There is no mental model 'law' as such.
Mental Models are a cognitive psychology concept which goes back to the 1940s. Of the articles linked by Sheff I'd read this one:
Susan Weinschenk is a psychologist and gets the detail's right:
This is a useful definition of Mental Models - particularly the bit about 'intuitive perceptions'. (Mental models are always an approximation to reality).
A mental model represents a person’s thought process for how something works (i.e., a person’s understanding of the surrounding world). Mental models are based on incomplete facts, past experiences, and even intuitive perceptions. They help shape actions and behavior, influence what people pay attention to in complicated situations, and define how people approach and solve problems.
Weinschenk however uses the term 'conceptual model' which I just think is a bit confusing ( ie what's the difference between 'mental models and conceptual models' ).
I prefer the term coined by Don Norman in his book The Design of Everyday things (1989) of 'System Image'.
The 'System Image' is basically the representation on screen which the user gets to interact with (see diagram in this article which is taken from Don's book.
Eg: The Graphical User Interface popularised by the early Mac computers (which was where computers were at at in 1989) would have popularised the concept of making the system image look like real world things such as waste baskets, so the 'conceptual model' being got over was that if you put files in the bin then they would have been deleted.
One of the huge failures of this at the time (ie where the designer's idea/ model and the users idea/ model completely failed to line up) was that putting a floppy disk icon in the bin ejected it - rather than deleted its contents...
Mental Models aren't just a label for diagrams, but are a good explanation of how the brain actually processes information and predict how people are seen to perform. Mental models affect how we interact with everything in the world: not just computer interfaces.
If you want to know more about this a specialist cognitive psychology book will give more informaition: Eg
Cognitive Psychology and its Implications J R Anderson