In a double-slit experiment, the dark fringes are formed when there is destructive interference between the waves coming from the two slits. The condition for destructive interference at a specific angle is given by the equation:
d * sin(theta) = m * lambda
Where:
- d is the distance between the slits
- theta is the angle measured from the central maximum
- m is the order of the fringe (an integer)
- lambda is the wavelength of light used
In the case you mentioned, when a dark fringe is formed opposite to a slit on the screen, it means that destructive interference occurs for that particular angle. The order of the fringe (m) can be determined by substituting the values into the equation and solving for m.
If you specify the value of the angle theta or any other known quantities in the setup, I can help you calculate the order of the fringe.