## 2.4 - Basic oscillations

 Now the previous considerations allow a first calculation of a wave that builds up, as the earth, around a sphere. Still, it must be considered that the wave propagation takes place linear and not along the curved surface of the ball.
 After the Huygens-Fresnel principle the points A, B, C serve so as source points of the standing wave (blue) Electromagnetic waves propagate spherically from one point. (red) If point B is the starting point so the distance BDC = h is the way of the wave. Illustration 2.4.1 – wave propagation

 Because there is a stationary state, it is sufficient to look at the way of the wave from one source to the next source.
 This makes
 The triangle of MCD in the picture 2.4.1 is rectangular in the point D and apply:
 Shift the equation to h:
 Since now all sizes are known, the wavelength can be determined:
 2.4.1 - Equation:
 In the equation is R the radius of the Earth, n is the number of waves. This results in the frequency equation for basic oscillations
 2.4.2 - Equation: n = 1,2,3,4,...
 The basic frequency is for n = 1:
 2.4.3 - Equation: Illustration 2.4.2 – basic oscillation
 This is also the oscillation of a bar with free ends, which length is equal to the diameter of the ball.
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