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Waves transfer energy without a net transfer of the medium along the whole path. In transverse waves, oscillations are perpendicular to travel; in longitudinal waves they are parallel, producing compressions and rarefactions. Amplitude is maximum displacement, wavelength is distance between matching points and frequency is oscillations per second. The period is time for one oscillation, T = 1/f. Sound is a longitudinal mechanical wave and needs a medium.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Compare transverse and longitudinal waves
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: In a transverse wave, what is the oscillation direction?
Perpendicular to travel. A wave on a string is a familiar example.
After watching 2: What stays constant when a wave refracts at a stationary boundary?
Frequency. The source sets the frequency.
What you'll be able to do
Waves transfer energy without a net transfer of the medium along the whole path. In transverse waves, oscillations are perpendicular to travel; in longitudinal waves they are parallel, producing compressions and rarefactions. Amplitude is maximum displacement, wavelength is distance between matching points and frequency is oscillations per second. The period is time for one oscillation, T = 1/f. Sound is a longitudinal mechanical wave and needs a medium.
Wave speed is v = fλ. Use metres for wavelength if speed is in m s⁻¹. Reflection changes direction at a boundary. Refraction occurs when wave speed changes between regions; frequency stays the same while wavelength changes. A change in direction occurs for non-normal entry, so bending is not the only evidence of refraction. Diffraction is spreading through gaps or around obstacles, with the effect depending on wavelength and gap size.
On a ripple tank or string, measure several wavelengths and divide by their number to reduce relative measurement uncertainty. Use a known frequency and calculate speed. For echoes, sound travels to the reflector and back, so distance is half speed times delay. Ultrasound uses reflected pulses for imaging or distance measurement. Distinguish time of flight from period; one concerns the journey while the other concerns repeated oscillation.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: An echo returns after 0.40 s. Sound speed is 340 m s⁻¹. Find reflector distance.
68 m.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.