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Cambridge IGCSE0625

General properties of waves

Physics 0625 Chapter Notes

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General properties of waves
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In this chapter

  1. 1Waves transfer energy, not matter
  2. 2Features of a wave
  3. 3Wave speed, frequency and wavelength
  4. 4Transverse waves
  5. 5Longitudinal waves
  6. 6Reflection, refraction and diffraction
  7. 7Diffraction and wavelength

1. Waves transfer energy, not matter

Waves transfer energy without transferring matter. A wave sent along a rope carries energy from one student to the other. The particles in the rope are not transferred.

Wave motion can be seen as vibrations in ropes and springs, and in water waves in a ripple tank.

Worked example 11 mark

The student at P moves one end of the rope up and down to send a wave along the rope towards the student at Q. Which row shows what is transferred from P to Q by the wave and what is not transferred from P to Q by the wave?

A energy and particles in the rope, wave peaks and troughs · B energy, particles in the rope · C particles in the rope, energy and wave peaks and troughs · D particles in the rope and wave peaks and troughs, energy

  1. 1

    energy is transferred from P to Q

  2. 2

    particles in the rope are not transferred, so the answer is B [1]

2. Features of a wave

A wave is described by its wavefront, wavelength, frequency, crest (peak), trough, amplitude and wave speed. Wavelength is measured between matching points, for example from one crest to the next. Amplitude is measured from the middle of the wave to a crest, not from trough to crest.

Examiner insight

“Some candidates confused the amplitude of the wave with the peak to peak value of the wave.” “Few candidates understood the term ‘wavefront’. Some candidates confused the term with ‘wavelength’.”

3. Wave speed, frequency and wavelength

Wave speed, frequency and wavelength are linked by v = fλ. State the equation first, then rearrange it.

Before you substitute, put every quantity in Hz, m and m / s. Divide powers of ten by subtracting the indices.

v = fλ

f = v/λ

λ = v/f

Examiner insight

“The most common error was to use an incorrect rearrangement of the equation, e.g. some candidates divided the frequency by the wavelength or the wavelength by the frequency.” “Some candidates were unable to divide the powers of ten.”

Worked example 13 marks

The wave speed of a gamma ray is 3.0 × 10⁸ m / s. The frequency of the gamma ray is 5.0 × 10²⁰ Hz. Calculate the wavelength of the gamma ray.

  1. 1

    v = f × λ [1]

  2. 2

    λ = (3.0 × 10⁸)/(5.0 × 10²⁰) [1]

  3. 3

    = 6.0 × 10⁻¹³ m [1]

4. Transverse waves

In a transverse wave the direction of vibration is at right angles to the direction of propagation.

Electromagnetic radiation, water waves and seismic S-waves can be modelled as transverse. A student makes a transverse wave on a rope by moving it up and down.

Examiner insight

“The most common error was not stating that the oscillations of the transverse wave were at right angles to the direction of propagation of the wave. Many candidates gave vague statements such as “the wave is at right angles to the wave”.” “Most candidates had the misconception that water waves are longitudinal.”

Worked example 12 marks

All electromagnetic waves are transverse waves. Describe the direction of vibration in a transverse wave.

  1. 1

    the vibration is perpendicular / at right angles [1]

  2. 2

    to the direction of propagation / energy transfer [1]

5. Longitudinal waves

In a longitudinal wave the direction of vibration is parallel to the direction of propagation.

Sound waves and seismic P-waves can be modelled as longitudinal.

Examiner insight

“Only the strongest candidates were able to recall a correct definition of longitudinal waves. Weaker candidates found it difficult to give a clear definition and many did not refer to vibrations.” “Most weaker candidates thought that sound was an example of a transverse wave.”

Worked example 12 marks

Complete the sentence to describe a longitudinal wave. In a longitudinal wave, the direction of vibration is … to the direction of propagation.

Give one example of a longitudinal wave.

  1. 1

    parallel [1]

  2. 2

    sound waves OR seismic P-waves [1]

6. Reflection, refraction and diffraction

Waves can undergo reflection at a plane surface, refraction due to a change of speed, and diffraction through a narrow gap.

A ripple tank shows all of these with water waves: reflection at a plane surface, refraction when a change in depth changes the speed, diffraction at a gap and diffraction at an edge.

