निम्नलिखित में से किन तरंगों को संचरण के लिए किसी भौतिक माध्यम की आवश्यकता नहीं होती है?

The equation of a plane progressive wave is given by y = 0.6 sin 2π(t - x2). On reflection from a denser medium, its amplitude becomes 23 of the amplitude of the incident wave. The equation of the reflected wave is:

A transverse wave y = 2 sin(0.01x + 30t) moves on a stretched string from one end to another end in 0.5 second. If x and y are in cm and t in seconds, then the length of the string is: