Teaching Physics with the Physics Suite

Edward F. Redish

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Closer than they appear

In the movie Jurassic Park, a car is being chased by a T-Rex and it is shown in the car’s passenger side external mirror – the one that says, “objects in mirror are closer than they appear.” Let’s analyze whether this is really true.

A. In the figure below is shown a mirror curved like the outside passenger-side mirror of a car. The mirror is cut from part of a sphere whose radius is R. The center of this sphere is marked by a circular dot at the far right. An object is a distance do from the mirror, as shown. On the figure, draw a careful ray diagram and indicate on your diagram the distances that correspond to the distance of the image from the mirror (di) and the image size (h’). (Use h for the object size.)

TRex in mirror

B. If the radius of the mirror is 2 meters and the T-Rex is 10 meters from the mirror and stands 5 meters tall (the ray diagram is not to scale), how big is the image and how far is it from the mirror?

C. Did you find that the image is bigger or smaller than the original object? Is it farther from the mirror than the object or closer? Does your result support or contradict the statement on the mirror? If it contradicts the statement, explain why they say it.

Page last modified May 2, 2010: OP42