When you draw waves, draw the direction of travel first. Wavefronts are perpendicular to it.

Examiner insight

“The most common error was to confuse refraction and diffraction.” “Very few candidates recalled that refraction was caused by a change in speed of the wavefronts.”

Worked example 12 marks

A vibrating rod produces a water wave in a ripple tank. The water wave travels from shallow water to deep water. Complete Table 5.1 to show what happens to the properties of the water wave.

wave propertyfrom shallow water to deep water
wavelengthincreases
frequency
speed
  1. 1

    frequency stays the same [1]

  2. 2

    speed increases [1]

7. Diffraction and wavelength

A wave diffracts more through a gap when the gap is made smaller or the wavelength is increased. Maximum diffraction occurs when the gap is about the same size as the wavelength.

A gap much wider than the wavelength gives very little diffraction, and the waves stay straight. The wavelength does not change when a wave diffracts. Wavelength also affects diffraction at an edge.

Examiner insight

“A significant number thought that increasing the amplitude of the wave would increase the diffraction.” “The presence of a much wider gap would mean very little diffraction so they were required to state that they would pass through the gap remaining as straight waves.”

Worked example 11 mark

When a person speaks, sound waves leave through the person’s mouth and spread out in many directions. Which row states the physical process that causes the sound waves to spread out and the reason why this happens as the sound waves leave an open mouth?

A diffraction, sound wavelengths are small compared to the size of the mouth · B diffraction, sound wavelengths are large compared to the size of the mouth · C refraction, sound wavelengths are small compared to the size of the mouth · D refraction, sound wavelengths are large compared to the size of the mouth

  1. 1

    the process is diffraction

  2. 2

    sound wavelengths are large compared to the size of the mouth, so the answer is B [1]

Recap

  • Waves transfer energy without transferring matter.
  • Amplitude is measured from the middle to a crest. Wavelength is measured between matching points.
  • Wave speed: v = fλ. Convert to Hz, m and m / s first.
  • Transverse: vibration at right angles to the direction of propagation. Light, water waves and S-waves.
  • Longitudinal: vibration parallel to the direction of propagation. Sound and P-waves.
  • Reflection, refraction (change of speed) and diffraction (gap or edge) are shown in a ripple tank.
  • Extended: a smaller gap or a longer wavelength gives more diffraction.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what a wave transfers from one place to another.1 mark
  2. State how the direction of vibration compares with the direction of propagation in (a) a transverse wave and (b) a longitudinal wave.2 marks
  3. Name one example of a transverse wave and one example of a longitudinal wave.2 marks
  4. The highest crest of a water wave is 6.0 cm above its lowest trough. The undisturbed water level is halfway between them. (a) State the amplitude of the wave. (b) Five complete waves fit into a length of 2.0 m. Calculate the wavelength.2 marks
  5. Sound of frequency 1.5 kHz travels through air at 330 m / s. Calculate its wavelength.3 marks
  6. In a ripple tank, straight water waves change direction when they pass from one depth of water to another. Name this effect and state what causes it.2 marks
  7. A buoy rises and falls 20 times in 40 s as waves pass it. Neighbouring crests are 3.0 m apart. Calculate the speed of the waves.4 marks
  8. Water waves in a ripple tank spread out after passing through a gap in a barrier. State two changes that would make the waves spread out more.2 marks
  9. A loudspeaker inside a room gives out two sounds at the same time. One has a frequency of 170 Hz and the other has a frequency of 8.5 kHz. The speed of sound in air is 340 m / s. The sounds leave the room through an open doorway 0.80 m wide. (a) Calculate the wavelength of each sound. (b) State which sound spreads out more after it passes through the doorway. Explain your answer.5 marks
  10. A vibrating rod makes straight water waves of frequency 12 Hz in a ripple tank. The waves travel at 0.30 m / s towards a gap 2.5 cm wide in a barrier. (a) Calculate the wavelength of the waves in cm. (b) State how much the waves diffract at the gap. Give a reason. (c) Some water is taken out of the tank. In the shallower water the waves travel at 0.18 m / s. The rod vibrates at the same frequency. Use a calculation to explain how the diffraction at the gap changes.7 marks

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