I was taking pictures of a desk in an office with overhead lighting. The light reflected off the desk and created shiny spots on the desk so that it wasn't uniform. Is there a way to prevent this? Would a polarizer work?
Yes, a polarizer can help, if the light source subtends a fairly small angle and if the reflection is from a dielelctric (i.e. non-conductor), such as glass, plastic or lacquer.
The simplest way to check is to test it with a polarizer on the camera and observe the reflection in the viewfinder or LCD. if you have a camera with separate viewfinder, hold the polarizer to your eye; rotate it to reduce the reflection; then put it on the camera lens in the same orientation.
If that doesn't do the job, tack a white sheet over the lights to diffuse them.
While a polarizer may help, if your office is typical, there will be too many lights in too many directions to eliminate all of them.
Things that may help:
A: Use a telephoto and shoot from further away. This will tend to reduce the number of lights that can reflect at you, and the angles will be closer to the same, so the polarizer will be more effective.
Note, that you have to have a top notch polarizer on a telephoto, as the lens will magnify any distortions in the polarizer.
B: Move around. You may find a position that minimizes reflections.
C: Drown them out. Use so much lighting of your own, that the office light reflections don't matter.
D: Turn the office lights off.
E: Strategically remove certain bulbs so the reflections are eliminated. Or pages of newsprint and masking tape to cast shade.
F: This one may not work, and will require some testing. Use something like beeswax or furniture polish and make the desk more of a matte finish.