Wavelength is the distance between the crest of one wave and the crest of the next wave. Wavelength is the distance between one trough to the next crest of the next wave. Frequency is the number of waves that pass by at a point each second.
Shorter wavelength means greater frequency for a wave.
The closer you get to a light souce, the less intensity it has. Heat waves, also called infrared waves are invisible. Light waves, also called white light is visible. Radio wave, also called microwaves are visible.