06:17 1 4GO A QIYUN ZHANG.pdf . . . 0083 23 An observer is standing on a railway platform. A train passes the observer at constant speed while emitting sound of constant frequency / from its whistle. What does the observer hear? A sound of a decreasing frequency as the train approaches and of an increasing frequency as Enin it moves away B sound of a higher frequency than f as the train approaches and of a lower frequency than / as the train moves away C sound of a lower frequency than f as the train approaches and of a higher frequency than / as the train moves away D sound of an increasing frequency as the train approaches and as it moves away All entry details, Including the spelling of you zinme and date ba checkel for accurad 24 Which microorganisms have a length that is equal to the wavelength in free space of an electromagnetic wave that is visible to the human eye? A algae of length 0.5 um B bacteria of length 5.0 MUN ZHANG C fungi of length 50 jim you D protozoa of length 100 um 3126 25 Two progressive waves overlap What is an essential condition for the two waves to form a stationary wave? A The waves are longitudinal. Emin Dads Tine Ration The waves are polarised. C The waves travel in opposite directions D The waves travel in the same direction