Question

Why do scientists claim that light can be considered a wave?

Answer 1

Scientists claim that light can be considered as a wave because it exhibits features which are characteristics of waves. These features include reflection, refraction, diffraction, interference and Doppler's effects. Thus, light behavior is consistent with the conceptual definition of wave.<span />

Answer 2

Scientists claim that light can be considered a wave because it has some properties or behaviors that are the same of waves: reflection, refraction, difraction, interference and doppler effect.

The reflection of light is what permits the formation of the image in the mirror.

The refraction of light is the bending of the light when it goes from one media to other, for example from air to water, and it causes that the objects inside the water seen from outside the water seem to be in a different position than their current position.

You can search for the other effects difraction, interference and Doppler. The fact is that the behavior of light is physically and mathematically compatible with the characteristics of waves.

Remember that for other purposes light alse has characteristics of particles.  This is known as duality: the light has dual behavior as a particle and as a wave.