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Old Is Gold Class 12
UNIT-1. WAVES AND OPTICS
A. WAVE MOTION
SHORT ANSWER QUESTIONS
1. We can't hear echo in a small room.Why?
Ans: We can't hear echo in a small room because in the sound can'tbe reflected back. For an echo of a sound to be heard, the minimum distance between the source of sound and the walls of the room should be 17.2m. Obviously, in a small room echoes cannot ve heard.
2. Frequency is the most fundamental property of a wave.Why?
Ans: As when a wave travels from one medium to another medium, wacelength and the speed of the wave changes whereas frequency remains unchanged.So, frequency is the most fundamental property of a wave.
3. If you are walking on the moon surface, can you hear the cracking sound behind you? Explain.
Ans: No, we can't hear the cracking sound behind us, if we are walking on the moon surface. This is so because there is no atmosphere on the moon. So, there is lack of medium for the propagation of the soubd waves on the moon. As we know that soubd waves need medium for propagation, it cannot propagate in vacuum
4. Distinguish between progressive waves and standing waves.
Standing wave: Wave profile doesnot move. Particles between two adjacent nodes vibrates with different amplitudes. Particles at nodes do not vibrate at all. Produced by the superposition of the two waves moving in a opposite direction. Doesnot transmit the energy.
5. Distinguish between light waves and sound waves.
Ans: Light wave: Velocity of light in air is approximately 3*10 to the power 18 m/s.
Light wave can travel through vaccum. Light wave are transverse wave. Light wave is a type of electromagnetic wave.
Sound wave: Velocity of sound waves in air is approximately 330 m/s. Sound requires medium for propagation and hence cannot travel through vacuum. Sound wave is longitudinal. Sound waves are mechanical wave.
6. Can two persons on moon hear the sound of each other? Explain.
Ans: No, two person cannot hear each other on moon because sound needs material or medium to travel. On moon their isn't any atmosphere so they unable to talk to each other.
7. Which types of wave propagate in liquid, explain.
Ans: Both transverse wave and longitudinal wave are possible in liquid. Between these two waves major component of the wave is longitudinal which dominates transverse wave. A liquid can have bulk modulus of elasticity but it doesn't have modulus of rigidity.
8. Do sound waves undergoes reflection, refraction and polarization phenomena?Explain.
Ans: Being a wave, the sound wave goes under reflection and refraction. These phenomena are exhibited by both transverse wave and longitudinal wave. But polarization phenomena is exhibit polarization phenomena.
9. How are stationary waves formed?
Ans: Statinary or standing wave are formed in a medium when two waves having equal amplitude and frequency moving in opposite direction along the same line interfere in a confined space. OR,
It is formed by superposition of the forward wave and reflected wave.
SOME IMPORTANT QUESTIONS
1. Transverse wave motion is possible in solid medium. Why?
Ans: Because, transverse waves can be produced in a medium which has elasticity of shape(rigidity). Only solid have rigidity, therefore, transverse wave motion is possible only in solids.
2. In which type of media (solids,liquids or gas) is longitudinal wave motion possible?
Ans: All types of media.
3. Why do longitudinal waves propagate only in gases?
Ans: For transverse wave motion, a medium must have elasticity of shape. Gases do not have elasticityof shape. So transverse wave cannot pass through it. For longitudinal wace motion, elasticity of shape of a medium is not necessary, so longitudinal wave can pass through the gases.
4. Longitudinal waves are also called pressure waves. Why?
Ans: Because, these waves propagate in a medium due to alternate compressions and rarefactions, i.e. they travel due to variation in pressure.
5. What is a harmonic wave function?
Ans: A periodic wave function whose functional form is in term of sine or cosine is called harmonic wave function.
6. We cannot hear explosions on other planets. Why?
Ans: Sound waves can travel only through matrials medium between the earth and other planets therefore we cannot hear the explosions on the other planets.
7. We see lightning before we hear thunder. Why?
Ans: Because, speed of light (3*10 to the power 8 m/s) is much higher than the speed of sound (332m/s).
