Refraction of waves is the change in the direction of propagation of waves caused by the change in the velocity of waves when propagating from one medium to another. In water waves, variations in water depth alter wave speed and wavelength, while the frequency remains unchanged. This concept explains natural occurrences such as sea waves curving to follow shorelines and sound being heard more clearly at night than during the day.
See how wave direction, speed and wavelength change between deep and shallow regions, and how this relates to wave behaviour at capes and bays.
Refraction of waves is the change in direction of propagation of waves caused by the change in the velocity of waves when the waves propagate from one medium to another.
Speed of water waves is influenced by the depth of water. Speed of sound waves is influenced by the density of air. Speed of light waves is influenced by the optical density of the medium.
Since frequency \(f\) remains constant during refraction: \(\frac{v_1}{\lambda_1} = \frac{v_2}{\lambda_2}\) or \(\frac{v_1}{v_2} = \frac{\lambda_1}{\lambda_2}\).
Wave speed is given by \(v = f\lambda\), so \(f = \frac{v}{\lambda}\).
Step 1: Draw an arrowed line \(AB\) perpendicular to the incident wavefronts to represent the direction of propagation of the incident wave.
Step 2: Draw the normal line perpendicular to the boundary between the deep and shallow regions at point \(B\).
Step 3: Draw an arrowed line \(BC\) representing the refracted ray. From deep to shallow, \(BC\) is bent nearer/towards the normal than \(AB\). From shallow to deep, \(BC\) is bent away from the normal.
Step 4: Draw parallel lines perpendicular to line \(BC\) to represent the refracted wavefronts. For deep to shallow, refracted wavefront lines are drawn closer together. For shallow to deep, refracted wavefront lines are drawn farther apart.
Sound during day: Air closer to Earth's surface is hotter than the cooler air above. Sound travels faster in hot air than in cold air. Sound waves are refracted upwards, away from the ground. Sound is heard weaker and less clearly by an observer on the ground.
Sound at night: Air closer to Earth's surface is colder than the warmer air above. Sound travels slower near the ground and faster in the warmer upper layer. Sound waves are refracted downwards, towards the ground. Sound is heard louder and more clearly by an observer on the ground.
Sea waves at shoreline: Capes are shallow water regions, while bays are deeper water regions. Wave energy converges towards the cape, causing higher waves. Wave energy diverges away from the bay, causing calmer water with smaller amplitude waves.
Answer in your mind, then press “Check Answer”.
What wave characteristic remains unchanged when a plane water wave is refracted from a deep region into a shallow region?
When a plane water wave propagates from a deep water region to a shallow water region at an angle of incidence of \(45^\circ\), how does the angle of refraction compare to \(45^\circ\), and in which direction does the wave bend?
A water wave with a wavelength of \(4\text{ cm}\) moves at a speed of \(16\text{ cm s}^{-1}\) in deep water. What is its speed in shallow water where its wavelength decreases to \(2.5\text{ cm}\)?
Refraction only occurs if the wave hits the boundary at an angle.
Speed and wavelength change whenever a wave enters a medium of different depth or optical density, even at normal incidence (\(i = 0^\circ\)). However, a change in the direction of propagation only occurs when the angle of incidence is greater than zero (\(i > 0^\circ\)).
Waves at bays have larger amplitude because bays are deeper.
Wave energy converges at the capes (shallow water), resulting in higher wave amplitude. At bays, wave energy diverges and spreads over a wider area, resulting in smaller wave amplitude and calmer waters.
Objective: Test your understanding of wave refraction, wave characteristics and natural phenomena.
1 What happens to the wavelength and speed of a plane water wave when it propagates from a shallow region into a deep region?
2 Why can sound from a distant source be heard more clearly and louder at night than during the day?
3 During the refraction of waves, the property of the wave that remains completely unchanged across different media is its .
4 When sea waves approach the shoreline, wave energy converges towards the cape and diverges at the .
Drag each term to the correct meaning. If using a phone, tap the answer first, then tap the matching box.
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