# Homework Help: Some multiple choice questions.

1. Sep 30, 2015

### goonking

1. The problem statement, all variables and given/known data
Hey, I just need a quick check on these questions.

1) In a regular sound wave, the displacement of the medium is:

A. perpendicular to the wave velocity
B. sometimes perpendicular and sometimes parallel to the wave velocity
C. parallel to the wave velocity.
D. neither parallel nor perpendicular to the wave velocity but at some angle relative to the wave velocity

2)When light passes from one medium to another with a lower index of refraction,

A. its wavelength increases in the new medium.
B. its wavelength decreases in the new medium.
C. its frequency increases in the new medium.
D. its frequency decreases in the new medium. E. its frequency increases and its wavelength decreases.

3)In order to form a real image of an object, a single lens must :

A. be able to refract light away from the optical axis.
B. have a focal length longer than the object distance.
C. be able to refract light towards the optical axis.
D. have no obstructions between it and the object.

4)If two sources of sound have the same frequency but have different initial phase constants,

A. they will always create destructive interference effects no matter where they are positioned.
B. they will always create constructive interference effects no matter where they are positioned.
C. they will create both destructive and constructive that are dependent on the positions of the sources.
D. they won't interfere with other, because interference is only observed in transverse waves.
E. they won't interfere with other, because interference is never observed in transverse waves.

5)

2. Relevant equations

3. The attempt at a solution
1) I answered A, sound wave is a transverse wave so medium is displaced perpendicular to velocity of wave.
2) I answered A, since frequency doesn't change when a wave enters a new medium, so if lower index of refraction, the V increases and so does wavelength.
3) I answered C because you need light to pass through the lens to have an image in the first place
4) I answered C
5) I answered A

2. Sep 30, 2015

### andrewkirk

Sound waves are longitudinal, not transverse.

In 5, what sort of displacement (ie in what direction) did you have in mind? Can that displacement occur at the ends of the tubes, as the graph would indicate if it were a graph of displacement?

3. Sep 30, 2015

### goonking

ok, i corrected number 1, should be same direction as the wave's velocity.

for 5, i imagine the wave in a closed tube to be longitudinal so displacement is parallel to the velocity. the displacement cannot happen at the end of closes tubes so the answer should be B then.

are the rest correct?