Ygthkb

A mage has created an amulet of light. It shines light from one side, and the user is capable of controlling the color and intensity of that light.

In an effort to avoid abuse, the mage has limited the amulet to only produce wavelengths in the visible range.

Perhaps less cleverly, they did not think to put a limit on the amount of light that can be produced at once.

A foolish thief has gotten their hands on the amulet and started cranking up the intensity. At what point will their actions start to cause noticeable physical affects, and what will those effects be?

My guesses is that at some point objects will become hot enough to catch fire, radiation pressure will start pushing objects (and the amulet) around, and light will shine through normally opaque objects. Are these guesses correct, and are there any other dangers my thief should be wary of?

Bonus: Are some wavelengths of visible light safer than others? Which are they?

If the light coming out of the amulet is well collimated (i.e. it can form a narrow beam that does not disperse - like a laser) and the light is emitted continuously rather than in pulses, then what it does will depending on the aperture (i.e. the cross sectional area) of the amulet.

Assuming a diameter of 2-3 cm (wristwatch or locket sized), an intensity of 10s to 100s of watts could cause blindness if pointed at someone's eyes, and kilowatts would be enough to set light to a flammable target. 10s of kW would cut through stuff. If the emission moves up to megawatts then the surface of the target will heat up and ablate so rapidly that it will effectively act like an explosion - the target might be thrown backwards but this is due to evaporation at the super-heated surface, not radiation pressure. Somewhere around 10^12-10^14 W, the air in the path of the beam would almost instantaneously ionize. This would be bad for the holder of the amulet as it means that the air directly in front of them would glow more brightly than the surface of the sun - the amulet holder would get very bad sun-burn.

At vastly higher energy the beam could convert materials to plasma, break up atomic nuclei and eventually convert all matter to a quark-soup like that found in the first few nanoseconds of the big-bang. Radiation pressure would now be evident - but only if you used the amulet in a vacuum as the reaction with any nearby matter would overwhelm the effect otherwise. At stupidly higher energy, you might reach the Plank limit beyond which physicists can only guess - but more likely the energy density would be so great that local space along the beam would instantly collapse into a black hole.

If the amulet fires out a narrow beam - 0.1 mm diameter perhaps, then all the power requirements drop by around 4 orders of magnitude. Probably harder to blind an enemy then but it would still be effective for cutting/drilling through things at a distance. If it was pulsed, then the relative danger to the user may be reduced.

If the amulet fired out a dispersed wide angle multi-frequency beam like a hand-held torch, then bad things would happen to the wielder (the sun-burn and explosions resulting from heating of the air immediately in front of the amulet) before it would do much harm to any target more than a few 10s of metres away.

If the light is not coherent, all the things described for the coherent beam would still happen, but at higher energy required for an effect on a target, and a lower energy required for the 'effects' on the wielder.

Somewhere in the kilowatt range nearby objects will start heating appreciably and possibly even catching fire (see for example Wicked Laser's FlashTorch). It will depend on their reflectivity, the material they're made of, beam dwell time, and so on.

Much sooner than that, you'll get temporary flash blindness from reflected light. I haven't been able to find a reliable source on the intensity required, but I came up with an approximate ballpark figure of 14000 lumen (that's around 150 W with high-efficiency LEDs, which the magic device appear to resemble) from data on nuclear explosions.

Radiation pressure doesn't come into play until much, much later.

A big "if" hangs on the quality of light. It might be in the visible octave, and still be almost-monochromatic coherent laser light.

At what point will their actions start to cause noticeable physical affects, and what will those effects be?

Projecting light is itself a physical effect. Starting at a flux of 0.0001 flux in an dark place, the light will be enough for the naked eye to see, which is equivalent to starlight on a moonless night.

But of course you didn't mean that.

The effects visible light has on things, besides making them visible to us, depend on the target. In general visible light can slow down or more usually speed up chemical reactions (think of polaroids) and warm stuff up.

Regular laser pointers are usually within the 3 to 5 miliwatt range. It can blind people. There is a chronic mental disease that causes people to point such lasers at aircraft for kicks, which is how you can use a very little amount of light to take hundreds of lives in a very grim way.

A 100 miliwatt laser pointer with a tight beam width at close range can set fire to matches and gasoline, so you may emulate that. More power makes it easier and faster to set stuff on fire.

Black Holes?

You amulet can generate black holes. Given the radius of your amulet $R_s$ we can calculate the amount of mass necessary to collapse it into a black hole.

$$M = frac{c^2}{2G}R_s$$

But your amulet produces light not mass. Lucky, Einstein can help us with its mass energy equivalence:

$$E = mc^2$$

So, we need:

$$E = c^2frac{c^2}{2G}R_s$$

That is the amount of energy you need to make a black hole. If your intensity were that amount of watts (and the light didn't move), in a single second you would be able to produce a black hole.

If you want the radiant intensity, knowing that $4pi$ Steradian is a sphere:

$$W/sr = frac{c^2frac{c^2}{2G}R_s}{4pi}$$

You need that amount of W/sr (also know as watts per steradian).

Or, given a candela is $frac{1}{683}$ W/sr:

$$text{Cd} = 683*frac{c^2frac{c^2}{2G}R_s}{4pi}$$

That is the amount of light need to perform a black hole. But, taking into account that light moves very fast, you should rather produce that output in a single burst of light instead of over a single second. Otherwise, you will need better maths.