Reflection occurs when waves strike an obstacle or barrier and bounce back into the same medium. Learn how wavefronts behave, why wave speed, frequency and wavelength remain unchanged, and how reflection is used in SONAR and medical ultrasound.
See how crests and troughs form bright and dark fringes, and how incident and reflected wavefronts obey \(i = r\) with constant wavelength.
Light passing through crests acts like convex lenses and converges to form bright fringes. Light passing through troughs acts like concave lenses and diverges to form dark fringes.
The incident ray and reflected ray make equal angles with the normal. The reflected wavelength is equal to the incident wavelength.
A wavefront is a line that joins points that are in phase in a wave. The direction of propagation is always perpendicular to the wavefront, and the distance between two consecutive wavefronts is the wavelength, \(\lambda\).
When a wave is reflected by a plane barrier, the angle of incidence equals the angle of reflection: \(i = r\). Both angles are measured from the normal line.
Wavelength, frequency and wave speed remain unchanged because the wave stays in the same medium and comes from the same source. Only the direction of propagation changes.
For a reflected pulse, the total distance travelled is twice the distance to the reflector: \(2d = vt\), so \(d = \dfrac{vt}{2}\).
Reflection / Echo Formula
\(d\) = distance to reflecting barrier or depth of sea (m), \(v\) = wave speed (m s⁻¹), \(t\) = total time between transmission and reception (s).
Answer in your mind, then press “Check Answer”.
What happens to the wavelength and speed of a water wave when it undergoes reflection?
What is the angle between the wavefront and the direction of wave propagation?
An ultrasonic signal is sent from a ship to the seabed. The echo is received \(0.06\text{ s}\) later. If the speed of sound in water is \(1\,500\text{ m s}^{-1}\), what is the depth of the sea?
Wave speed changes after reflection because the wave bounces back.
Wave speed depends only on the properties of the medium. Since reflection occurs within the same medium, speed does not change.
Frequency changes after reflection.
Frequency is determined strictly by the vibration frequency of the wave source, which remains unaffected by reflection.
Objective: Test your understanding of wave reflection, wavefronts and echo calculations.
1 Which of the following wave characteristics changes when a water wave is reflected by a plane barrier?
2 A pulse of ultrasound travels at \(1\,500\text{ m s}^{-1}\) in water. It is reflected by the seabed and detected after \(120\text{ ms}\). What is the depth of the sea?
3 The direction of wave propagation is always to the wavefront.
4 In reflection of waves, the angle of incidence is always equal to the angle of .
Match each step of drawing water wave reflection to its correct description. If using a phone, tap the answer first, then tap the matching box.
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