Thin-Film Interference AP Physics 2 MCQ – Exam Style Questions etc.
Thin-Film Interference AP Physics 2 MCQ
Unit 14: Waves , Sound , and Physical Optics
Weightage : 15–18%
Exam Style Practice Questions ,Thin-Film Interference AP Physics 2 MCQ
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
Two sources, in phase and a distance d apart, each emit a wave of wavelength λ. See figure below. Which of the choices for the path difference ∆L = L1 – L2 will always produce destructive interference at point P?
A) d sin θ B) (x/L2)d C) λ/2 D) 2 λ
Answer/Explanation
Ans:D
Solution: By definition, when the path difference equals ½ λ or any odd multiple of ½ λ’s for sources of the same λ, there will be destructive interference.
Question
Observations that indicate visible light has a wavelength much shorter than a centimeter include which of the following? (Select two answers.)
(A) A colored pattern is observed when light reflects off a thin film.
(B) Bright and dark fringes are observed when laser light passes through a diffraction grating.
(C) Light is polarized through a liquid crystal display (LCD).
(D) Redshift is observed when observing a receding galaxy.
Answer/Explanation
Ans: (A) and (B).
Thin films and the spacing between slits in a diffraction grating are on the order of microns,\ which correspond to wavelengths of light. Neither phenomenon can occur if light had wavelengths greater than a centimeter. Choices (C) and (D) are not wavelength dependent.
Question
A technician sets up a red laser pointer and directs light at a strand of hair suspended between two blocks in such a way that the light falls on a screen behind the hair as shown in the figure. This produces a pattern of alternating light and dark fringes on the screen. The technician makes one change to the setup and repeats the procedure. This change produces a similar pattern, but the light and dark fringes are spaced farther apart. Which of the following could account for this different pattern?
(A) The hair was replaced with one of larger diameter.
(B) The screen was moved farther away from the hair.
(C) The red laser was replaced with a blue laser.
(D) The red laser was moved farther away from the hair.
Answer/Explanation
Ans:
B—From the equation that models this interference pattern {d sin q = mX), we see that in order to make the fringe spacing farther apart, we need the angle q to get larger. This could be accomplished by decreasing d (the width of the hair) or increasing 2 (the wavelength of the laser). Neither of these is listed. The spacing could also be spread farther apart by simply moving the screen farther away from the hair.