Page 141 - Understanding NCERT Science 09

Fig. 11.6(b) represent the density and
Heinrich Rudolph Hertz
pressure variations, respectively, as a sound
was born on 22 February
wave propagates in the medium.
1857 in Hamburg,
Compressions are the regions where Germany and educated at
particles are crowded together and the University of Berlin. He
represented by the upper portion of the curve confirmed J.C. Maxwell’s
in Fig. 11.6(c). The peak represents the region electromagnetic theory by
of maximum compression. Thus, his experiments. He laid the
compressions are regions where density as H. R. Hertz foundation for future
well as pressure is high. Rarefactions are the development of radio, telephone, telegraph
regions of low pressure where particles are and even television. He also discovered the
spread apart and are represented by the photoelectric effect which was later
valley, that is, the lower portion of the curve explained by Albert Einstein. The SI unit of
in Fig. 11.6(c). A peak is called the crest and a frequency was named as hertz in his honour.
valley is called the trough of a wave.
The distance between two consecutive Frequency tells us how frequently an
compressions (C) or two consecutive event occurs. Suppose you are beating a
rarefactions (R) is called the wavelength, as drum. How many times you are beating the
shown in Fig. 11.6(c), The wavelength is drum in unit time is called the frequency of
usually represented by λ (Greek letter your beating the drum. We know that when
lambda). Its SI unit is metre (m). sound is propagated through a medium, the

Fig. 11.6: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents
graphically the density and pressure variations.

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