What does interference really mean?

[sona1177]

Homework Statement

At a particular point on a screen illuminated by two slits, the path length from that point to the bottom slit is greater than to the top slit. Which of the following statements are true about the path-length difference between the two waves arriving at the screen?

a dark line will appear if the difference in path lengths is two wavelengths FALSE
a dark line will appear if the difference in path lengths is 3/2 wavelengths TRUE
a bright line will appear if the difference in path lengths is half a wavelength FALSE
a bright line will appear if the difference in path lengths is three wavelengths TRUE
a dark line will appear if the difference in path lengths is 1/4 of a wavelength TRUE

Homework Equations

I am not getting the problem correct, so something is wrong here.

I thought for destructive interference, the path difference is an odd number of half wavelengths. And for constructive, the path difference is an integral multiple of wavelengths.

That's what I used to get my answers.

What have i done wrong?
thanks!

The Attempt at a Solution

 

[ehild]

The last answer is wrong. Is 1/4 of a wavelength is an integer multiple of the half-wavelength?

ehild

 

[sona1177]

Thanks!