Electromagnetic waves are transverse and can travel through a vacuum at about 3 × 10⁸ m/s. From lowest to highest frequency the spectrum is radio, microwave, infrared, visible, ultraviolet, X-ray and gamma. Frequency increases as wavelength decreases; only the visible part is directly detected by human eyes. Example: Order visible, radio and X-rays by increasing frequency. Radio, visible, X-rays. Use the stated conditions to connect the scientific model with the observation. Justify your answer.