They move in a waves motion
15.49 should be the answer if that is 12 watt battery.
Answer:
Explanation:
a ) Slit separation d = .1 x 10⁻³ m
Screen distance D = 4 m
wave length of light λ = 650 x 10⁻⁹ m
Width of central fringe = λ D / d
= 
= 26 mm
b ) Distance between 1 st and 2 nd bright fringe will be equal to width of dark fringe which will also be equal to 26 mm
c ) Angular separation between the central maximum and 1 st order maximum will be equal to angular width of fringe which is equal to
λ / d
= 
= 6.5 x 10⁻³ radian.
They have the same velocity because their displacements (shortest line from point A to point B, which is a straight line) are the same and they meet at the same time.