Amonochromatic laser is exciting hydrogen atoms from the n=2 state to the n=5 state. part a what is the wavelength λ of the laser?

The energy levels of the hydrogen atom are given by

where n is the level number.

In order to make transition from n=2 state to n=5 state, the electron should acquire an energy equal to the difference between the two energy levels:


Keeping in mind that , we can convert this energy in Joules


This is the energy the photons of the laser should have in order to excite electrons from n=2 state to n=5 state. Their frequency can be found by using

where h is the Planck constant and f is the photon frequency. Re-arranging it, we find


and finally we can use the relationship between frequency, wavelength and speed of light which holds for photons, in order to find their wavelength:

and this is the laser wavelenghth.

Aform of energy that is reflected or emitted from objects in the form of electrical and magnetic waves that can travel through space. there are many forms of  electromagnetic  energy including gamma rays, x rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.

Amethod of procedure that has characterized natural science since the 17th century, consisting in systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses.