Year 9 Science · Unit 4 · Lesson 4
Challenge Worksheet
Learning Goals
Comparing wave frequencies
The bar chart below shows the typical frequency of five different wave types. Study the chart carefully, then answer the questions.
Note: bar heights represent relative magnitude. Actual values shown above each bar.
(a) Assuming all five waves travel through the same medium at 343 m/s (speed of sound in air), calculate the wavelength of the ultrasound wave (f = 40,000 Hz). Show your working and state which wave type has the shortest wavelength.
(b) Which of the five wave types cannot be heard by humans? Explain your answer using the human hearing range (20 Hz to 20,000 Hz).
(c) A radio wave of frequency 1,000,000 Hz travels at the speed of light (3 × 10⁸ m/s), not at 343 m/s. Contrast how wavelength changes for sound waves versus light waves as frequency increases. Why are the wavelengths so different even for the same frequency?
Evaluate the claim
A student claims...
"If I double the frequency of a wave, I double its speed. So a sound wave at 880 Hz travels twice as fast as a sound wave at 440 Hz in the same room."
(a) Use v = fλ to explain whether doubling the frequency doubles the wave speed. What actually happens to the wavelength when frequency is doubled in the same medium?
(b) Describe a situation where changing frequency would change wave speed. (Hint: think about what happens when a wave moves from one medium to a different medium.)
(c) A concert at the Sydney Opera House has instruments playing at 440 Hz and 880 Hz simultaneously. A sound engineer claims the higher-pitched instruments will reach the back of the auditorium first. Assess this claim using what you know about wave speed in a given medium.
Wrap Up
In one sentence, what was the main idea of this lesson?