5th Class Science Lights, Sound and Force Sound

     Sound

Sound is a form of energy that gives the sensation of hearing. The sound that reaches your ear travels through the air.  

          Human Ear

Sound waves enters into ear and vibrates the eardrum, these vibrations are carried to brain through the nerves. Brain interprets the sound and sends the message to the receptor through motor nerve. There are 3 small bones in the ear. These are malleus, incus and stapes. Stapes is the smallest bone in the body lies in our ear.

        Pleasant Sounds

           Unpleasant Sound

         Reflection of sound

Reflection of sound waves is the phenomena of bouncing off sound waves to the same medium, after striking the surface. Sound waves also follow the law of reflection, as followed by light waves. Sound waves can be reflected from walls of building, root, clefts etc.

Sound waves on striking a surface can:

Reflect from the surface.

Diffract around an obstacle.

Diffraction phenomena — Unlike light rays, sound wave does not travel in straight line, rather it bends and scatters around obstacles like corners of doors, windows opening etc. A diffraction phenomenon enables us to hear the conversation of the people sitting in adjacent room, as sound waves manage to reach us through the door openings.

         Echo

Echo phenomena occurs when the reflected sound waves reaches the source in more than 0.1 seconds after the original sound wave was produced. When we talk in an empty room, we'll hear our talk after a short while. This occurs because of the reflection of the sound waves back towards the source in 0.1 sec. Echo can't be heard everywhere. For echo to be heard, at least the distance of source from the reflecting surface should be 17.2 m.

The reflected sound can be heard only if it takes more than 0.1 sec to reach the observer.

Absorption of sound waves (Acoustics): It means the absorption of sound waves by a material. A surface that absorbs sound waves does not reflect them. A good sound absorber is generally a porus material i.e. glass, wool etc.

         Levels of Sound

Sound is measured in decibel. A table is given below in which source of sound and decibels are given.

Sounds of intensity more than 80 db is termed as noise. Deaf people can communicate with each other with the help of the sign languages. They use their different body parts to make signs.

Note : Human cannot hear sounds of frequency less than 20 Hz and more than 20, 000 Hz.

     How does a bat find its food?

(a) High pitched sounds produced by the bat bounce off the food and then come back to the bat

(b) Bat finds the food with the help of eyes

(c) Smell of the food helps the bat to locate it

(d) All of these

(e) None of these

Answer: (a)

Explanation 

Bat can't see in the dark hence, he locates food by producing high pitched sound that bounce off the food and then comes back to the bat.

Therefore, option (A) is correct and rest of the options is incorrect.

       Which one of the following statement is incorrect?

(a) Sound needs medium to travell         

(b) Sound can travel! through vaccum

(c) Sound can't travel! through vaccum

(d) Both A and B

(e) None of these

Answer: (b)

Explanation

Sound can't travell through vaccum. Therefore, option (B) is correct and rest of the options is incorrect.

No sound can be heard on moon because there is no atmosphere.

Speed of sound is 4 times faster in water than in air.

Straight line propagation of light is called rectilinear propagation.

The highness or lowness of sound is described as pitch.

Sound waves below 20 waves per sec is subsonic and above 20,000 waves per sec. is ultrasonic.

Visible light ranges from violet, indigo, blue, green, yellow, orange, red in an light spectrum.

Violet has minimum wavelength and red has maximum.

The study of light is called optics and the study of sound is called acoustic.

Light can travel through a vacuum.

Even though light travels at the speed of 3 x 10⁸ m/s, it would take million years to travel across the vast distances in space.