If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Physics library

### Course: Physics library>Unit 14

Lesson 2: Interference of electromagnetic waves

# More on single slit interference

See how there's actually constructive (rather than just destructive) interference at some points on the screen. Created by David SantoPietro.

## Want to join the conversation?

• so is there an actual constructive spot in the very center where you have that large large peak?
• Correct. You can see this by looking at the formula.
Where theta=0 there will be no path difference. Consequently, there will be no way for the two wave fronts to get out of phase, and they therefore interfere constructively.
• What is the reason we get any additional constructive points besides the middle peak? If there are destructive interferences everywhere, why do we still see a pattern of peaks rather than just one peak in the middle?
• Because they are not completely destructive everywhere. Where they are completely destructive there we see a dark spot. And in other places we find mild bright places.
• At why are the waves in different phases? Aren't they from the same source?
In the double slit experiment we assumed that they are from the same source since they originate from the same laser, so why not here as well?
• Because they are travelling different distances.
• Oh! I'm super cooper CONFUSED!!
Is the bright spot in the middle made of constructive or destructive waves? If destructive waves cancel each other out as said in the first video,then how do they form a spot out there on the screen?
• Good question

Lets start by saying that ALL waves can be constructive and destructive; it depends on the way that they meet. Have a look at the topic again just to get really clear,

For the bright spot (or 'central maximum' this is where the waves will be meeting in a constructive way. (ie they are IN PHASE). If they were meeting in a totally destructve way, then there would be a dark spot. (ie a MINIMUM)

:-)
• if one peak meets one crest then they cancel out each other,is this violate the law that energy cannot be created nor destroyed since light is a form of energy?
• Nope ,
When a crest and a trough from two travelling wave meets , they just cancel out each other at that moment , after that specific moment there will be two waves again.

In a interference pattern , there are constructive and destructive interference points , making the energy(amplitude) at some points being " cancelled " while some being " doubled " , make the system's net energy not changing.
• what's the differrance between single slit interference and diffraction grating?
they both have independant waves in very short distance and i cant find the differance between placing many holes on the board to make many waves and considering wave as a group of independant waves
• A diffraction grating has many, many slits. Why don't you watch the videos about the double slit and the diffraction grating.
• what happens if we take consecutive points instead of alternate ones?
• I wanna know that if the points are at the same wavefront. Don't they also have the same phases?
• yes, I think they would be in phase. if they belonged to the same wave front. interesting question

you could almost use that as a definition for a wave front...