When two sound waves of similar frequency interfere with each other, there is a fluctuating sound heard which is periodic and repeating. This fluctuating sound is called beat. Beat frequency( number of beats per second) is equal to the difference of the frequency of the two sound waves.
So, sound waves of frequency 400 Hz and 404 Hz will produce 4 beats per second.
Answer:
1. The change in temperature of the material.
2. The length of the material.
3. The type of material.
Explanation:
The term linear expansion is defined as the increase in length of any conductor when there is a change in temperature. The new length of any wire or rod is given by :

is the initial length of the rod
is the coefficient of linear expansion
is the change in temperature
It is clear that the linear expansion of a material depends on :
1. The change in temperature of the material.
2. The length of the material.
3. The type of material.
So, all options are correct.
We will make the comparison between each of the sizes against the known wavelengths.
In the case of the <em>hydrogen atom</em>, we know that this is equivalent to
m on average, which corresponds to the wavelength corresponding to X-rays.
In the case of the <em>Virus</em> we know that it is oscillating in a size of 30nm to 200 nm, so the size of the virus is equivalent to the range of the wavelength of an ultraviolet ray.
In the case of <em>height</em>, it fluctuates in a person around
to
m, which falls to the wavelength of a radio wave.