What is the difference between “looking at oneself” and “watching oneself on”?

Question

Example sentences

(a) He’s so obsessed with himself. He keeps looking at himself in the mirror.

(b) He’s so obsessed with himself. He keeps watching himself on the mirror.

As a native speaker of English, I know that sentence a is correct, and that sentence b feels wrong. Why? Well, that’s an important question that I need to figure out the answer to. Oxford Learner's Dictionary gives:

Look at (phrasal verb)

1. To examine something closely.
2. To think about, consider or study something.
3. To view or consider something in a particular way.

Watch (verb)

1. To look at somebody/something for a time, paying attention to what happens.
2. To take care of somebody/something for a short time.
3. To be careful about something.
4. To pay attention to a situation so that you notice any changes.
5. To observe somebody’s activities without their knowledge, especially over a period of time.

What the above definitions tell me is that while it makes sense to “look at yourself in the mirror,” it does not make sense to “watch yourself on the mirror.” Why? I think it is because nothing happens when you look at yourself in the mirror. If you substitute mirror for TV, it would be fine, because you be watching footage of yourself doing something.

I would appreciate any answers that I can get on this.

Answer 1

First of all, let's get one thing out of the way: As Kate Bunting points out in a comment, "We say in the mirror, never on." (Well, if the mirror is horizontal, then I guess that you could place something on it—perhaps a "watch"!)

There is nothing wrong with the phrase "watching [oneself] in the mirror". It can be used if one is observing something happening:

He’s obsessed with dancing and likes to watch himself in the mirror when he tries out new moves. (I adapted this example from fev's comment.)

It can also be used even if "nothing happens when you look at yourself in the mirror":

He drank the potion and is now watching himself carefully in the mirror to see whether he starts turning into a frog.

Of course, "watching [oneself]" is not exactly the same as "looking at [oneself]", but the dictionary definitions already explain that.

Answer 2

Watch and look both work with in - neither works with on

1. He keeps looking at himself in the mirror.
2. He keeps watching himself in the mirror.

Are both fine while neither works with “on”.

Similarly,

1. He keeps looking at himself on the TV.
2. He keeps watching himself on the TV.

Also work, but neither work with “in”.

This has nothing to do with the verb choice and everything to do with the prepositions and the nature of mirrors and TVs.

Mirrors produce the illusion of a three dimensional mirror world that the reflections inhabit. They are “in” the mirror. Physically, that’s complete nonsense, all the light is reflected off the mirrored surface so there is no “in” but the brain doesn’t care about the real world, just the model it makes of it.

TVs on the other hand produce a two dimensional image on the screen and the brain “knows” there is no “inside”, so people are “on” TV, not “in” TV. At least, when referring to the TV as an object, it’s perfectly fine to refer to someone as being in TV when referring to the television industry.