Seeing with echoes

Science · Waves, Light & Sound · ages 15-16

Name ______________________ Date ____________

Why must the return time be halved?
- The timer runs too fast
- The pulse travels to the boundary and back
- Waves slow down at night
What counts as ultrasound?
- Sound above about 20 kilohertz
- Sound below a whisper
- Any sound in water
When does an ultrasound reflection happen?
- Anywhere in open water
- At a boundary between two materials
- Only at the surface
Why is gel spread on the skin for a medical scan?
- To cool the patient
- To remove air gaps that would reflect the pulse early
- To make the image colourful
Sound runs at 1500 m per s in seawater. A pulse returns after 2 s. How deep is the sea floor in m?
Answer: ______________
A wreck survey repeats a sounding with double the return time. What happens to the depth?
- It doubles
- It halves
- It stays the same
A maker claims ever-higher frequency sees ever deeper. What is wrong?
- Frequency never affects images
- Low frequencies cannot travel at all
- Higher frequencies fade faster, so each job picks its own balance
A pulse in steel runs at 6000 m per s and returns after 0.001 s. How deep is the reflecting crack in m?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

The pulse travels to the boundary and back · The measured time covers twice the depth, down plus back.
Sound above about 20 kilohertz · Ultrasound starts past the top of human hearing.
At a boundary between two materials · Only a change of material sends part of the pulse back.
To remove air gaps that would reflect the pulse early · Air gaps would bounce the pulse before it even enters.
1500 · 1500 times 2 divided by 2 is 1500.
It doubles · Depth scales with time, so double time means double depth.
Higher frequencies fade faster, so each job picks its own balance · Sharper costs shallower: short waves lose energy faster in the material.
3 · 6000 times 0.001 divided by 2 is 3.