0

Sound and light - class-VIII

Description: sound and light
Number of Questions: 23
Created by:
Tags: world of sounds propagation of sound waves acoustics physics
Attempted 0/23 Correct 0 Score 0

Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. This is because:

  1. speed ofsound is greater than speed oflight

  2. speed of sound is equal to the speed of light

  3. speed of light is much greater than the speed of sound

  4. none of these


Correct Option: C
Explanation:

It is because light travels faster  ($3\times10^{8}m/s$) than sound ($332m/s$) , so we hear thunder after a few second of the flash .

Correct comparison between light waves and sound waves is:

  1. Both are mechanical waves

  2. Both need a material medium for propagation

  3. Both can travel through vacuum

  4. None


Correct Option: D
Explanation:

Sound waves are mechanical waves, light waves are electromagnetic waves. Light can travel through vacuum, sound cannot.

Sound waves travels _________ than light in vacuum.

  1. faster

  2. slower

  3. not slower

  4. Can't travel


Correct Option: D
Explanation:

Light waves are non mechanical waves. They don't need a material medium for their propagation. So they can travel through vacuum. But sound waves can't travel through vacuum.

Sound waves travels _____  light in air

  1. faster than

  2. slower than

  3. at same speed as

  4. either faster than or with same speed as


Correct Option: B
Explanation:

Light waves are non mechanical waves. Speed of non mechanical waves is much higher than mechanical waves.

The distance between tow adjacent particles which are in the same phase in a progressive wave is 20 cm. determine the velocity of the wave if its frequency is 10 Hz.

  1. $2 {ms}^{-1}$

  2. $40 {ms}^{-1}$

  3. $8 {ms}^{-1}$

  4. $80 {ms}^{-1}$


Correct Option: A

Light and Sound waves

  1. both are mechanical waves

  2. both need material media for propagation

  3. both can travel through vacuum

  4. are two different types of waves


Correct Option: D
Explanation:

Sound waves are mechanical waves which are longitudinal in nature and require material media to pass through.

Light waves are electromagnetic waves which are transverse in nature and do not require any material medium to pass through.

The thunder of a cloud is heard later compared to flash of lightning because :

  1. speed of sound is greater than speed of light

  2. speed of light is greater than speed of sound

  3. speed of sound is the same as speed of light

  4. sound is obscured by the rain


Correct Option: A
Explanation:

The light travels much faster than the sound. The thunder travels at the speed of sound while the light from the flash travels at the speed of the light. The speed of light is about 340 m/s through air, while the speed of light is about 300 million m/s.So the thunder is heard after the flash of lightening.

You see lightning before you hear the thunder because

  1. Lightning is much brighter than thunder.

  2. Thunder is heavier than lightning.

  3. Light can travel through air but sound cannot.

  4. Sound travels slower than light.


Correct Option: D
Explanation:

Answer is D.

Lightning and thunder are produced simultaneously, but the thunder is heard a few seconds after the lightning is seen. This is because the speed of light in air is more than the speed of sound in air.The speed of light is given as $3\times { 10 }^{ 8 }m/s$. The speed of sound is 340 m/s.
Hence, the flash of lightning is seen before the thunder is heard as the speed of sound is less than speed of light.

Speed of sound wave in a gas is $v _{1}$ and tins speed of molecules of the gas at the same temperature is $v _{2}$. Then :

  1. $ v _{1} = v _{2}$

  2. $ v _{1} < v _{2}$

  3. $ v _{1} > v _{2}$

  4. $v _{1}\le\ v _{2}$


Correct Option: A

A source of frequency 500Hz emits waves of wavelength 0.2m.How long does it take to travel 300 m?

  1. 70 s

  2. 60 s

  3. 12 s

  4. 3 s.


Correct Option: D

A worker lives at a distance of 1.32 km from the factory. If the speed of sound in air is 330 m$s^{-1}$, how long will the sound of factory siren take to reach the worker?

  1. 1 s

  2. 2 s

  3. 3 s

  4. 4 s


Correct Option: D
Explanation:

Given,


Distance $=1.32\,km=1320\,m$

Speed of sound in air $=330\,m/s$

We have,

$Time=\frac{Distance}{Speed}$

$Time=\frac{1320}{330}=4\,s$

Thus it would take $4\,s$ for the sound of the factory siren to reach the 
worker.

A thunder tap is heared 5 second after the lightening flash. The distance of the flash is (velocity of sound in air is 345 m/sec.) :

  1. 3560 m

  2. 1725 m

  3. 1780 m

  4. 1815 m


Correct Option: B
Explanation:

Given,

$v=345m/s$
$t=5sec$
The distance of the flash is,
$d=vt$
$d=345\times 5=1725m$
The correct option is B.

