In this physics SAT prep guide, we will talk about waves that are among the many ways energy can be transferred. We will also go on to examine topics such as the properties of waves, the concepts of reflection and refraction, and the Doppler effect.

This detailed analysis is meant to help students clear up their concepts when they prep for their SAT subject tests and exams.

So far we have also covered topics explored by pupils in the SAT Physics Syllabus.

Wave Properties

One of the most interesting topics of the SAT Physics subject test is waves. They are one of the ways energy is transferred, but it is important to understand here that waves do not move matter.

In essence, a wave consists of displacement, rest position, peak, trough, amplitude, wavelength, frequency, and time.

Waves travel in crests and troughs. In the case of longitudinal waves, these are called compressions and rarefactions.

The crest denotes when the wave is at its maximum positive displacement from its point of equilibrium. In contrast, a trough is the point of maximum negative displacement from its equilibrium point.

The wavelength of the wave, denoted by the Greek alphabet lambda, is the distance between two successive crests or troughs. Greek alphabets routinely feature in physics problems and equations.

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Transverse And Longitudinal Waves

There are two kinds of waves: transverse and longitudinal. Let us see the difference between the two.

Transverse Waves

This is where the particles move perpendicular to the direction of the wave itself. Plucking a string cause vibrations that travel the length of the string. The string oscillations are perpendicular to the direction of the wave.

Some examples of a transverse wave include radio waves, microwaves, ripples in a pond, electricity, and seismic P-waves. In a transverse wave, the vibrations are at right angles to the wave direction.

An example of transverse waves around us that we use for countless applications is electromagnetic waves. Learn more about magnetism and electromagnetism here.

Electromagnetic waves are transverse waves that travel through a vacuum at a speed of 300 million meters per second. The electromagnetic spectrum shows how these waves can have a variety of uses like:

  • Radio waves- used for communication
  • Microwaves- used in household microwaves
  • Infrared- used in infrared cameras
  • X-rays- used in medical procedures to detect broken bones

On the other hand, a longitudinal wave is a wave in which the particles move parallel to the wave's direction. Imagine you are holding a slinky in both hands. If you jerk it towards you from one side, it will transmit a pulse horizontally, causing it to oscillate back and forth.

Rarefactions are areas where a longitudinal wave is spread out. At the same time, compression is an area of high density in the wave. Sound waves consist of a mix of low-pressure and high-pressure regions called rarefactions and compressions.

In a sound wave, the frequency of the wave is known as its pitch.

Periodic Motion

The time period of a wave is a reciprocal of its frequency. It is measured in seconds, and the frequency is measured in Hertz. The unit of Hertz has an inverse relationship to second and can be applied to other situations as a measure of how frequently an event occurs.

The maximum displacement of a wave from its equilibrium position is known as amplitude. When one complete cycle of motion and the wave returns to its equilibrium point is known as the period of oscillation. The frequency is the number of cycles the waves completes in one second.

Let your physics and maths tutor help you practice these concepts until you have fully mastered them. With the right kind of tutoring, you can surely ace your physics tests.

Superposition

When two or more waves interact with each other, a phenomenon called superposition occurs. The waves can be travelling in the same or different directions.

Imagine two experiments holding opposite ends of the same string, each shake their side of the string. What will happen when the waves meet each other in the middle? They will either reinforce or cancel out each other.

Superimposition means that waves cannot alter each other's frequency, wavelength, or direction. They are simply superimposed on each other.

Destructive Interference

What if one experiment yanks the string downwards, while the other pulls up by the same amount? The two waves will cancel each other completely, and the total displacement will be zero. It does not mean that any wave is destroyed. After they pass each, they will continue just like they did before they met.

Constructive Interference

Conversely, if both experiments yanked the string upwards by the same amount, the two waves will meet each other and the displacement will be double.  And then they will continue on as they did before interacting.

Beats

When you hear two musical notes of slightly different pitch played at the same time, you will notice the phenomenon of interference. The beat will come due to repeated cycles of constructive and destructive interference, interspersed between each other. The number of beats per second is determined by the frequency difference between the two sound waves.

Standing Waves And Resonance

Until now, we focused on traveling waves, where a wave travels a distance through a medium. We will now look at waves that do not travel anywhere but simply oscillate in place.

These waves are called standing waves. Many of the terms used for traveling waves apply to standing waves as well, but there are few peculiarities you should know.

Reflection

different types of reflection
A diffuse reflection can be noticed on a body of water. The reflection is a bit blurred because the surface is unsteady. (Source: pixabay)

Suppose you have a wave traveling on a string attached to a wall. When the wave hits the wall, it will reflect and travel back to its source. A reflected wave is the mirror image of the original wave. So a trough on the original wave will be a crest on the reflected wave.

A reflected wave will interact with any waves it meets on its way back. If the string length is a multiple of half of the wavelength, then the superposition will result in standing waves. Here, the wave will appear to be still, and the crests and troughs will not seem to move on the wave.

Instead, the wave will have points known as nodes and antinodes. These are points where the wave will experience complete destructive and constructive nodes, respectively. The distance between successive nodes and antinodes will be half of the wavelength.

A common example of reflection you can observe in nature is moonlight. The moon does not have any natural light but rather reflects light from the sun. Read more about the moon, the sun, and other planetary bodies in the topic of space physics.

Resonance and Harmonic Series

An electric guitar with strings
Guitar strings are fixed at both ends therefore they oscillate. (Pixabay)

The strings on a musical instrument oscillate as standing waves since they are fixed at both ends. They can only vibrate at certain frequencies.  The longest wave is called resonance and has two nodes at the end and one antinode at the centre.

The string can support several waves, as long as they have an integral number of nodes. This series of standing waves is called the harmonic series of the wave.

To make it easier for you to grasp, let us include a real-life example of harmonic series. A skilled physics and maths tutor will make sure to include everyday examples in their physics lessons.

A real-life example of this is when you blow into bottles. The sound comes out of them because your breath creates a series of standing waves in the bottle.

The pitch of the sound is inversely proportional to the wavelength. At the same time, the length of the standing wave is directly proportional to the wavelength. The longer the wavelength, the longer the standing wave, and the lower the frequency.

That is why musicians add water to the bottles to alter the pitch. The water inside the bottle decreases the wavelength.

The Doppler Effect

Until now, we only discussed scenarios where the source of the waves was stationary. What about the cases where the source of the wave is also moving?

There are many such cases where the wave source moves through the medium it is propagating the waves. A very common example would be a speeding police car or ambulance.

The speed of the waves depends on the properties of the medium, such as its temperature and density. It does not depend on the source's motion; the waves will travel at the same speed regardless of how fast the source is moving.

Police car chasing with the siren on
The waves speed from a moving police car with the sire ON depends on the density and temperature, and not motion. (Pixabay)

However, what can alter due to the wave source are the wave's frequency and wavelength. This change is called the Doppler effect

If you want to view learn topics beyond this syllabus for your physics exam, you can check out other sources on the internet.

If you are a physics enthusiast, you should check out our SAT articles on particle model of matter, and atomic structure. The topics may not necessarily be part of your SAT physics test syllabus.

Head on over to Superprof to get the best tutors for your SAT II Physics prep. Sign up now and find teachers who can help you understand waves along with numerous other physics-related topics.

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