During the years I was on the road, doing science performances at schools and museums, I could count on being asked that question at least once a week. After all, isn't that the official uniform for all "real" scientists?
To understand why not all scientists wear white lab coats, lets start by thinking about why some scientists WOULD wear one. After all, the standard, white, lab coat is not the most stylish outfit, so there must be other reasons.
- To keep from damaging or staining their clothing. Just as many cooks wear an apron to protect their clothes, scientists who are working with chemicals that are dangerous or that stain usually wear something to protect their clothes.
Often, a lab coat isn't enough. After all, it is just made of cloth. Many scientists also wear an apron, made of material that resists acids, bases, stains, etc.
- To prevent contamination of their work or equipment. Imagine that you have just enjoyed a chili dog for lunch, and are now back to work on the bacterial cultures you have been carefully growing. As you lean over, a bit of chili falls from your shirt into the culture. There goes all your hard work! Putting on a lab coat as you enter the lab helps prevent that by covering up the clothing you wear outside the lab.
- To prevent contamination from spreading outside the lab. Just as you don't want a bit of chili to fall into a bacterial culture, you don't want to wear a bit of that bacterial culture on your sleeve when you go to lunch. If scientists are working with dangerous chemicals or microbes, they remove their lab coats before they leave the lab. They also start with a fresh lab coat the next day. Wearing the same, dirty lab coat defeats the purpose.
- To "look like" a scientist. I put "look like" in quotes because wearing a white, lab coat does not make you a scientist any more than wearing a football jersey makes you a professional quarterback. Still, the news media will often ask scientists to wear a lab coat during an interview for this very reason.
The white color also helps you spot anything you happen to spill while you are in the lab, letting you know it is time for a fresh lab coat. Of course, that doesn't work if what you spill happens to be white. For that reason, some scientists wear light green lab coats, unless they also work with light green chemicals.
My scientific background is in geology, and like many geologists, I rarely had the need to wear a lab coat. The same thing was true through my many years in museum education and as a traveling science educator. Of course, now that I am producing science videos, there are times when I feel that I need to wear something to make me look more scientific. That is why I have a rabbit costume.