If people type into Google “cat has a sore eye” as people do then in my opinion they are referring to one of two conditions (a) conjunctivitis and (b) a painful eye as evidenced by symptoms such as squinting and excessive tearing
The reason why I have decided that people are referring to conjunctivitis is because this is an inflammation of the membrane covering the back of the eyelids and on the surface of the eyeball and it is one of the most common eye conditions in cats. Importantly you’ll see a inflammation as demonstrated by redness around the eye as you can see in the photograph below. For this reason the condition is called “pink eye”. When people see redness and inflammation they automatically assume that the area is sore.
In feline conjunctivitis there is not much pain but there is itchiness. I’m just providing layperson’s advice. You will need to see a veterinarian. Conjunctivitis is a bacterial infection following on from a cat cold (a viral infection) which is more technically referred to usually as a herpesvirus infection. There will be other issues to discuss with your veterinarian such as whether another virus is involved. I’m just providing an introduction.
Once again, if a cat owner is typing into Google “cat has a sore eye” they may have observed their cat showing signs of a pain eye including excessive tearing, squinting, tenderness to the touch and the avoidance of light. With a painful eye, a cat may paw at it. The usual causes are injuries to the cornea and diseases of the inner eye. There may be health problems which cause impaired vision such as glaucoma and uveitis. Once again if you see these sort of symptoms you should see a veterinarian as promptly as possible.
The purpose of this article is simply to raise some questions, to provide an alert because I can only guess what people are searching for in terms of information if they search for “cat has a sore eye”. I think I am right though.
P.S. The reason for Charlie’s tearing was cancer behind the eye.