Why are Many Bottles Brown?
Bottles come in many colors. Often the choice of color is cosmetic. In other cases, as with brown and amber bottles, the color is selected on the basis of more than looks. So why are bottles often brown?
The short answer is that brown helps protect contents from light. But there’s a good bit of science behind that answer.
Visible light has a wavelength in the range of 380 to 740 nanometers. This only represents a small portion of the full range of all the frequencies and wavelengths of the electromagnetic spectrum. See below for a diagram of the full spectrum.
Notice that directly to the left of visible light is UV — known as UV light or ultraviolet radiation or — which has slightly higher frequency than visible light. It is the UV light that most commonly affects the integrity of a bottle's contents. A classic example is “skunky” beer: when beer in a clear glass is exposed to light, the taste goes bad, due to photo-oxidation. This is one specific example of a “photochemical” reaction, a chemical reaction that is caused by the absorption of light by atoms or molecules. There are many different photochemical reactions that can take place. A common photochemical reaction problematic in the packaging business is photodegradation, which is basically a breakdown of molecules initiated by the absorption of light photons. UV light, which has higher energy than visible light, often accelerates photodegradation.
The color of any object is the frequency of the wavelength that reflects from the objects surface. Amber and brown are a low-intensity, low-frequency hues: basically, darkened yellow, orange, or red. The low intensity blocks out some light and the hue enables some frequencies in the red to yellow range to pass through. Lower frequencies are effectively blocked, including ultra-violet frequencies. This is precisely why brown/amber is the best color range for allowing some transparency while still blocking harmful UV light!
Not all substances will have a photochemical reaction when exposed to light; those that do are considered “photosensitive.” Examples of things that are photosensitive include many vitamins, many medications, beer, hydrogen peroxide, alkali salts, and photography film. In fact, so many different things in the pharmaceutical industry are photosensitive that brown and amber pill bottles and medicine bottles have become the norm.
Now you know why some bottles are brown.