In a longitudinal wave, the motion of the medium is parallel to the direction of the wave. This means that the particles move left and right which in turn makes the other particles start to oscillate.
The UNIA founded by Marcus Garvey has a constitution which defines red, black, and green as the Pan-African colours: red representing the noble blood that unites all people of African ancestry, the colour black for the people, green for the rich land of Africa..
Black symbolizes evil, emptiness, death, and dirtiness, but also power, dignity and strength. Since black symbolizes death, it is the color worn by many cultures in times of mourning. At the extreme opposite is white. This color symbolizes purity, sterility, innocence, and goodness, but also sterility in most cultures.