Motion, Heat and Waves
Waves, Sound & Light
13 minStudent plan
One set of rules covers sound, light, radio and ultrasound, including the sensor that measures distance by timing an echo.
After this lesson you can
- Use v = f x lambda for sound, light and radio alike
- Work out a distance from an echo time
- Explain what an ultrasonic sensor is measuring
The wave equation
Every wave obeys v = fλ: speed equals frequency times wavelength. Because the speed is fixed by the medium, frequency and wavelength trade off against each other, raise one and the other falls.
Sound travels at about 343 m/s in air, so a 1 kHz tone has a wavelength of about 34 cm. Light and radio travel at 3 × 10⁸ m/s, so a 100 MHz FM signal has a wavelength of 3 m, which is why its antenna is about that size.
Waves come in two kinds. In a transverse wave the oscillation is perpendicular to the direction of travel, as with light. In a longitudinal wave it is along the direction of travel, as with sound, which is a pressure wave, and therefore cannot cross a vacuum.
In the full lesson
- The wave equation
- Reflection, and measuring distance
- Interference and the inverse square law
This lesson comes with the Student plan. The first module of Physics for Engineers is free.