Wave and Optics Short Questions Answers

1. Why is a diver underwater unable to hear the sound produced in the air outside?

When sound travels from air to water, a large portion of sound energy produced in the air reflects from the water surface and only a little fraction of incident intensity (= 0.1 %) enters water suffering refraction. So, the diver underwater cannot get a distinct impression of any sound.

2. What are mechanical waves and electromagnetic waves? Give some examples.

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 waves that can propagate even in empty space, where there is no medium. Eg: Light waves, Radio waves, Infrared and ultraviolet radiation, etc.

3. What are transverse and longitudinal waves?

If the particles of the medium vibrate perpendicular to the direction of propagation of the wave, the waves are called Transverse Waves. A transverse wave travels forming crests and troughs.

If the particles of the medium vibrate along (to and fro) the direction of propagation of the Waves, the waves are called Longitudinal Waves. A longitudinal Wave travels forming compressions and rarefactions.

4.  Longitudinal waves are also called pressure waves. Why?

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 refraction 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.

Progressive Waves are those waves that carry energy along with them from one place to another. The transfer of energy occurs along with crest – troughs or compression – rarefactions.

 

Stationary Waves are those which can not carry energy from one end to other. Energy in stationary waves is confined between nodes.

 

6. Do sound waves undergo reflection, refraction, and polarization phenomena? [HSEB 2062]

Sound wave undergoes reflection and refraction but it does not follow polarization. Since a sound wave is a longitudinal wave, the vibration of particles occurs to and fro in the direction of the propagation of the wave. The vibrations of such type cannot be limited or controlled by any barriers and so polarization is not possible in them.

7. Do sound waves need a medium to travel from one point to other in space? What properties of the medium are relevant? [HSEB 2062]

The plain answer is ‘YES’. Sound waves are mechanical waves and they travel by means of push and pull existing between particles, without which the compressions and rarefactions cannot transmit. The relevant properties are the elastic and inertial properties, as is shown by various relations for velocities of sound as v = √k/p = √γP/ρ (in air or gas)• v = √γ/ρ (in solid bars), v= √T/m (in stretched strings), etc.
The elastic properties determine how fast or slow the molecules return ed to the same position after being displaced by the neighboring particle. The inertial property determines how difficult it is to move the particle or stop it after it has started moving.

8. Although the density of a solid is high, the velocity of sound is greater in a solid, explain.

The velocity of sound in a solid is given by
v = √E/ρ

Where E and ρ are the elasticity and density of the solid respectively. For a solid, E takes a higher value than ρ i.e. the ratio of E/ρ will be more. so, in selling, the velocity of sound is the greater velocity of sound is greater although its density is high.

9. Explain why the flash of light reaches the earth before the sound comes from the same thunder.

This phenomenon depends on the comparative speed of light and sound through air. The velocity of tight from lightning through the air is almost 3×108 m/s, so the flash reaches earlier. However, the velocity of the sound of lightning travels with a speed of around 340m/s according to the V(in the air) = √γP/ρ . So, this sound reaches later than the flash.

10. What discrepancy was there in Newton’s formula for the velocity of sound in the air?

Isaac Newton assumed that when sound traveled through the air, the vibration of air molecules cause isothermal expansion and compression because he observed that the temperature of air remained constant. He then obtained a formula for the determination of the velocity of sound, expressed as V(in the air) = √P/ρ .

When standard values were used for P (air) and ρ (air) (1.01 × 10^5N/m^2) and 1.293 kg/m^3 respectively), the velocity of sound came to be 279.49m/s. This value did not agree with the experimentally observed value of about 330m/s. So, it was argued that the very theoretical basis of Newton’s formula should have been wrong.

11. A person walking on a railway line hears two sounds of the same explosion occurring far away, why?

The phenomenon is due to two paths used by sound to reach the listener – one is the metal some form of iron) of the railway line and the other is the air. When the sound of an explosion travels through metal, the velocity will be high (about 5000m/s), making the time required to travel very less. For sound traveling through the air, the velocity of sound will be less (about 330m/s) and so • takes more time to travel. So, the listener will hear the sound twice.

12. When a person hammers at one end of a metal pipe and a listener places his ear at the other end, two di ct sounds are heard. Why?

The phenomenon is also due to two paths used by sound to reach the listener – one is the metal pipe itself and the other is the air. When the sound of an explosion travels through metal, the velocity will be high (about 2000 to 5000m/ s), making the time required to travel very, less. For sound traveling through the air, the velocity of sound will be less (about 330m/ s} and so it takes more time to travel. So, the listener will hear the sound twice.

13. Velocity of sound increases on a cloudy day. Why?  [HSEB 2066]
Such phenomenon is due to the addition of moisture on-air and an increase in velocity of sound in such air. Clouds contain a lot of water vapor in them. If a day is cloudy, the added water vapor in the air reduces the density. Then according to the relation V(in the air) = √γP/ρ, the velocity of sound will be more.

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