If a wave travels at a constant speed, the greater its wavelength, the lower its frequency.

Here's why:

* Wave Speed: The speed of a wave is determined by the medium it travels through. For a given medium, the wave speed is constant.

* Wavelength: The wavelength is the distance between two consecutive crests or troughs of a wave.

* Frequency: The frequency is the number of waves that pass a point in a given time.

The relationship between these three is:

Speed = Wavelength x Frequency

Since the speed is constant, if the wavelength increases, the frequency must decrease to maintain the equation.