Wavelength

Wavelength is the distance traveled by a wave during one complete period of oscillation, measured between two points in phase.

Discover FizziQ

How to measure it in class

With FizziQ and an open tube, students can determine the wavelength of a sound using acoustic resonance.

Steps:

Open FizziQ and select the white noise emission from the sound library. Prepare a tube (test tube, cardboard tube) of known length.
Place the smartphone at one end of the tube and emit the white noise. Record the sound spectrum at the other end with a second smartphone or the same device.
Observe the resonance peak on the frequency spectrum. The fundamental resonance frequency of an open tube corresponds to a wavelength equal to twice the length of the tube.
Calculate the wavelength: λ = 2L. Then deduce the speed of sound: v = λ × f. Compare with the theoretical value of 343 m/s at 20 °C.

Scientific activities on this topic

Several experiments that are easy to carry out with a smartphone, a tablet or a computer allow you to measure and understand wavelength.

Learn more

A bit of history

The notion of wavelength emerged in the 17th century with the work of Robert Hooke and Christiaan Huygens on the wave nature of light. In 1801, Thomas Young performed his famous slit experiment, which demonstrated light interference and made it possible to measure the wavelength of visible light for the first time.

The electromagnetic spectrum

The wavelengths of the electromagnetic spectrum cover an immense range. Radio waves measure several kilometers. Visible light lies between 380 and 780 nm. X-rays have wavelengths on the order of a nanometer, and gamma rays go below a picometer.

Wavelength and diffraction

Diffraction occurs when a wave encounters an obstacle or an opening whose size is comparable to its wavelength. This is why sound bends around obstacles (λ on the order of a meter) while light does not at our scale (λ on the order of 500 nm).

Complementary experiments

The Tube effect experiment measures the speed of sound using the resonance frequencies of a tube, which are directly related to the wavelength.

Formula

λ = v × T = v / f

Meaning:

λ: wavelength (m)

v: propagation speed of the wave (m/s)

T: period (s)

f: frequency (Hz)

Application examples

The color of visible light is determined by its wavelength: approximately 700 nm for red, 400 nm for violet
FM radio waves have a wavelength of approximately 3 meters
A low-pitched sound of 100 Hz in air has a wavelength of 3.4 meters
Microwave oven waves have a wavelength of 12 cm, which explains the appliance’s dimensions
Ocean waves have wavelengths ranging from a few meters to several hundred meters

FAQ

Q: How do you calculate the wavelength of a sound? A: Use the formula λ = v/f. For a 1,000 Hz sound in air at 20 °C, the wavelength is 343/1000 = 0.343 m, or 34.3 cm.

Q: What is the wavelength of audible sounds? A: Audible sounds (20 Hz to 20,000 Hz) have wavelengths between 17 m (very low-pitched sounds) and 1.7 cm (very high-pitched sounds) in air.

Q: Does wavelength change depending on the medium? A: Yes. The propagation speed depends on the medium. Sound travels faster in water than in air. At the same frequency, the wavelength in water is therefore larger.

Q: Why do bass frequencies pass through walls better? A: Low-pitched sounds have a large wavelength. When the wavelength is large compared to the obstacle, the wave bends around it more easily through diffraction. This is why you mainly hear bass frequencies behind a wall.

Q: What is the difference between wavelength and amplitude? A: Wavelength is a spatial distance related to frequency. Amplitude is the maximum height of the oscillation, related to signal intensity. These are two independent quantities.

Frequency - Period - Wave Speed - Interference