Hi!
Cat lovers should love this, hlee!
A minor correction: the diamond shaped green area in the middle of the eye is actually the cat's pupil and the yellow is the iris. I think the small round object in the middle there is a reflection but it's definitely not a pupil.
Unlike a human's pupil which is a round hole, a cat's pupil is a vertical slit that can enlarge to cover the whole front of the eye in darkness. When its full opened, its round but in lighter conditions, the pupil is smaller and the slit is very apparent. Its very common not to see the whites of a cat's eye at all.
Here is a diagram that compares the cat's eye to a human eye.
Can you see the difference?
Many animals eyes share this trait: frogs, snakes.