Answer : The energy of one photon of hydrogen atom is, 
Explanation :
First we have to calculate the wavelength of hydrogen atom.
Using Rydberg's Equation:
Where,
= Wavelength of radiation
= Rydberg's Constant = 10973731.6 m⁻¹
= Higher energy level = 3
= Lower energy level = 2
Putting the values, in above equation, we get:
Now we have to calculate the energy.
where,
h = Planck's constant = 
c = speed of light = 
= wavelength = 
Putting the values, in this formula, we get:
Therefore, the energy of one photon of hydrogen atom is,
Explanation:
A projectile (Cannon ball) is launched at an angle to the horizontal and rises up to a peak while moving horizontally. When it reaches the peak, the projectile starts to fall.
The time described above is known as the waves Period.
The time which it takes for a particle to complete one full cycle is known as the period. Period is normally measured in seconds. Frequency on the other hand is the number of cycles which are completed in a given period of time e.g a second. periodic time T is given by reciprocal of frequency (1/f).
