# 7.1.1 Progressive Waves

### Wave Motion

• Energy is transferred through moving oscillations or vibrations. These can be seen in vibrations of ropes or springs

Waves can be shown through vibrations in ropes or springs

• The oscillations/vibrations can be perpendicular or parallel to the direction of wave travel:
• When they are perpendicular, they are transverse waves
• When they are parallel, they are longitudinal waves

#### Ripple tanks

• Waves can also be demonstrated by ripple tanks. These produce a combination of transverse and longitudinal waves

Wave effects can be demonstrated using a ripple tank

• Ripple tanks may be used to demonstrate the wave properties of reflection, refraction and diffraction

### General Wave Properties

• Displacement (x) of a wave is the distance from its equilibrium position. It is a vector quantity; it can be positive or negative
• Amplitude (A) is the maximum displacement of a particle in the wave from its equilibrium position
• Wavelength (λ) is the distance between points on successive oscillations of the wave that are in phase
• These are all measured in metres (m)

Diagrams showing the amplitude and wavelength of a wave

• Period (T) or time period, is the time taken for one complete oscillation or cycle of the wave. Measured in seconds (s)

Diagrams showing the time period of a wave

• Frequency (f) is the number of complete oscillations per unit time. Measured in Hertz (Hz) or s-1

Frequency equation

• Speed (v) is the distance travelled by the wave per unit time. Measured in metres per second (m s-1)

#### Phase

• The phase difference tells us how much a point or a wave is in front or behind another
• This can be found from the relative positive of the crests or troughs of two different waves of the same frequency
• When the crests or troughs are aligned, the waves are in phase
• When the crest of one wave aligns with the trough of another, they are in antiphase
• The diagram below shows the green wave leads the purple wave by ¼ λ

Two waves ¼ λ out of phase

• In contrast, the purple wave is said to lag behind the green wave by ¼ λ
• Phase difference is measured in fractions of a wavelength, degrees or radians
• The phase difference can be calculated from two different points on the same wave or the same point on two different waves
• The phase difference between two points:
• In phase is 360o or 2π radians
• In anti-phase is 180o or π radians

#### Exam Tip

When labelling the wavelength and time period on a diagram, make sure that your arrows go from the very top of a wave to the very top of the next one. If your arrow is too short, you will lose marks. The same goes for labelling amplitude, don’t draw an arrow from the bottom to the top of the wave, this will lose you marks too.

### Wave Energy

• Waves transfer energy between points, without transferring matter
• When a wave travels between two points, no matter actually travels with it:
• The points on the wave simply vibrate back and forth about fixed positions
• Waves that transfer energy are known as progressive waves
• Waves that do not transfer energy are known as stationary waves

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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