# What is wave speed

## Big Bang HTL 1, textbook

a4b2ka Selected phenomena 107 Wave basics 1 12 12.3 Ocean and puddle of water Wavelength and speed This section deals with further basic properties of waves: length and speed. We will look at these using the example of light, sound and water waves. What do we understand by the terms amplitude, period of oscillation and frequency? Read in chap. 11.2. If you throw a stone into the water and watch the first ring, you will see that it will soon split into several rings. Why? How fast is a tsunami wave? And what is its wavelength? What do you think? If you count the seconds between lightning and thunder and then do some math, you know how far away the lightning was! How exactly does it work and why? How long does it take for us on earth to feel a change in light and gravity when someone (e.g. Supersuperman) suddenly removes the sun? At sprint starts in athletics, each participant has their own loudspeaker behind their starting block. This is very important, especially in the 400 m run! Why? L A sand scorpion can precisely locate a beetle from a few decimeters away without having to see or hear it. How does it work? It has to do with the speed of the waves! L F11 A2 F12 A2 F13 A2 F14 A2 F15 A2 F16 A2 F17 A2 And the period of oscillation of the wave is the reciprocal of the frequency (see section 11.2). There is a fundamental relationship between wave speed, frequency and wavelength that is always valid. If you know two quantities, you can calculate the third immediately (Tab. 12.2): Formula: general wave speed vfv or T = ⋅ = λ λ v ... speed in m / sf ... frequency in Hertz λ ... wavelength in m wave λ [m] v [m / s] f [s -1] T [s] green light 5 · 10 -7 c ≈ 3 · 10 8 6 · 10 14 1.7 · 10 -5 “concert pitch” 440 Hz 0.77 ≈ 340 440 2.3 · 10 -3 10,000 Hz 0.034 ≈ 340 10,000 10 -4 carrier waves FM4 2.9 c ≈ 3 · 10 8 102.5 · 10 6 9.8 · 10 -9 Tsunami 2 · 10 5 200 0.001 1000 Tab 12.2: Five examples of wavelengths. Light has an extremely short wavelength. Sound and radio waves are in the centimeter or meter range. Tsunamis have wavelengths of around 200 km! What can the speed of a wave generally depend on? 1) The type of wave, 2) the nature of the medium, 3) the mode of vibration of the wave and 4) the wavelength. However, these criteria do not always apply at the same time! Let's look at a few examples. 1) The wave speed depends on the type of wave (Tab. 12.1, Section 12.2). The speed of light c is the upper limit of all speeds in the universe. Light, radio signals and gravitational waves cannot be faster. If the sun were gone at this moment, you would only notice the disappearance of light and gravity in 8 minutes (F15). Fig. 12.16: Definition of the wavelength using the example of a longitudinal and transverse earthquake wave F Two new terms appear when describing waves: wavelength and wave speed. The wavelength is always measured between two points of the same phase, that is, between two points that are just equally strongly deflected from the rest position. In principle, it doesn't matter which points you take. Mostly, however, one measures between two wave crests (Fig. 12.16 a) or between points of maximum compression (b). The wavelength always bears the letter λ (lambda), which is a small Greek L. The wave speed (v) is the speed at which the wave propagates. This can be as great as the speed of light (c) (see also Tab. 12.1, Section 12.2). Furthermore, three terms apply to waves that you already know from vibrations: The amplitude indicates how strongly the wave is deflected. The frequency indicates how many wave crests or points of maximum compression pass an observer per second. For testing purposes only - property of the publisher öbv