Two waves of wavelengths $\lambda$ and $(\lambda + \triangle \lambda)$ produce $6$ beats per second. If $\dfrac {\triangle \lambda}{\lambda} = \dfrac {\lambda + \triangle \lambda}{51}$, find the speed of sound wave in a gas in which these waves produce beats?

  1. $316\ m/s$

  2. $310\ m/s$

  3. $306\ m/s$

  4. $326\ m/s$


Correct Option: D

Flash and thunder are produced simultaneously. But the thunder is heard a few seconds after the flash is seen. Why?

  1. The speed of sound is greater than the speed of light.

  2. The speed of sound is equal to the speed of light.

  3. The speed of light is greater than the speed of sound.

  4. The speed of light is less than the speed of sound.


Correct Option: C
Explanation:

Flash and thunder are produced simultaneously. But the flash is seen first and then the sound is heard. This is because the speed of sound in air is very less than the speed of light in air.

Speed of sound in air $=334\,m/s$
Speed of light in air $=3\times 10^8\,m/s$

The speed of light in vaccum is....

  1. $3 \times {10^8}m/s$

  2. $0.3 \times {10^8}m/s$

  3. $9 \times {10^8}m/s$

  4. $30 \times {10^8}m/s$


Correct Option: A

The time interval between a lightning flash and the first sound of thunder was found to be $5 \ s$. If the speed of sound in air is $330  \ ms^{-1}$, find the distance of flash from the observer.

  1. $1650 \ m$

  2. $165 \ m$

  3. $330 \ m$

  4. None of the above


Correct Option: A
Explanation:

The speed of the sound, distance traveled and the time taken relationship is given as follows.


$Speed\; of\; sound=\dfrac { Distance\; travelled}{ Time\; taken} $

From this equation the Distance traveled is derived as 
$Distance\; travelled=Speed\; of\; sound\times Time\; taken$


That is, $Distance\; travelled=330\; m/s\times 5\; seconds$, given that the speed of sound is 330 m/s.

= 1650 meters.

Hence, the distance of flash from the observer is 1650 meters.

The speed of sound in a certain medium is $960 m/s$. If $3600$ waves pass over a certain point in 1 minute, the wavelength is,

  1. $2 m$

  2. $8 m$

  3. $4 m$

  4. $16 m$


Correct Option: D

When a tuning fork of 500 Hz is used, the first and second resonating lengths of closed air column are found to be 18 cm and 54 cm respectively. Find the velocity of sound in S.I. system.

  1. $120{ms}^{-1}$

  2. $240{ms}^{-1}$

  3. $360{ms}^{-1}$

  4. $480{ms}^{-1}$


Correct Option: C

An observer measures speed of light to be C when he is stationary with respect to the source. If the observer moves with velocity $\displaystyle v$ towards the source then the velocity of light observed will be :

  1. $\displaystyle c- v$

  2. $\displaystyle c + v$

  3. $\displaystyle \sqrt{1-v^2/c^2}$

  4. $\displaystyle c$


Correct Option: D
Explanation:

Speed of light does not change with the frame of reference.  Option 'D' is correct.

The frequency of fundamental mode of vibration of an air column enclosed in a closed end pipe is 250 Hz. If its length is 33 cm, then find the velocity of sound in air.

  1. $390 {ms}^{-1}$

  2. $160 {ms}^{-1}$

  3. $330 {ms}^{-1}$

  4. $150 {ms}^{-1}$


Correct Option: C

A wave has a frequency of $5$ GHz.
What is the period of the wave?

  1. $200$ ps

  2. $0.2$ ns

  3. $20$ ns

  4. $20000$ $\mu s$


Correct Option: B
Explanation:

Given that,


Frequency $f=5 GHz=5\times10^{9}\ Hz$


The period of the wave is reciprocal of frequency.

$T=\dfrac{1}{f}$

$T=\dfrac{1}{5\times10^{9}}$

$T=2\times10^{-10}$

$T=0.2\ ns$

The period of the wave is $0.2\ ns$

Hence, B is correct option.

Which of the following product of $e, h, \mu, G$ (where $\mu$ is the permeability) be taken so that the dimensions of the product are same as that of the speed of light?

  1. $h e^{-2} \mu^{-1} G^0$

  2. $h^2 e G^0 \mu$

  3. $h^0 e^2 G^{-1} \mu$

  4. $h G e^{-2} \mu^{0}$


Correct Option: A
- Hide questions