The grid on the photo, the camera field of view, the landscape, sounding rocket contrails, etc, etc are all of known size. It's like using a beer can for scale in a photo.
You must be too young to have used a home film projector. Even in a little 8mm projector, the light bulb is very hot, to get the brightness needed. Any film that sits in front of the bulb will simply melt from the heat. The only reason it normally doesn't is because the film is being moved through the projector so quickly that it doesn't have time to get that hot, but when the advancing mechanism gets stuck, it only takes a few seconds for that frame of the film to melt away.