# OCR B AS PHYSICS - CHAPTER 6, WAVES

- Created by: Rebecca Fernandes
- Created on: 26-05-14 11:29

Chapter 6 - Waves

**Superposition and Coherence:**

**Superposition occurs when 2 or more waves pass through each other.**

At the instant the wars cross, the displacements due to each wave combine. Each wave then continues on in the original direction.

Principle of superposition = when 2+ waves cross, the resultant displacement is the vector sum of the individual displacements.

**Interference can be constructive or destructive.**

A crest + A crest = a bigger crest & a trough + a trough = a bigger trough. ∴ constructive.

A crest + a trough (of equal size) = 0. The 2 displacements cancel each other out completely ∴ destructive.

If crest /trough aren't the same size the inference isn't total. For interference to be noticeable the amplitudes should be almost equal.

**You can use phasors to show superposition**

You can use little rotating arrows to represent the phase of each point on a wave.

The phasor rotates ANTICLOCKWISE through one whole turn as the wave completes a full cycle.

To find resultant at timer, add the phasors tip to tail.

**In PHASE means In STEP, 2 points in PHASE interfere CONSTRUCTIVELY.**

2 points on a wave are in phase if they are both at the same point in a wave cycle.

Often use one wave cycle as 360 degrees (2π radians).

Points that have a phase difference of 0 or a multiple of 360 degrees are in phase ∴ phasors are pointing in same direction.

Points with a phase difference of odd-number multiples of 180 degrees are exactly out of phase. This is called **ANTI PHASE.** Their phasors point in opposite directions.

**Points can have a phase difference of any angle.**

You can talk about 2 different waves being in phase. In practice this happens because both waves came from the same oscillator.

In other situations, nearly always be a phase difference between 2 waves.

**To get interference patterns the 2 sources must be coherent.**

Interference happens in a jumble when observing waves of different wavelength and frequency.

To get clear interference patterns the 2+ sources must be coherent.

**2 sources are coherent if they have the same wavelength and frequency and a fixed phase difference between them.**

**Constructive/Destructive Interference depends on path difference.**

Constructive/Destructive depends on how much further one wave has travelled that the other wave.

The amount by with one waves path is longer is called the path difference.

**Constructive interference occurs when path difference = number x wavelength**

p=nλ

**Destructive Interference occurs when path difference = (number+1/2) x wavelength**

p=(n+1/2)λ

**Standing Waves:**

**You get standing waves when a progressive wave is reflected at a boundary**

**A standing wave is the superposition of 2 progressive waves with the same wavelength, moving in opposite directions.**

Unlike progressive waves, standing waves transmit no energy.

You can demonstrate standing waves by setting up driving oscillator at the end of a stretched string with the other end fixed. Wave generated is reflected back and forth.

For most frequencies the resultant pattern = jumble. If…

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