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Continual light is possibly one of the most common spells in an AD&D fantasy world. It lasts forever (or nearly so), and can be used to light up everything from houses to dungeons without danger. An otherwise medievalish world can have the equivalent of modern indoor lighting, streetlights, flashlights, and so on - and they never even need to change the batteries.

An important thing to know about light is that it is just one section of the electro-magnetic (EM) spectrum. The whole EM spectrum includes microwaves, infrared radiation, X-rays, and so on. Light (and microwaves, infrared radiation, etc.) is made of photons, which travel like waves (and like particles, but that’s not important at the moment). A photon’s wave has a frequency (how many times it peaks within a certain amount of time) and a wavelength (the actual length of the wave). These two properties are inversely related to each other. That means when one goes up, the other goes down. So a wave with a long wavelength has a low frequency, and one with a short wavelength has a higher frequency.

http://www.jsc.mil/images/speccht.jpg This website has a nice picture of the EM spectrum. On the left are the long wavelength, low frequency waves. On the right are the short wavelength, high frequency waves. You probably recognize X-rays on the right and radio waves on the left; the same microwaves that are in your microwave oven are about where the VHF and UHF labels are in yellow.

Some PCs want to use continual light spells to make infra-lanterns or ultra-lanterns for their demihuman characters to use. This can have unexpected consequences.

The easiest, least dangerous option is to allow the infra- and ultra-lanterns, with the rationale that demihuman mages would be aware of these regions of the spectrum. But the light rocks will now be warm (or even hot, for an added wrinkle) and can even give the PCs sunburn! A clever, nasty PC might want to research a “UV only” version of this spell and cast it in the home of an unloved NPC (or other PC). Unwitting long-term exposure to enough UV light can cause serious second-degree burns!

Also there comes the other problem. The nasty mage above has learned that shifting his spell this way causes burns. What if he shifts it some more? Soon, he’ll be in the X-ray region, and may start causing cancer in others! (It’s not an immediate game effect, but it’s a silent and unexpected way to effectively poison someone over the course of a few years). Shift it the other way and he’s got a portable microwave oven.

While wooden shutters might block out visible light and its near neighbors, you’d need lead to block X-rays and their kin. Special shielding is also needed for microwaves. It can get nasty, fast!

Flashlights

Some PCs think that by dropping a light rock down a scroll tube, they have invented the flashlight. This is not so. They will have decreased the total light output of their light rock (so the traditional 60’ radius might be decreased), but the light coming out of the end of the scroll tube will tend to diffuse just like a torch.

The secret of the flashlight is a special curved mirror behind the light source that reflects the light and casts it forward again. It’s not just a semicircular mirror either; it’s of a sort called parabolic. Without this mirror, you won’t get flashlight-like behavior.

Lasers

It is true: the light in a laser does bounce back and forth between two special mirrors for some time before emerging from the other end. However, putting a continual light rock between two such mirrors does not a laser make. Laser stands for “Light Amplification by Stimulated Emission of Radiation.” My advice to you if your PCs attempt to build a Wand of Lasery Death: tell them that when they can tell you what laser stands for, explain what all the words in the acronym mean, and how they intend to achieve that stimulated emission of radiation, let them attempt to build it.

Spyglasses

While lenses were available in Europe in the 1300s and 1400s, the first description of a telescope dates from 1608. It had a power of 3 or 4 (it magnified things 3 or 4 times regular size). Galileo was a telescope-making master, quickly progressing to a 3-power telescope to an 8 to a 20-power. (It was with this last one that he was able to see the moons of Jupiter. Other scientists couldn’t verify his claims of Jovian moons because they didn’t have telescopes that were good enough yet!)

The point: telescopes (or “spyglasses”) aren’t really medieval. Most GMs allow them anyway; that’s fine. Historically, they didn’t start out too powerful - more like the sort of toy telescopes you’re likely to pick up for a child. It’s something to keep in mind when the PCs want to pick out the fine detail on the heraldry of the rider they see as a dot on the horizon.

Lightening Bolts

If you’ve ever seen a lightening bolt, you know the AD&D spell of that name looks nothing like one. So we won’t take the spell effect as a literal lightening bolt, but rather as an electrical discharge that proceeds in a straight line from point of origin to target.

The (2nd edition) description states that a lightening bolt that strikes a wall before reaching its full length will double back on itself, rebounding towards its point of origin. Since we’re talking about a magical spell, there’s nothing inherently wrong with that. It seems to make slightly more physical sense, though, if the bolt bounces off the wall at an angle. Why is this our gut feeling?

The Law of Reflection states that the “angle of incidence” of light on a surface is equal to its “angle of reflection.” This just means that, if I shine a ray of light onto something, it should make a symmetric V-shape as it bounces off. (For instance, the \ side might be me shining the light, and the / side the reflected light.)

Magical lightening bolts are not light rays, but we imagine that they look the same. The GM may use the Law of Reflection to allow “bank shots” with lightening bolts. It does mean that the PCs can and will try to angle their bolts to take out as many NPCs as possible. Just remember that the NPCs can do the same.