Reflection and Echoes
Waves bounce off surfaces.
About this topic
Waves bounce off surfaces following the law of reflection: the angle of incidence equals the angle of reflection. An echo is reflected sound heard after a delay of at least about 0.1 s, requiring a surface at least 17 m away.
Echo-sounding measures sea depth by timing reflections of sound pulses, and bats and dolphins find prey by echolocation. Hard, flat surfaces reflect sound well, while soft materials such as curtains and carpet absorb it. Concert halls are shaped and lined to control reverberation, which is many overlapping reflections that blur sound if too long.
Worked example: a ship sends a pulse and hears the echo after 0.8 s. With sound at 1500 m/s in water, the round trip covers 1200 m so the depth is 600 m, half the total path. A shout in an empty gym may sound like a muddy blur because reflections return in under 0.1 s and merge with the original.
Reflection applies to all waves, including light, radio and water waves hitting a harbour wall. Parabolic dishes reflect waves to a focus, as in satellite dishes. A misconception is that the angles are measured from the surface; they are measured from the normal, the line at right angles. Remember to divide the total time by two for the one-way distance.
Key ideas
- Angle in equals angle out
- Echoes need distance of about 17 m
- Sonar uses echoes
Quick check
Use cases: where Reflection and Echoes helps
- Homework: review the key idea behind Reflection and Echoes before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Reflection and Echoes without notes.
- Visual learning: play with the interactive lab to see how Reflection and Echoes changes when you adjust its inputs.
- Teaching a friend: explain Reflection and Echoes in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Reflection and Echoes?
Waves bounce off surfaces.
What will I learn in Reflection and Echoes?
Angle in equals angle out. Echoes need distance of about 17 m. Sonar uses echoes.
Is Reflection and Echoes part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Reflection and Echoes lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Reflection and Echoes?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.