Maharashtra Board Class 12 Physics Chapter 6 Superposition of Waves PDF Download

6. Superposition of Waves

Can You Recall?

1. What is wave motion?

2. What is a wave pulse?

3. What are common properties of waves?

4. What happens when a wave propagates?

5. What are mechanical waves?

6. What are electromagnetic waves?

7. How are mechanical waves different from electromagnetic waves?

8. What are sound waves?

6.1 Introduction

You may be familiar with different waves like water waves, sound waves, light waves, mechanical waves, electromagnetic waves etc. A mechanical wave is a disturbance produced in an elastic medium due to periodic vibrations of particles of the medium about their respective mean positions. In this process, energy and momentum are transferred from one particle to another. Thus, a wave carries or transfers energy from one point to another, but there is no transfer of matter or particles of the medium in which the wave is travelling.

Another type of waves, known as electromagnetic waves, do not require material medium for their propagation; these are non-mechanical waves. We have studied sound waves (which are mechanical waves), their properties and various phenomena like echo, reverberation, Doppler effect related to these waves in earlier classes. In this Chapter, we will study mechanical waves, reflection of these waves, principle of superposition of waves, various phenomena like formation of stationary waves, beats, and their applications.

6.2 Progressive Wave

Have you seen ripples created on the surface of water when a stone is dropped in it? The water is displaced locally where the stone actually falls in water. The disturbance slowly spreads and distant particles get disturbed from their position of rest. The wave disturbs the particles for a short duration during its path. These particles oscillate about their position of rest for a short time. They are not bodily moved from their respective positions. This disturbance caused by the stone is actually a wave pulse. It is a disturbance caused locally for a short duration.

A wave, in which the disturbance produced in the medium travels in a given direction continuously, without any damping and obstruction, from one particle to another, is a progressive wave or a travelling wave e.g., the sound wave, which is a pressure wave consisting of compressions and rarefactions travelling along the direction of propagation of the wave.

6.2.1 Properties of Progressive Waves

1) Each particle in a medium executes the same type of vibration. Particles vibrate about their mean positions performing simple harmonic motion.

2) All vibrating particles of the medium have the same amplitude, period and frequency.

3) The phase, (i.e., state of vibration of a particle), changes from one particle to another.

4) No particle remains permanently at rest. Each particle comes to rest momentarily while at the extreme positions of vibration.

5) The particles attain maximum velocity when they pass through their mean positions.

6) During the propagation of wave, energy is transferred along the wave. There is no transfer of matter.

7) The wave propagates through the medium with a certain velocity. This velocity depends upon properties of the medium.

8) Progressive waves are of two types - transverse waves and longitudinal waves.

9) In a transverse wave, vibrations of particles are perpendicular to the direction of propagation of wave and produce crests and troughs in their medium of travel. In longitudinal wave, vibrations of particles produce compressions and rarefactions along the direction of propagation of the wave.

10) Both, the transverse as well as the longitudinal, mechanical waves can propagate through solids but only longitudinal waves can propagate through fluids.

Teacher's Note

When you drop a stone in water, you see ripples moving outward. These ripples are progressive waves carrying energy from one place to another, just like sound travels from a speaker to your ear.

Exam Trick

Remember: Progressive wave = wave moves forward. In progressive waves, the particle vibrates but does not move with the wave. Energy moves, not particles.

Points to Remember

A mechanical wave needs a medium to travel.
Progressive waves carry energy from one place to another.
Particles vibrate up and down, but the wave moves forward.
All particles have the same amplitude and frequency.
Progressive waves can be transverse or longitudinal.

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