# Double slit interference of light

The typical textbook explanation of the interference fringes in a double slit experiment goes like this. If the difference between the path lengths travelled by waves from each slit to a point on the screen is a whole number of wavelengths then the two waves interfere constructively at the point and we have a bright spot.

But is this right? Consider two waves which begin from the two slits in phase with the amplitudes of their electric fields equal to zero. They arrive at a point on the screen at which the difference of their path lengths is one wavelength. This also means that the difference between the time taken to reach the point is one period. So they arrive in phase at the point but since the amplitude of their electric fields is zero the point is a dark spot, not a bright spot.

What is the error in this argument?

T/2 later you have amplitudes at max . The intensity of light is proportional to $$A^2$$ . when you "see" light you dont see the maxima and minima, you just get the mean energy. So as in the two slits you have at the point oscillation of the electric field with double amplitude.