Why does the disk of a spiral galaxy appear dark when viewed edge on?

Kinetic energy = 1/2mv^2. since the mass is constant we can form a ratio of it's kinetic energy at 60 divided by that at 30 to see ke(60)/ke(30) = +(60^2)/30^2). this may be said to 300% increase.

Ray 2 should cross through the focal point on the right side of the lens

Um idk im just doing this for points so just ask google or something