In a TV program, a photographic negative is introduced as evidence in a court tr
ID: 1403730 • Letter: I
Question
In a TV program, a photographic negative is introduced as evidence in a court trial. The negative shows an image of a house (now burned down) that was the scene of the crime. At the trial the defendant’s acquittal depends on knowing exactly how far above the ground a window was. An expert called by the defense claims that this height can be calculated from only two pieces of information: (1) the measured height on the film, and (2) the focal length of the camera lens. Explain whether the expert is making sense, using the thin-lens and magnification equations to guide your thinking.
Explanation / Answer
We only know hi (the measured height on the film) and the focal length of the camera lens.
Thin Lens Equation
1/do + 1/di = 1/2
Magnification Equation
M = -di/do = 3/ho
We need to calculate ho, but that cannot be calculated by knowing f & hi alone.
So, the expert is not making sense.