Amplitude & Intensity

Overview

This lesson connects the size of an oscillation to the rate at which wave energy is transferred. You should be able to calculate intensity and compare intensities when the amplitude of a progressive wave changes.

What You Need to Know

• Amplitude is the maximum displacement from the equilibrium position.
• A progressive wave transfers energy, so intensity describes how much power passes through each square metre.
• Use intensity = power / area when power and area are given.
• For a progressive wave, intensity is proportional to amplitude squared.
• If amplitude doubles, intensity becomes four times larger. If amplitude is halved, intensity becomes one quarter as large.

How to Work Through It

1. Revisit amplitude on a displacement graph and connect it to energy transfer.
2. Use intensity = power / area in direct calculation questions.
3. Practise proportional comparisons using I proportional to A^2.
4. Combine both ideas in questions that ask for explanation as well as calculation.

Check Your Understanding

• What are the units of intensity?
• What happens to intensity if the same power is spread over a larger area?
• Why does doubling amplitude not just double intensity?
• Which quantity is measured from the centre line to a peak?

Common Mistakes

• Using peak-to-peak height instead of amplitude.
• Treating intensity as the same quantity as power.
• Forgetting to square the amplitude ratio when comparing intensities.
• Giving a proportionality answer without stating which quantity has changed.

Next Steps

• Practise both direct intensity calculations and amplitude-ratio comparisons.
• Keep the amplitude-squared rule ready for the polarisation lesson and Malus’s law.