Here's why:
* Good magnification: It provides a good balance between magnification and field of view. You can see enough detail for most common bacteria, yeast, and mold, while still being able to visualize a decent portion of the sample.
* Versatility: It's suitable for a wide range of applications, from examining basic morphology to observing simple structures like flagella.
* Accessibility: It's usually included as part of the standard set of objectives on most microscopes.
While other objectives like the 10x (low power) and 100x (oil immersion) are also important, the 40x objective is the most frequently used for routine microbiological observations.
