Question

Propagation of anything requires energy. Then how a ray of light travels millions of km without any change in energy??

Answer 1

the thing we must know is that light has energy. you have said that propagation of anything requires energy, and yeah, the light too has energy. light is composed of particles called PHOTONS. but you could ask that if photons have mass. but the answer is NO. because, photons are the massless particles, and they have only energy and momentum to travel. the unique feature of light is that it travels 3 lakh kilometres per hour with constant speed, without any change. so this made light a different source from all the other sources.

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