Move the screen to various distances from the bulb and measure how many squares are illuminated at various distances. Graph distance from the bulb vs. number of squares illuminated. When the distance is doubled, the number of squares illuminated is quadrupled, etc. Graph distance from bulb squared vs. number of squares illuminated. This yields a straight line.
Shadows can be much bigger that the source of the shadow.
The further a light source is away, the dimmer it appears. Astronomers can use this relationship to estimate how far away a distant star is, if they know how bright the star actually is.