The eye has a transparent lens, which concentrates the rays of light on the retina to form a clear image. Clouding of the natural lens is called cataract. In case of cataract developement, the decrease in visaul acuity is gradual and slow. A sudden loss of vision is caused by other factors and requires immediate referral to a doctor. Cataract is not caused by watching/reading strain and usually occurs in both eyes – not necessarily simultaneously.
Clouding of the lens interferes with the passing of light rays through it, thus causing everything to appear dimmer. For example, when the room is lit and the rest of the people see well, the person with the cataract will ask to "turn on" the light. Another sign of the cataract's progress is the difficulty to distinguish between an object and its background. For example, the "bumps" disappear from the pavement, the wrinkles on the face, the crumbs from the floor and the dust from the furniture. That's why sometimes the patients after the surgery, when the vision improves, jokingly wish their cataract back
A certain type of cataract causes the light to scatter within the eye and thus causing glare. In these cases driving with the sun behind is easy, but driving into the sun is very difficult, and for the same reason other car's headlights make the night driving almost impossible.
Of note, cataract is not a disease, but rather, a process of lens aging, which occurs more frequently after the age of 60. Yet, there are diseases, medical treatments, trauma and congenital defects that can lead to cataract formation.