A source of sound S is moving with a velocity of $50 \mathrm {~m} \mathrm {~s} ^ { - 1 }$ towards a stationary observer. The observer measures the frequency of the source as 1000 Hz . What will be the apparent frequency of the source when it is moving away from the observer after crossing him? (Take velocity of sound in air is $350 \mathrm {~m} \mathrm {~s} ^ { - 1 }$ )\\
(1) 750 Hz\\
(2) 857 Hz\\
(3) 1143 Hz\\
(4) 807 Hz