Which quanity tells you about the brightness of a light wave?

Question

the frequency, wavelength, speed, or amplitude? why?

Answer 1

Amplitude tells you the brightness. Frequency (or wavelength) tell you the color of the wave. The product of these two is speed.

Answer 2

Amplitude describes the strength of a given wave (the height of from trough to peak). The higher the amplitude, the more strength is behind it, hence the brighter the light.

Wavelength corresponds to color - in order from longest (visible light) to shortest:
Infrared (not visible)
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Ultraviolet (not visible)

When light passes through a medium, say a prism, there is a miniscule change in speed and a corresponding bend. This often will result in the light separating into the typical rainbow.

Hope this helps!

Tom