Short Answer Questions : Wave
1. Why is a diver under water unable to hear the sound produced in air outside?
ans: When sound travels from air to water, large portion of sound energy produced in air reflects from the water surface and only a little fraction of incident intensity around 0.1% enters water suffering refraction. That’s exactly the reason why a diver under water can’t get a distinct impression of any sound.
2. What are mechanical waves and electromagnetic waves? Give some examples.
ans: The waves which require some material medium to travel are called Mechanical Waves. Eg: sound waves, Waves in a string or a rope under tension etc.
Electromagnetic waves are those which can propagate even in empty space, where there is no medium. Eg: Light waves, Radio waves infrared, ultra violet radiations etc.
3. What are transverse and longitudinal waves?
ans: If the particles of the medium vibrate perpendicular to the direction of propagation of wave, the waves are called Transverse waves. The propagation of transverse waves are the from of crests and troughs. i.e they form crests and troughs as they travel.
If the particles of the medium vibrate along ie in a to and fro motion of the propagation of waves, the waves are called longitudinal waves. A longitudinal waves travels forming compression and rarefractions.
4. Longitudinal waves are also called pressure waves. Why?
ans: If the particles of the medium vibrate along (to and fro) the direction of propagation of the Waves, the waves are called Longitudinal waves. When they vibrate to and fro, they cause compression in the region in front of the vibrating particles and rarefraction to the backside. So, wherever they go, they create regions of high pressure and low pressure. So, they are called pressure waves.
5. Distinguish between progressive and stationary waves.
ans: Progressive waves are those waves which carry energy along with them from one place to another. The transfer of energy occurs along with crest – trough or compression – rarefractions. It may be either transverse or longitudinal. E.g. light wave, sound waves, water waves etc are progressive waves.
Stationary waves: The resultant waves formed due to superposition of two progressive waves of same amplitude and frequency travelling in opposite directions is called stationary wave. The waves cannot carry energy from one end to other. Energy in stationary waves is confined between nodes. For example, electrons orbiting around the nucleus of an atom is considered stationary wave.
6. When a stone is thrown into a pond and the resulting ripples spread in ever-widening circles, the amplitude decreases with increasing distance from the center. Why?
ans: When a stone is thrown into a pond and the resulting ripples spread in ever-widening circles, the amplitude decreases with increasing distance from the center.This is because, as the ripples travels in the form of waves and the wave-forms is transverse waves, the energy is transformed from one particle to another particles and with the increase in distance from the source of disturbance, there is loss of energy. This energy is used to vibrate the neighbouring particles.Moreover, the vibrating particles experience resistance forces like inertia, viscosity etc. that lowers the vibration of particles. This ultimately results in decrease in amplitude.
7. How are stationary waves formed?
ans: Stationary waves are those which cannot carry energy from one end to other. Their formation is possible when two waves of same frequency travelling with same velocity in opposite directions interact with each other. This makes the resultant velocity, it gets locked in between the nodes. This makes their amplitude increase to very high values.
8. Do sound waves need a medium to travel from one point to other point in space? What properties of the medium are relevant?
ans: Of course, a medium is necessary for the propagation of a sound wave. The medium must meet two requirements . First the medium should have elasticity so that particles can return to equilibrium or mean position after having been disturbed. The medium must have inertia so that particles are able to maintain their previous state of rest or motion.
9. When water is poured into an extremely hot vessel, it gives a sizzling sound why?
ans: Such phenomenon is more related to the sudden high pressure created in the water i.e. the vessel boundary. It is to be noted that sound is produced due to compression and rarefactions produced in the producing substance and then in the medium, which requires that there be some pressure difference. When water comes in contact with hot vessel, water suddenly reaches boiling point there and turn to vapour. It tries to expand suddenly causing a large pressure. This pressure pushes neighboring atoms and suddenly starts a series of compressions and rarefactions which then gives the impression of sound.
10. What are the types of waves propagate in liquid? Explain.
ans: Liquid molecules are mobile and also can easily slide ie shear over one another. Besides they are highly incompressible. So, inside the liquid, the waves are longitudinal, the velocity being higher that in air. At the surface however, the waves are transverse.
11. Can we talk to each other on the surface of the moon?
Or
If you are walking on the surface, can you hear the cracking sound behind you?
ans: ‘NO’. The answer is no as it is impossible to talk to each other. Sound propagates only through the material medium. There is no atmosphere on the moon. Due to the absence of medium sound waves can’t travel there.
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