Here's why:

* One wavelength: This indicates that all the light waves have the same frequency and therefore the same wavelength.

* Crests and troughs aligned: This means the waves are in phase with each other. All the crests (highest points) and troughs (lowest points) of the waves line up.

Coherent light is essential for many technologies, including:

* Lasers: Lasers produce highly coherent light, which is why they can be focused to a very small spot and used for precise applications like cutting and surgery.

* Holography: Holography relies on the interference patterns created by coherent light to record three-dimensional images.

* Interferometry: Interferometry uses the interference of coherent light waves to measure distances and other physical properties with great precision.

Important Note: While the description is accurate, the term "aligned" is usually reserved for describing the polarization of light, which refers to the direction of the electric field oscillations.