How does a photograph become a photo?

On the old fashioned camera's (SLR) a photo is created through a chemical reaction made of silver halide crystals being exposed to light, silver halide crystals are a light sensitive chemical that changes colour and forms any image that is being projected onto it. the film in the camera is glazed with this light sensitive chemical. in the camera the film is directly behind the lens and shutter, so when the camera shutter opens it lets any light from the outside into the camera throug the lense and it hits the film at the back. (the duration the shutter is open for varies, considering on how powerful the light is outside, for example if you are taking a picture of the sun, the shutter only needs to be open for roughly half a mili-second, where as if you were taking a picture of a darkroom the shutter would have to stay open for a longer duration like 3 seconds.)