Physics: Waves & Optics Flashcards

A thin film of oil (n = 1.4) on water (n = 1.33) has a thickness of 200 nm. For normal incidence, which visible wavelength (in air) experiences constructive interference in reflection?

Answer: 560 nm

Constructive reflection with one phase inversion: 2nt = mλ → λ = 2(1.4)(200)/1 = 560 nm.

A converging lens of focal length 15 cm forms a real, inverted image twice the size of the object. How far is the object from the lens?

Answer: 22.5 cm

Magnification m = −2 means di = 2do; substituting into 1/f = 1/do + 1/di gives do = 1.5f = 22.5 cm.

A police siren emits 800 Hz. As the car moves away from a stationary listener at 20 m/s (vsound = 340 m/s), what frequency does the listener hear?

Answer: 754 Hz

f' = f(v/(v+vs)) = 800(340/360) ≈ 756 Hz, closest to 754 Hz.

In a diffraction grating with 500 lines/mm, light of wavelength 500 nm is incident. At what angle does the second-order maximum appear?

Answer: 30.0°

d = 1/500 mm = 2000 nm; sin θ = mλ/d = 2(500)/2000 = 0.5, θ = 30°.

Which property of a wave changes when it crosses from one medium to another while the frequency remains constant?

Answer: Wavelength

When a wave crosses media, frequency is unchanged but speed changes, so wavelength (λ = v/f) must change.

An open-pipe organ pipe resonates at its fundamental frequency. If the length is doubled, what happens to the fundamental frequency?

Answer: It is halved

For an open pipe, f = v/2L; doubling L halves the fundamental frequency.

Polarized light passes through a second polarizer whose axis is 60° from the first. What fraction of the original intensity is transmitted?

Answer: 1/4

By Malus's law, I = I₀ cos²60° = I₀(0.5)² = I₀/4.