Among the following options, whose action causes a shift in the focal length of an eye lens?

Ciliary muscles are smooth muscle fibres. When the ciliary muscles relax the lens becomes flat and when they contract the lens grows thicker. The focal length of the eyes changes as the curvature of the eye lens changes.

• When the ciliary muscles relax, the eye lens thins and the focal length increases, allowing you to view distant objects.
• When the ciliary muscle contracts, the eye lens thickens and the focal length decreases. It allows us to perceive items that are close by.

A pupil is a hole in our eye that allows light to enter.

The iris is a muscle in our eyes that regulates the size of our pupils. It dilates and contracts, causing the pupil size to change. It regulates the amount of light that enters our eyes.

The retina is where images are created.