L3 Waves Concepts

Question 1

What’s a diffraction grating?

Answer 1

List all qualities:
- Causes multiple sources of interference
- Higher chance for destructive interference
- Smaller chance for constructive interference, must be in phase to create this
- Pattern is small, bright dots
- Light is concentrated to smaller points.

Question 2

Define beats!

Answer 2

An alternating soft/hard sound on pulse. When two waves of the same amplitude but slightly different frequencies meet:
* In phase: constructive interference occurs, displacements add, loud sound heard
* 180/out of phase: destructive interference occurs, displacements cancel, no sound heard.

Question 3

What are the equations for beats?

Answer 3

• Beat frequency |f1 - f2|
• Resultant frequency (f1+f2)/2

Question 4

What is diffraction?

Answer 4

The bending of waves around or through a barrier/gap.

Question 5

What happens when waves pass through a smaller gap?

Answer 5

Waves going through smaller gaps will result in more diffraction, which means they bend and curve more. The closer to the wave length the gap size is, it will curve more.

Question 6

Why are only odd-numbered harmonics produced in open-closed pipes?

Answer 6

• Boundary conditions require that at the closed end, air cannot move freely so it forms a node, same vice versa for the open end
• The fundamental harmonic has 1/4 of a wavelength in the pipe.
• Next standing wave must still have a node at closed and antinode at open. Only way to do this is fitting 3/4 of a wavelength in the pipe. (skips 2nd harmonic).

Question 7

Explain why frequency increase occurs when the source of the sound is moving towards a stationary observer.

Answer 7

• Source of sound advances on previously emitted wavefronts
• Wavefronts in front of source bunch up as a result
• Apparent wavelength observed is decreased. v=fλ
• v stays constant, so when λ decreases, f increases

Question 8

When standing waves occurs in strings, what would you have to do to increase the waves frequency?

Answer 8

• Tighten the string - this increases the speed of the wave in the string. As v increases, λ stays the same, f increases
• Applying pressure on one part of the string to shorten the length (on guitar/violin). This shortens the wavelength. v stays constant, λ decreases, f increases
• Changing the thickness of the string - If we decrease the density of the string (thinner) then v will increase. Since λ stays constant, f will increase.

Question 9

When standing waves occur in open-open pipes, how do you increases the waves’ frequency?

Answer 9

• Blow harder to increase the speed of the air.
• “Warm up” the instrument by blowing warm air into it. Mist also increases speed of air
• Lift finger or push a button. This decreases the length which decreases λ.

Question 10

What’s the difference between diffraction gratings and double slit?

Answer 10

Diffraction gratings:
The pattern uses many closely spaced slits. This in turn produces multiple sources of interference and many waves interfere at certain angles. Higher chance destructive - 180/out of phase, causes wide spaces of dark spots. Lower chance of constructive - in phase, produces well-defined bright dots. Only at very precise angles do all waves add up constructively.

Double slit:
Pattern uses two slits, and this produces broad and evenly spaced fringes. They are relatively less sharp and blend in more than diff gratings.
Dark fringes - destructive interference, 180/out of phase, amplitudes cancel.
Bright fringes - constructive interference, in phase, amplitudes add up