AP Physics Physics: Waves & Optics 3

Question 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?

Accepted Answer

0.6 m

Answer

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

Question 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?

Accepted Answer

0.34°

Answer

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

Question 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?

Accepted Answer

−12 cm

Answer

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

Question 4

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

Accepted Answer

4

Answer

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

Question 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?

Accepted Answer

0.2 m/s

Answer

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

AP Practice Test