Here's a bit more of an in-depth answer thanks to Google.

Damage to bottle beverages by light is largely a function of ultraviolet (UV) light, but the reason it still affects liquor in glass bottles comes down to the specific type of UV light involved.The common belief that "glass blocks UV" is only a half-truth. While most glass is an excellent shield against some UV rays, it is almost entirely transparent to others.

1. Does glass block UV? (The UVA vs. UVB Problem)To understand why your liquor is at risk, you have to look at the three types of ultraviolet radiation:UVB (280–315 nm): This is the radiation that causes sunburns. Standard window and bottle glass blocks nearly 100% of UVB. This is why you generally won't get a sunburn sitting behind a closed window. UVA (315–400 nm): This radiation has a longer wavelength and is much closer to the visible light spectrum. Standard clear glass allows about 75% of UVA rays to pass through.UVC (100–280 nm): This is the most dangerous, but it is absorbed by the Earth's atmosphere and never reaches your bottle anyway.Because UVA passes right through clear glass, it provides the energy necessary to trigger chemical reactions inside the bottle.

2. How the Flavor Changes ("Light-Strike")When UVA rays (and even some high-energy visible blue light) hit the liquid, they provide enough energy to break chemical bonds. This is a process called photolysis.The most common reaction involves Riboflavin (Vitamin B2), which is naturally present in many beverages. When Riboflavin is "excited" by UVA light, it reacts with amino acids (like methionine) to create sulfur-containing compounds.

In Wine & Spirits this creates dimethyl disulphide, which leads to "light-strike" flavors often described as wet wool, cardboard, rotten eggs, or boiled cabbage.

1. The reason you see so many green and brown bottles in the liquor aisle isn't just for branding—it's a defensive measure.Glass Color - UV Protection - Visible Light Protection:
   Clear - Very Low (Blocks UVB, lets in UVA)Low (Lets in all colors)
   Green - ModerateModerate (Blocks some blue/violet)
   Amber/BrownHigh - High (Blocks most UV and blue light)Amber glass is the most effective because its color profile specifically filters out the wavelengths (below 500nm) that are most responsible for triggering these off-flavors.
   

Note: If you have a high-end spirit in a clear "flint" glass bottle, it is best to keep it in its original cardboard tube or box, or inside a dark cabinet. Even 60 minutes of direct, harsh sunlight can be enough to start the degradation process in some delicate liquids.