Answer :
Answer:
Option B
Explanation:
As we know
Frequency (F) * wavelength (W) = C (speed of light - can also be taken as constant)
Hence,
[tex]F1 W1 = C\\F2W2 = C[/tex]
Or [tex]F1W1 = F2W2[/tex]
Substituting the given values, we get -
[tex]840 *2.5 = 500 *XX = 4.2[/tex] m
Hence, option B is correct