Question
A trumpet player notices a string on a harp is beginning to vibrate when he plays a certain note on his trumpet. What is this an example of?
(A) Constructive interference
(B) Destructive interference
(C) Refraction
(D) Resonance
Answer/Explanation
Ans:
(D) Resonance is the sympathetic vibration of an object when impacted by a wave with the same frequency as its own fundamental frequency of oscillation.
Question
A shadow is formed by a point source of light. Upon closer inspection, the edges of the shadow seem to be diffuse and fuzzy. This phenomenon is probably caused by
(A) dispersion as the different wavelengths of light focus at different points
(B) refraction as the rays are bent by their contact with the shadow-forming surface
(C) diffraction as the waves nearest the edge of the shadow-forming surface are sources for waves going into the shadow region
(D) dispersion as the different wavelengths are traveling at slightly different speeds in the new medium
Answer/Explanation
Ans:(C)
Diffraction is the bending of wave fronts around obstacles or through openings. In this case, diffraction occurs around the edges of the object causing the shadow. This will cause the edge of the shadow to be slightly less than fully dark at the edges.
Question
Which of the following waves cannot be polarized?
(A) Sound waves because they are longitudinal
(B) Waves on a string because they lack amplitude
(C) X rays because they are too short
(D) Microwaves because they have too low of a frequency
Answer/Explanation
Ans:(A)
Polarization is a property of transverse waves. Sound is longitudinal.
Question
The colors observed in thin films like soap bubbles are caused by
(A) reflection and interference
(B) refraction and reflection
(C) diffraction and interference
(D) polarization and reflection
Answer/Explanation
Ans:(A)
The interference between the ray reflecting off of the top boundary and another ray bouncing off of the lower boundary either produces some bright colors (constructive interference) or does not produce certain colors (destructive interference).
Question
As the number of lines per cm on a diffraction grating is increased (or the slit spacing is decreased in a two-slit diffraction pattern),
(A) the spacing between the spectral lines increases
(B) the spacing between the spectral lines decreases
(C) the intensity of the spectral lines increases
(D) the intensity of the spectral lines decreases
Answer/Explanation
Ans:
(A)
Remember that d sin θ = mλ . Since d is getting smaller as the number of lines per cm increases, sin θ is getting bigger—spreading out the peaks.