Waves, Sound, and Physical Optics practice quiz

Key ideav = fλ. The wave speed is set by the medium, and the frequency is set by the source.

Common mistakeThinking a higher frequency changes the wave speed in the same medium. It shortens the wavelength instead.

Key ideaLight is a transverse wave moving at c in vacuum. A polarizer passes half the intensity of unpolarized light; for polarized light at angle θ to its axis it passes I = I₀cos²θ.

Common mistakeUsing cos θ instead of cos²θ, or forgetting the factor of ½ for unpolarized light.

Key ideaWhen the source and observer approach, the observed frequency is higher; when they recede, it is lower.

Common mistakeThinking the source’s motion changes the speed of the sound. The medium sets the wave speed; a moving source changes the wavelength, and a moving observer meets the wave crests at a different rate.

Key ideaStanding waves need nodes at fixed ends (and antinodes at open ends), so only certain wavelengths fit: λ = 2L/n for a string fixed at both ends.

Common mistakeUsing the closed-pipe pattern for an open pipe, or forgetting that a pipe closed at one end has only odd harmonics.

Key ideaBright fringes appear where the path difference is mλ: d sin θ = mλ, or y ≈ mλL/d for small angles.

Common mistakeThinking moving the slits closer together brings the fringes closer. It spreads them out.