A laser beam of wavelength 720 nm shines through a diffraction grating that has 750 lines/mm and observed on a screen 1.4 m behind the grating. For the steps and strategies involved in solving a similar problem, you may view a Video Tutor Solution. Part A How many bright fringes can be observed on a screen

m_max = 5

Explanation:

In order to calculate the number of bright fringes on the screen, you first take into account the diffraction grating equation:

           (1)

d: distance between slits

m: order of a bright fringe

λ: wavelength of light = 720nm = 720*10^-9m

θ: angle between the normal to the grating and the mth bright fringe

The maximum number of fringes is obtained when the angle θ is a maximum, that is, for θ=90°

The distance between slits is calculated by using the following formula:

N: number of slits per meter = 750 lines/mm

You solve for m in the equation (1)m, and replace the values of d and θ for the maximum number of bright fringes over the normal to the screen.

The maximum number of bright fringes is an integer, then you approximate m = 2. This means that there are two bright fringes above the central peak.

The total number of fringes is twice the previous value of m plus the central peak:

There are 5 bright fringes in the diffraction pattern