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Physics: Waves & Optics Flashcards

7 cards from real AP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. A string fixed at both ends vibrates in its third harmonic. If the string length is 0.9 m, what is the wavelength of the standing wave?

    Answer: 0.6 m

    In the nth harmonic, λ = 2L/n = 2(0.9)/3 = 0.6 m.

  2. Light of wavelength 600 nm passes through a single slit of width 0.1 mm. What is the angle to the first diffraction minimum?

    Answer: 0.34°

    sin θ = λ/a = 600×10⁻⁹/10⁻⁴ = 6×10⁻³, so θ ≈ 0.34°.

  3. A convex mirror has a focal length of −20 cm. An object is placed 30 cm in front of it. What is the image distance?

    Answer: −12 cm

    Using 1/v = 1/f − 1/u = 1/(−20) − 1/30 = −0.0833, so v ≈ −12 cm.

  4. Two sound waves of frequencies 440 Hz and 444 Hz are played simultaneously. How many beats per second are heard?

    Answer: 4

    Beat frequency = |f1 − f2| = |440 − 444| = 4 beats per second.

  5. A transverse wave has amplitude A = 0.02 m and angular frequency ω = 10 rad/s. What is the maximum transverse speed of a particle on the string?

    Answer: 0.2 m/s

    Maximum transverse speed vmax = Aω = 0.02 × 10 = 0.2 m/s.

  6. Which color of visible light has the highest frequency?

    Answer: Violet

    Violet light has the shortest wavelength (~380–420 nm) and therefore the highest frequency in the visible spectrum.

  7. A plane wave strikes a small circular obstacle. According to Huygens' principle, what appears directly behind the obstacle?

    Answer: A bright spot at the center of the shadow

    Each point on the wavefront acts as a secondary source; waves diffract around the obstacle and constructively interfere at the center, creating a bright Poisson spot.