Question

Geometric optics describes the interaction of light with macroscopic objects. Why, then, is it correct to use geometric optics to analyse a microscope’s image?

Short answer

We use geometric optics approximation because the size of the object is much larger than the wavelength of light.

Step-by-step solution

Geometric optics Approximation.

One approximation that geometric optics makes is that the waves (rays) travel in straight lines until they hit a surface. When the ray encounters a surface it can either bounce back (reflect) or bend (refract) but then continues to travel in a straight line.

Step 2:Comparison of the wavelength of light with the size of an object.

The size of the object we see in the microscope is of the order of $\mu m\left({10}^{-6}m\right)$. The wavelength of the light used in the microscope is of the order of few 100 nm . So the wavelength of the light is much smaller than the size of the object, hence we can use the geometrical optics approximation.