Coherent light of frequency 6.32 * 10^14 Hz passes through two thin slits and falls on a screen 85 cm away. You observe that the third bright fringe occurs at +- (plus or minus) 3.11 cm on either side of the central bright fringe. (a) How far apart are the two slits? (b) At what distance from the central bright fringe will the third dark fringe occur?

Question

Coherent light of frequency 6.32 * 10^14 Hz passes through two thin slits and falls on a screen 85 cm away. You observe that the third bright fringe occurs at +- (plus or minus) 3.11 cm on either side of the central bright fringe.  (a) How far apart are the two slits?  (b) At what distance from the central bright fringe will the third dark fringe occur?

once again, I am confused!

bobpursley · Answer

calculate lambda. n*Lambda/s=w/d for the bright fringes. N*lambda/2=w/d for the dark fringes. Now be careful on this one. http://www.matter.org.uk/schools/Content/Interference/formula.html