cataract anatomy

Cataracts are opaque patches in the normally clear lens such that it is like looking through increasing smudges on spectacles.

Red Eye Effect

The red reflex is created by shining a light in the eye and seeing the red light which is reflected back from the retina.
This is commonly seen in 'red eye' caused by flash photography.
If a cataract is present then a dark patch will appear to be present somewhere within the otherwise 'red' pupil.