Goal: To understand the basics of interference

Objectives:1) What is interference

2) Constructive interference3) Destructive interference

4) Path differences for complete interference

What is interference?

• Interference is when a defender attacks a receiver before the ball reaches him.

• Err wait sorry wrong sport.

• Interference is when 2 waves reach the same spot.

• These waves will add together – thus interfering with one another.

• Depending on how they interfere this can be a good thing or a BAD thing.

Constructive interference

• If two waves land on top of each other they can add together if their positives and negatives match up (this is called “in phase”).

• If the waves peak at exactly the same time, they will add together making a bigger wave.

• This is called constructive interference.

Destructive interference

• If two waves land on top of each other they can subtract if their positives meet with the others negatives (this is called “out of phase”).

• If the waves peak at exactly opposite times, they will subtract making a smaller wave.

• If the waves have the same amplitude (or strength) then they can COMPLETELY cancel each other out!

• This is called destructive interference.

Uses?

• Think of 1 possible GOOD use for both constructive and destructive interferrence.

Path differences for complete interference

• If a source emits photons of a specific wavelength in 2 different directions it is possible (due to reflection or something else) that those two paths will cross.

• When they cross you can possibly get interference.

• However, what type of interference will you get?

Depends

• The interference type depends on the difference in distances that each wave has to take.

• Lets say a radio wave is 1 m long.• What must be true about the difference in

distances if two paths meeting produce constructive interference?

• What about for destructive interference?

Depends

• The interference type depends on the difference in distances that each wave has to take.

• Lets say a radio wave is 1 m long.• What must be true about the difference in distances if

two paths meeting produce constructive interference?• The difference in paths has to be equal to an exact

number (i.e. an integer) of wavelengths.• So, 1 m, 2 m, 17m, ect, but not 1.1 m.

• What about for destructive interference?• Has to be a difference of half waves – such as 0.5 m,

1.5m, 2.5 m, 175.5 m, but not 0.4 m, or 0.6 m, and def not 3.0 m.

Example

• This is from HW 10• A radio tower emits radio waves with a

wavelength of 400 m.• These waves travel 15.3 km to a radio receiver.• One path is directly to the receiver.• The 2nd path goes past the receiver and reflects

of the hills behind the receiver and come back.• If no the reflection causes no phase change in

the waves then what is the smallest distance away the hills can be from the receiver if total destructive interference is caused?

Example

• A radio tower emits radio waves with a wavelength of 400 m.

• These waves travel 15.3 km to a radio receiver.• One path is directly to the receiver.• The 2nd path goes past the receiver and reflects of the

hills behind the receiver and come back.• If no the reflection causes no phase change in the waves

then what is the smallest distance away the hills can be from the receiver if total destructive interference is caused?

• The path difference has to be a half wave, or 200 m.• But, since the radio waves have to go to the hill and back

the path difference is twice the distance to the hills.• So, the distance to the hills is 100 m.

Conclusion

• We quickly looked at what constructive and destructive interferences are.

• We saw to get them we needed to add waves either in phase or out of phase.

• If they all come from the same place then we need to have the path differences be some number of wavelengths for constructive interference, or some half number for destructive.