The Physics of Waves: Light Waves, Sound Waves and Ultrasound

Sound cannot travel through a vacuum, but all EM radiation, including light, can.

The greater the amplitude, the greater the energy carried.

Frequencies less than 20 Hz are call infrasound. Frequencies greater than 2 kHz (2000 Hz) are still audible. The best answer is 2 MHz. While this is frequency typical of medical ultrasound, any frequency greater than 20,000 Hz is inaudible and called ultrasound.

If you have an idea of the approximate value of the speed of sound in air, choices A, C and D can be easily rejected.

In choice D, the different speeds with which different frequencies of light travel through media are due to the different refractive indices of the different frequencies.

Ultrasound is a mechanical wave. Its depth of penetration in human tissue decreases as its frequency increases.

v = fλ so f = v/λ = 3 × 10⁸ m/s ÷ 500 nm = 3 × 10⁸ m/s ÷ 500 × 10−⁹ m = 1500 × 10⁸− −⁹ = 1500 × 10¹⁷ = 1.5 × 10¹⁴ Hz.

In myopia, the image forms ventral (in front of) the retina. A diverging lens will extend the focal length of a myopic eye so that the image falls onto the retina.

First determine the frequency: f = 1/T = 1/(2 × 10−¹⁵) = 0.5 × 10¹⁵ Hz. Then rearrange the wave equation for wavelength: v = fλ so λ = v/f.
Then substitute the numbers: λ = v/f = 3 × 10⁸ m/s ÷ 0.5 × 10¹⁵ Hz = 6 × 10−⁷ m.

Period = 1/frequency, hence = 1/1000 = 0.001 s