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Background

The term wave is commonly used in our day to day life. Some natural occurrences are as a result of waves. At the coast line, people enjoy the actions of the sea waves while having fun. Waves can sometimes be useful in our day to day living like in the case of tidal waves being used to produce electricity and at other times they can be destructive for example tsunami.

Objectives

By the end of the topic, you should be able to:

  • Describe experiments to illustrate properties of waves
  • Sketch wave fronts to illustrate properties of waves
  • Explain the phenomena of constructive and destructive interference
  • Describe experiments to illustrate stationary waves












Introduction

In your form two syllabus, you learnt about the formation and classification of waves. In this chapter, we are going to look at the properties exhibited by waves. Such properties will include reflection, refraction, diffraction and interference.

Wave properties

The following are some of the wave properties: Reflection, refraction, diffraction and interference.

Reflection

Waves are reflected according to the same laws of light reflection. The following animations show how waves behave before and after reflection when they hit differently shaped reflectors.

(a) Plane waves hitting a straight reflector at 90O

When plane waves hit a straight reflector at an angle of 90o, they are reflected as shown in the animation below. Click on the play button and make your observations.

b. Plane waves hitting a reflector at an angle

When the plane waves meet a straight reflector at an angle, they are reflected as shown in the next animation. Play the animation to observe the behaviour of the waves.

c. Plane water waves hitting a concave reflector

The animation below shows what happens when plane water waves meet a concave reflector. Click on the play button and make your observations. What do you observe?

Observations

The waves converge at a point in front of the reflecting surface.

Conclusion

This shows that the concave reflector has a real focus.

d. Plane water waves hitting a convex reflector

Play the animation below and observe how plane waves are reflected on hitting a convex reflector.What do you observe?

Observations

The waves appear to be diverging from a point behind the reflector showing that the convex reflector has a virtual focus.

e. Circular water waves hitting a concave reflector

Play the animation below to observe how circular waves incident on a concave reflector behave on hitting the reflector.

Refraction of waves

This can be demonstrated using a raised platform in water when a wave created at the deep end is observed as it moves through to the shallow end. At the shallow end, the speed and wavelength decreases. Click on the play button to observe this from the video below.

Discussion

The frequency of the wave remains constant. Light behaves in a similar manner when it moves from a rarer to a denser medium. For sound, refraction occurs due to the change in speed of the wave. The speed is also affected by temperature. As sound passes through layers of air at different temperatures, it is refracted to other directions. On a hot day, the range of sound is less because the waves are bent upwards from the hot earth where they otherwise travel faster. In the evening or when cold, air near the ground becomes cool and refraction makes it easier to hear distant sounds across the land.

Diffraction

This is the spreading of waves around obstacles or as they pass through openings/gaps. Play the video below to observe this.


Discussion

Diffraction of the waves is greatest when the wavelength of the waves is similar to the width of the slit (gap). For a given wavelength, the narrower the gap, the greater the bending of the waves. Sound easily spreads round corners because of its long wavelength. During diffraction, the velocity, wavelength and frequency of the wave remain constant. Sounds of shorter wavelength are least diffracted and hence more directional. It is for this reason that high pitched sounds are heard best in front of speakers and not behind them. Light has very short wavelength and requires a very short aperture to show significant diffraction. Radio transmission at long wave metre band can have good reception over a wide area.

Interference

This is a term given to effects of waves which occur when two or more separate wave trains overlap. There are two types of interference:

(a) Constructive interference and

(b) Destructive interference.

Constructive and destructive interference

Constructive interference

The production of increased amplitude when two waves are in phase is called constructive interference. Play the video below to observe this.


Observations

Bright and dark fringes (lines) are seen on the white screen.

Discussion

The formation of bright and dark lines shows that interference is taking place. The bright lines show the parts where the waves from the two sources interfere constructively while the dark lines show points where destructive interference takes place.

Conclusion

The production of increased amplitude when two waves meet in phase is called constructive interference while the reduced resultant amplitude caused by the meeting of a crest and a trough is known as destructive interference.

Stationary waves

Stationary waves can be produced on a string by either plucking it or creating a vibratory motion on it. Click the play button and make your observations.


Observation

A standing wave with nodes and antinodes is formed.

Explanation

The particles at the fixed end cannot vibrate and therefore undergo zero displacement forming a node. At the antinodes, the particles are free to vibrate horizontally and are therefore displaced thereby forming an anti-node.

Introduction

In your form two syllabus, you learnt about the formation and classification of waves. In this chapter, we are going to look at the properties exhibited by waves. Such properties will include reflection, refraction, diffraction and interference.

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