8. What causes the rolling sound of a thunder?
Ans: The multiple reflection of sound between the clouds and the earths surface causing rolling sound of thunder.
9. What is the nature of sound waves?
Ans: Longitudinal. It travels as longitudinal wave through a medium.
10. Why are stationary waves called so?
Ans: In these waves, particles of the medium vibrate about their mean position but disturbance (wave length) doesnot move in any direction hence the name stationary wave.
11. Two astronauts on the surface of the moon cannot talk to each other like on the earth. Why?
Ans: Sound propagates only through the material medium. There is no atmosphere on the moon as is on the earth. Therefore, sound can propagate through the air on the earth and we can hear sound here, but not on the moon.
12.Which is more elastic, steel or rubber?
Ans: Consider two identical wires, one steel and another rubber, suspended from rigid support. Suppose, equal masses are attached to the lower ends of each, therefore , tension in them is equal. We will find that the extension in the rubber thread is much greater than extension in steel wire. Since, Young's modulus is inversly proportional to the extension produced. Thus, magnitude of stress (Y*strain) for given value of strain is much larger in steel than in rubber. This means, if steel wire is deformed, large internal forces appear. In this sense, one can say that steel is more elastic than rubber.
13. Rubber pads are used as vibration absorber. Why?
Ans: Rubber has large elastic hysteresis, i.e. workdone by a deforming force on deforming a rubber is greater than the workdone by the rubber in returning to its original shape. Thus, a particular amount of energy is absorbed in the process of deformation and restoration. This loss of vibrational energy appears as heat. If a rubber pad is placed between the vibrating body and the floor, the rubber is compressed and released in every cycle of vibration. Mechanical energy is converted into internal energy which results in the rise in temperature. As a result, only a small part of energy which results in the rise in temperature. As a result, only a small part of energy of vibration is transmitted to the floor.
14. Why are springs made of steel and not copper?
Ans: Because, steel is more elastic than rubber.
15. Where does the work, which is used to stretch or compress a wire go?
Ans: This is stored as elastic potential energy in the wire.
16. Ploughing of the field helps to retain the moisture of the soil. Why?
Ans: Ploughing of the field breaks the small capillries formed in the soil. This stops the capillary action of water in the soil. Thus, water cointained in the soil cannot evaporate and moisture is retained.
17. Liquid droplets are spherical but larger drops have oval shape. Why?
Ans: For small droplets, the force of surface tension is much greater than force of gravity(i.e.weight), therefore, they acquire spherical shape. But, for larger drops, force of gravity is much greater than force of surface tension, hence, takes oval shape.
18. Why does oil spread over water surface while water doesnot do so on oil surface?
Ans: Oil has surface tension much less than that of water. Therefore, oil poured over water experiences tension tangential to the surface. As a result, oil spreads over the surface of water. But, the situation is opposite for water on the oil surface and water cannot spread.
19. Why does hot soup taste better than cold one?
Ans: Surface tension of hot soup is better thab that of cold soup so spreads over larger area and gives better taste.
20. In the absence of external force, a liquid drop always takes a spherical shape. Why?
Ans: In the absence of external force, only the force of surface tension acts on the liquid which modifies the shape of the liquid to have minimum for spherical shape. So it will take a spherical shape. Due to the same region, a freely falling rain drop and a soap bubble are spherical.
21. A steel needle may float on water but sinks when a small amount of detergent is added to water. Why?
Ans: It is so because detergent reduces the surface tension and the reduced surface tension cannot support the weight of needle.
22. Can the waves of sea be claimed by pouring oil on its surface?
Ans: The oil poured on the sea spreads over the surface which is driven forward by wind. A comparatively pure water is left behind. Since surface tension of pure water is greater than that of oil, it exerts backward pull to the water and prevents the waves from becoming very large.
23. Why should detergents have small angles of contact?
Ans: If angle of contact is small, it is easily dissolved in water.
Neb Solution of modern physics short questions of class 12
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