Light is radiant energy - energy travelling through space.
Light travels at a constant speed for any observer.
By Early 1900's became apparent light also acts like a particle - a photon, explained by Einstein. Depending on the type of experiment, light reveals either wave or particle properties. The light-gathering power of a telescope depends on the area of its objective lens or mirror. The better the resolution , the sharper the image. The larger the objective the better the resolution.