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# Some radio waves of frequency of about $1.5 \times 10^9$ Hz were received by a radio-telescope from distant star. If the speed of the waves is $3 \times 10^5 \: km/s$, then the wave- length of the wave will be:

$\begin {array} {1 1} (a)\;0.1 \: m & \quad (b)\;0.6\: m \\ (c)\;0.2\: m & \quad (d)\;0.46\: m \end {array}$

Frequency, $v = 1.5 \times 10^9\: Hz$
Speed of waves, $c = 3 \times 10^5\: km/s = 3 \times 10^8m/s$
So, wavelength of wave, $\lambda = \large\frac{c}{v}$
$= \large\frac{3 \times 10^8}{ (1.5 \times 10^9 )}$
$= 0.2 m$