Chapter 8 Waves
   
  1. Types of Waves
  2. Measuring Waves
  3. Moving from one medium to another
  4. Collision of waves
  5. Reflection, Refraction and Diffraction
  6. Chapter Quiz
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  Section Section 1: Types of Waves

There are three types of waves:

 

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  1. Mechanical waves require a material medium to travel (air, water, ropes). These waves are divided into three different types.
    • Transverse waves cause the medium to move perpendicular to the direction of the wave.
    • Longitudinal waves cause the medium to move parallel to the direction of the wave.
    • Surface waves are both transverse waves and longitudinal waves mixed in one medium.
  2. Electromagnetic waves do not require a medium to travel (light, radio).
  3. Matter waves are produced by electrons and particles.

By moving a rope once, a single vibration is produced. This is a pulse.
Click on the image to produce a pulse.
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By moving a rope regularly up and down, a traveling or periodic wave is produced.

 

  Section Section 2: Measuring Waves

Any point on a transverse wave moves up and down in a repeating pattern.

 

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The shortest time that a point takes to return to the initial position (one vibration) is called period, T. In this example, every vibration is marked with a short pause.
Point on a wave

The number of vibrations per second is called frequency and is measured in hertz (Hz). Here's the equation for frequency:
f = 1 / T

The shortest distance between peaks, the highest points, and troughs, the lowest points, is the wavelength, wavelength.

Example of wavelength

By knowing the frequency of a wave and its wavelength, we can find its velocity. Here is the equation for the velocity of a wave:

Velocity

However, the velocity of a wave is only affected by the properties of the medium. It is not possible to increase the speed of a wave by increasing its wavelength. By doing this, the number of vibrations per second decreases and therefore the velocity remains the same.

The amplitude of a wave is the distance from a crest to where the wave is at equilibrium. The amplitude is used to measure the energy transferred by the wave. The bigger the distance, the greater the energy transferred.

Amplitude

Example:
A radio wave has a frequency of 93.9 MHz (93.9 * 106 Hz). What is its period?
f = 93.9 * 106 Hz
f = 1 / T
T = 1 / f

T = 1 / 93.9 x 10
6 Hz
T = 1.06 x 10
-8 s

QUESTION: If the velocity of a radio wave is 300 000 000 m/s (light speed) and its wavelength is 3.108 m, what is the frequency of the wave?
MHz

 

  Section

Section 3: Moving from one medium to another

Once a wave (incident wave) has reached the end of a medium, part of the energy is transferred to the medium that is immediately next to it (transmitted wave) and part is reflected backward (reflected wave).

The energy transferred depends on the difference between the mediums. If there is a significant difference, almost all the energy will be reflected. If the mediums are similar, most of the energy will be transferred. However, the reflected waves will be inverted if the medium that comes next is more dense or it won't be inverted if the medium is less dense.

Transfering waves

 

  Section Section 4: Collision of waves

When two waves traveling in opposite directions through the same medium collide, the amplitude of the resulting wave will be the sum of the two initial waves. This is called interference and there are of two types:

 

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  • Constructive interference is when the amplitudes of the initial waves are in the same direction. The resulting wave will be larger than the original waves. The highest point of a constructive interference is called an antinode.
    Constructive interference
  • Destructive interference is when the amplitudes of the initial waves are opposite. The amplitude of the resulting wave will be zero. The point in the middle of a destructive interference is called a node and it never moves.
    Destructive interference

 

  Section

Section 5: Reflection, Refraction and Diffraction of waves of two dimensions (waves on water surface)

When a wave hits a barrier, it will be reflected depending on the direction of the barrier (normal). The angle between the incident wave and the normal is the same as the angle between the normal and the reflected wave.

 

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incident/reflected wave

When a wave enters a different medium (more shallow region) at an angle, the direction of waves changes. This change is called refraction.

When a wave travels through a small hole in a barrier, it bends around the edges. This is called diffraction.

 

  Section

Section 6: Chapter 8 Quiz

This is the quiz of chapter 8. Try it!


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