A lens diagram with a positive lens. Mess around with the lens strength (F distance) and object to see how the projected object changes size! This is correct to how it would work in real life! scales: 1 = 1cm. (so F distance is automatically 75cm)
The purple line is the object that is going through the lens. the black lines are rays of light that the object is reflecting, and they are used to calculate the other purple line: the object that is projected. if you were to place down a screen you would have to place it at the rightmost purple line. the F points are where the light rays would be focussed if parallel rays whent through the lens.