Re: Reflections in a revolving door

The question: "When I enter my workplace via the revolving door, I see a reflection of myself, and what is directly behind me. As I push on the door and it revolves, the angle of reflection doesn't change! I still see what is directly in front of the building. The image doesn't shift or distort in any way. How/why does this happen?"

You are experiencing a crude corner mirror. First, read the discussion about bicycle reflectors in this answer in our archives. "Corner cubes" are slightly different than the corner mirror formed by the revolving door, but reflections from 90-degree corners are at work for both the revolving door and the corner cube. You can find a huge amount of information on the Internet by searching for "corner cube" using any of the popular search engines. The corner-cube reflectors have three planes meeting mutually at 90 degrees, while your revolving doors have only two planes meeting at 90 degrees.

When a "ray" of light encounters reflectors at 90 degrees it is returned backwards upon its original angle (in two dimensions) after it has reflected twice, and if it encounters a corner cube then it is returned back onto its original angle, backwards, in all three dimensions. You can form your 2-plane corner by using two simple plane mirrors that are touching along one edge and are at (roughly) 90 degrees to each other. If you do so and look into the corner you will see yourself as others see you rather than as you usually see yourself in a plain flat mirror (such as your bathroom mirror). Check out this web page and this other web page that discuss corner mirrors. I found these pages by searching for "corner mirror" using the Google search engine.

It is apparent that your revolving door has a total of four doors connected along a central axis, so that it forms four corner mirrors. One of the most interesting, and initially surprising, things about a corner mirror is that the angle that the "rays" obtain upon the two reflections does not depend on the orientation of the original ray to the mirror, but will always be returned at the angle at which it arrives (Assuming the angle of the mirrors is actually exactly 90 degrees!). Perhaps you play billiards? Except for slightly different behavior of the billiard balls (compared to light "rays") due to rotation imparted by collision with the first rail, billiard balls also exhibit this "same angle" behavior after they encounter two rails. The phenomenon that you describe where the image does not shift even though the revolving door is revolving is because of this characteristic of the corner mirror to return the light "ray" back at the angle at which it arrived. The picture below shows how the "rays" can return at their arrival angle, for two different angles of view.

Good observation, and good question!

John Link, MadSci Physicist