MCAT Physical : Interference and Diffraction
Study concepts, example questions & explanations for MCAT Physical
All MCAT Physical Resources
Example Questions
Example Question #1 : Light
At a local concert, a speaker is set up to produce low-pitched base sounds with a frequency range of 20Hz to 200Hz, which can be modeled as sine waves. In a simplified model, the sound waves the speaker produces can be modeled as a cylindrical pipe with one end closed that travel through the air at a velocity of 
If the sound crew that set up the speakers accidently set up an additional identical speaker directly opposite the base and flipped the phase switch to produce base waves that can be modeled as cosine waves, how would the volume of the sound change at the midpoint of the two speakers?
The pitch heard decreases
It becomes silent
The pitch heard increases
It becomes quieter
It becomes silent
First, let’s look at what this question is asking us to consider. The question wants us to determine the relationship between waves produced from our sine phase base speaker and a cosine phase speaker accidently placed directly across from it. Two important facts come to mind when thinking about this scenario. First, the waves physically overlap in space because the speakers are pointed directly at each other. Second, the sine and cosine waves are exactly 180º out of phase. Remember that waves completely out of phase create destructive interference, meaning that the resultant wave will have no amplitude. The volume directly between the two speakers is zero because volume is directly correlated to wave amplitude.
Example Question #21 : Waves
Two waves of equal amplitude destructively interfere, resulting in a wave with zero amplitude. What is the phase difference between the two waves?
Two waves will cancel to zero amplitude when the relative shift between them is half a period. This corresponds to half of 
Example Question #1 : Light
Two people are singing a note at exactly 
The waves went through constructive interference and canceled each other out
The microphone was unable to pick up the sound because of the large amplitude
The singers' combined frequency was higher than the frequency response of the microphone
The microphone could pick up the same frequency coming from two different sound sources
The waves went through destructive interference and canceled each other out
The waves went through destructive interference and canceled each other out
Destructive inference occurs when the peak of one wavelength encounters the trough of another wavelength. This causes the waves to cancel each other out, essentially producing a much smaller audible wavelength. In contrast, constructive interference will result in the summation of the wave amplitudes, and an increase in the volume and intensity of the sound.
Example Question #4 : Interference And Diffraction
A red light (wavelength = 6.5 * 10-7m) shines on a diffraction grating, creating an interference pattern on a distant screen. If a blue light (wavelength= 4.75 * 10-7m) were shone on the same screen instead, the bright lines on the screen would __________.
remain the same
move closer together
move farther apart
become dimmer
become brighter
move closer together
The bright lines from a diffraction grating can be located with the equation 
Example Question #3 : Light
What term describes the phenomenon as a wave spreads into the region behind an obstruction?
Dispersion
Refraction
Diffraction
Reflection
Superposition
Diffraction
Diffraction occurs when a wave passes into a region behind an obstruction (or spreads out when passing through an aperture).
Certified Tutor
Certified Tutor
All MCAT Physical Resources
