The speed of light depends upon the medium through which it travels. The speed of light in a vacuum is a universal constant in all reference frames. The speed of light in a medium is always slower the speed of light in a vacuum. The speed of anything with mass is always less than the speed of light in a vacuum. The amplitude of a light wave is related to its intensity.
Intensity is the absolute measure of a light wave's power density.
Brightness is the relative intensity as perceived by the average human eye. The frequency of a light wave is related to its color. Color is such a complex topic that it has its own section in this book. Monochromatic light is described by only one frequency. Laser light is effectively monochromatic.