Bouncing Waves
Introduction

When you look in a mirror, you are seeing a light wave that has bounced, or been reflected. This is the reason you can see your clothes without looking down. The light coming from them reflects at the mirror and into your eyes.

You may also have noticed that waves in a swimming pool or on the sea are reflected when they hit a wall.

Whenever any wave hits a surface it gets reflected.

Reflection from a wall
Shake the beads in Fig.1 below. Watch what happens when the wave hits the wall.

Figure 1.   Waves on a string of beads attached to a wall.
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The direction the wave is travelling in changes. It turns around. As the wave cannot go on any further it bounces off the wall. This is called reflection. In the language of physics, we say that the wave has been reflected from the wall.

Figure 2.   Waves on a spring that is attached to a wall.
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Compress the spring in Fig.2.

Is the
longitudinal wave
In a longitudinal wave, the material moves in the same direction as the motion of the wave. Sound is an example of a longitudinal wave.
longitudinal wave
reflected when it reaches the wall?
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Reflecting plane waves
Generate some plane waves in the ripple tank in Fig.3 by clicking on the water. Watch what happens to these waves when they hit the barrier.

Figure 3.   Head-on reflecting waves in a ripple tank.
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We can draw a line at right angles to the waves. This line shows the direction the waves move in. The waves in Fig.4 have this line indicated in red. Just click on the water to make waves and change their angle of direction by moving the slider.

Figure 4.   Reflecting waves in a ripple tank.
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Try moving and rotating the ray box in Fig.5 below to investigate how light reflects from a mirror.


Click on the figure below to interact with the model.

 Figure 5.  Reflection of light from a mirror.

Reflecting circular waves
Circular waves are easy to make. Just throw a stone into a pond, or watch a raindrop land in a puddle. The circular waves grow and spread out from the point where the stone or the raindrop landed on the water. This point is the source of the waves.

Watch what happens when circular waves hit a plane barrier. Click on the water in Fig.6 to make circular waves.

Figure 6.   Reflecting circular waves.
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Like plane waves, circular waves are also reflected from a barrier.

What happens to the waves after they have been reflected?
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The reflected waves are similar to the initial waves but are moving in the opposite direction.

Where would you drop a stone to make waves which look just like the reflected waves?
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The point where the reflected waves appear to come from is called the virtual source of the waves. It is a
virtual source
The point from which reflected waves appear to come from is called the virtual source of the waves.
virtual source
, because it is not really the source of the waves.

Summary


Waves are reflected when they hit a surface.

The point from which reflected waves appear to come from is called the virtual source of the waves.
Exercises
1. What kinds of waves can be reflected?
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2. When are waves in a ripple tank reflected?
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3. Decide whether the factors below affect the angle at which a wave is reflected from a barrier.
  • The speed of the wave
    The wavelength of the wave
    The frequency of the wave
    The amplitude of the wave
    The angle the wave strikes the barrier
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Figure 7.  
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4. The wave in Fig.7 above hits the barrier. How is it reflected?
Figure 8.  
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5. The wave in Fig.8 above is reflected from the barrier. Which picture shows what happens?
Figure 9.  
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6. The wave in Fig.9 above is reflected from the barrier. Which picture shows what happens?
Figure 10.  
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7. The wave in Fig.10 above is reflected from the barrier. Which picture shows what happens?
Figure 11.  
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8. The wave in Fig. above is reflected from the barrier. Which picture shows what happens?
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