If a single atom of hydrogen is present, how many wavelength will be seen and why

Here's why:

• Single Transition: When an electron in a single hydrogen atom transitions from a higher energy level to a lower one, it undergoes one specific energy change.
• Single Photon: This single energy change results in the emission of one photon of light.
• Single Wavelength: Each photon has a unique energy, frequency, and thus a single corresponding wavelength.

The full hydrogen emission spectrum, which shows multiple distinct wavelengths (lines), is observed from a macroscopic sample of hydrogen gas. In such a sample, many atoms are simultaneously undergoing various different possible electron transitions, each contributing a photon of a specific wavelength to the overall observed spectrum.