Huygens principle and interference of light

Let y be the distance from the center of the central diffraction maximum to the first diffraction minimum. 

The angle T is related to this distance y and the distance to the screen, L, by the equation: 
 

Since the angle T is very small, cos T » 1. Thus, tan T » sin T. Then, combining the above equation with equation (*), we have sin T = l / a » y / L, or