निम्नलिखित में से कौन सी तरंग परिघटना प्रकाश तरंगों द्वारा प्रदर्शित की जाती है लेकिन हवा में ध्वनि तरंगों द्वारा नहीं?

A closed organ pipe and an open organ pipe are tuned to the same fundamental frequency. The ratio of their lengths is:

At what speed should a source of sound move away from a stationary observer so that the observer finds the apparent frequency equal to half of the original frequency? (where v is the velocity of sound)