurs, resulting in minimal or zero amplitude at those points.
8. Could both sound and light waves undergo destructive interference?
Yes, destructive interference can happen with any type of wave, including sound, light, and water waves, as long as the conditions for interference are met.
9. How might misconceptions about destructive interference affect a student's exam answers?
A common misconception is that destructive interference destroys energy. In reality, energy is redistributed, not lost—the total energy in the medium remains constant, as demanded by the law of conservation of energy.
10. If the amplitudes of interfering waves are not equal, can perfect destructive interference still occur?
No, perfect destructive interference happens only if the interfering waves have equal amplitudes. If not, the resultant amplitude equals the difference of the two, and total cancellation does not occur.
