An arrangement that uses a double slit to demonstrate the interference of light from a laser is shown.
The light from the laser has a wavelength of 
and is incident normally on the slits. The separation of the two slits is 
. The slits and screen are parallel and separated by a distance of 
.
An interference pattern of bright fringes and dark fringes is formed on the screen. The distance on the screen across 8 bright fringes is 
, as illustrated below.
The light waves emerging from the two slits are coherent. State what is meant by coherent.
Calculate the separation of the slits.
.
