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Unformatted text preview: Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨uttler Solutions to Conceptual Practice Problems PHYS 1112 InClass Exam #2A+2B Thu. Mar. 19, 2009, 11:00am12:15pm and 2:00pm3:15pm CP 2.01: In a twosource interference experiment two sources are oscillating in phase with the same period. The first intensity minimum, closest to the central intensity maximum M and to the left of M, is located at point P, as shown in Fig. 2.01. A wave crest A from source 1 and a wave trough B from source 2 arrive simultaneously at P. Therefore, A and B must have departed from their respective sources as follows: M P Source 1 Source 2 Fig. 2.01 l 1 l 2 (A) A departed 1 / 2 period before B. (B) B departed 1 / 2 period before A. (C) A departed 1 period before B. (D) B departed 1 period before A. (E) A departed 3 / 2 period before B. CP 2.01 Answer: (B) At P , the pathlength difference Δ l ≡ l 2 l 1 (see Fig. 2.01) must be half a wavelength: Δ l = 1 2 λ , since it’s the first interference minimum to the left of center M (with Δ l = 0 at M ). Hence, B has 1 2 λ further to travel than A, which takes an extra travel time of 1 2 period. So B must leave 1 2 period earlier than A to arrive simultaneously at P . [Recall: v = λ/τ ; so it takes 1 period τ for a crest or trough to travel 1 wavelength λ .] CP 2.02: A diffaction grating is illuminated with coherent (laser) light with a wavelength λ < d where d is the spacing between adjacent slits in the grating. The first intensity maximum, to the right of and closest to the central intensity maximum M, is located at point Q, as shown in Fig. 2.02. A wave crest A from slit R and a wave crest B from the neighboring slit S to the right of R, arrive simultaneously at Q. Therefore, A and B must 1 Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨uttler have departed from their respective slits as follows: M Q Diff. Grating Laser Beam Fig. 2.02 S R l R l S (A) A departed 1 / 2 period before B. (B) B departed 1 / 2 period before A. (C) A departed 1 period before B. (D) B departed 1 period before A. (E) A departed 3 / 2 period before B. CP 2.02 Answer: (C) At Q , the pathlength difference Δ l ≡ l R l S (see Fig. 2.02) must be one full wavelength: Δ l = λ , since it’s the first interference maximum to the right of center M (with Δ l = 0 at M ). Hence, A has 1 λ further to travel than B, which takes an extra travel time of 1 period. So A must leave 1 period earlier than B to arrive simultaneously at Q . [Recall: v = λ/τ ; so it takes 1 period τ for a crest or trough to travel 1 wavelength λ .] CP 2.03: A beam of coherent (laser) light of wavelength λ is incident upon a diffraction grating with line spacing d , with λ < d , as shown in Fig. 2.03. Assume  Δ y  is the distance (in cm) between the two 1storder intensity maxima, observed on a screen at a distance L on the other side of the grating. This distance  Δ y  will 2 Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨uttler...
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 Spring '08
 SEATON
 Physics, Electric charge, electric ﬁeld vector

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