Here's why:

* The relationship: Wavelength and frequency are inversely proportional. This means that as one increases, the other decreases.

* The equation: This relationship is defined by the following equation:

c = fλ

where:

* c is the speed of light (a constant)

* f is the frequency

* λ is the wavelength

* Explanation: If you decrease the frequency (f), to maintain the constant speed of light (c), the wavelength (λ) must increase.

Think of it like this: Imagine waves in a pool. If you make fewer waves per second (lower frequency), the distance between each wave crest (wavelength) will be greater.