SciTech

What’s the chemistry behind eerie Halloween lighting?

Kenneth Lyle is an instructor and outreach coordinator in the department of chemistry at Duke University. Here he breaks down the chemical components that illuminate many trick-or-treating outings. Questions and answers have been edited.

Q. Glow sticks almost seem magical. You bend them, hear a cracking noise, shake them, and they glow. How do they work?

A. Glow sticks use a chemical reaction called chemiluminescence to produce light. Inside each glow stick are small vials of a corrosive chemical called hydrogen peroxide. When those vials are broken, the hydrogen peroxide gives electrons to another chemical within the stick, such as luminol or Cyalume, bumping them up to a higher energy state. Those chemicals then transfer that energy to a chemical dye, which in turn releases the energy in the form of light that we can see. The color of the glow stick depends on the dye used. The reaction itself is very slow, so other chemicals that can act as catalysts to speed up the reaction – such as copper or iron ions – are usually added.

Q. Is it true that you can extend the life of glow sticks by putting them in the freezer?

A. Because the light is produced by a chemical reaction, glow sticks can only be used once. Once one of the chemicals involved in this process are used up, the reaction ceases. But they can last longer by storing them in a freezer to slow down the rate of reaction. One can observe this phenomenon by inserting one glow stick in a cup of ice cooled water and another in hot water, and then comparing their brightness to each another and to a third light stick at room temperature.

Q. What are glow-in-the-dark decorations like skeletons and skulls made of, and how do they hold their light?

A. Glowing skeletons and other such eerie objects involve a totally different process called phosphorescence. These objects contain a chemical, such as zinc sulfide, that absorbs light of a higher energy and releases light of lower energy, often in the visible range. The difference in energy absorbed and the energy emitted is taken up by the substance itself, which causes it to undergo internal changes. These changes occur slowly and are why we see the release of light over a period of time.

Q. How do the black lights in haunted houses and clubs create an eerie glow?

A. Black lights use mercury plasma, a substance that emits both visible and ultraviolet light when it is connected to a high-voltage source. The dark coating on the black light bulb is designed to block most of the visible light without blocking the ultraviolet. Ultraviolet is invisible to our eyes and is of higher energy. The ultraviolet light causes fluorescent and phosphorescent materials to glow. The main difference between these two types of materials is that fluorescent materials stop emitting light immediately after the source of light is removed whereas phosphorescent materials continue to emit light long afterward.

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