Discussion
Assertion (A) : Thermal noise is sometimes referred to as white noise.
Reason (R) : When the white noise is passed through a network, the spectral density will be altered by the shape of the network frequency response.
Answer: B
Both (A) and (R) are true but (R) is not the correct explanation of (A).
Thermal (Johnson) noise has a power spectral density of 4kTR that is flat over all frequencies used in electronics. A flat spectrum, like white light containing all colours, is why it is called white noise, so the assertion is true.
The reason is also true: passing white noise through a network shapes its output spectrum as \(S_{out}(f) = |H(f)|^{2} S_{in}(f)\), giving coloured noise. But that describes what happens after filtering, not why thermal noise is called white, so it does not explain the assertion.
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