Pop quiz, can you explain this?

Maybe this is an easy one to answer for photographers, but I and a bunch of other engineers I shared this with had a lot of fun figuring this out.

This image was taken at night so I needed a relatively long exposure of 1/20 of a second.

As you all know, that should result in the spent case being one long blur, but as you can see there are 6 distinct ghosts of the case.

There are two parts to this question.
1) what would cause the ghosts?
2) why are there 6 of them?

what kind of lighting was in use?

Looks like multiple exposures to me. Was there some kind of strobe light or something? Or a light that turned on and off 6 times?

It's the frequencies of the cycling of the light from the florescent lights at the range. When the pulse is at it's peak you get a frozen casing, in 1/20 of a second that appears to happen 6 times (probably a bit more as there's burning powder in the chamber), so the average means you're also getting peak exposures when nothing is there. Very similar to stroboscopic mode on flashes, or ghosting during long exposures. I'm a bit tired to do the math and explain the rest.

The shell casing is flipping end over end, hence it looks like there are multiples. There's 6 of them due to the duration of your shutter speed.

Motor On wrote in post #15535040
It's the frequencies of the cycling of the light from the florescent lights at the range. When the pulse is at it's peak you get a frozen casing, in 1/20 of a second that appears to happen 6 times (probably a bit more as there's burning powder in the chamber), so the average means you're also getting peak exposures when nothing is there. Very similar to stroboscopic mode on flashes, or ghosting during long exposures. I'm a bit tired to do the math and explain the rest.

^^This^^
As the casing is in different positions at each peak, it will show as different casings. The frequency of the lights will dictate how many casings are shown during exposure time. ie 1/20th sec / freq= no of images. (120Hz) freq?

If you look at an old CRT computer monitor reflected by a piece of glass you'll clearly see the phase of the cycling as it goes around. I don't know why it can only be seen well in a reflection but it sure shows what our eyes lie to us about. Our eyes interpret what we see as a natural view but seeing the reflection takes away something our brain is adding to the image. And what we see is the cycling of the light just as we can see how light cycles in a slow exposure photo. If we just watched the case eject with our natural eye we would never pick up on the way the light only peaks at intervals. But some people get headaches from fluorescent lights because of this effect. Their brain is telling them something is wrong but they can't see it well enough to know exactly what it is. So it stresses the brain and causes headaches.

light cycling or stroboscopic flash would be my guess.

He's in the matrix???
all kidding aside Motor On got it

Many manual flashes have a stroboscopic mode which will give this result.

I ran into an interesting phenomenon tonight. I shot a night photo and I got a reflection off my lens then off my filter. I ended up with 2 moons in every photo I took until I took the filter off.

Motor On wrote in post #15535040
It's the frequencies of the cycling of the light from the florescent lights at the range. When the pulse is at it's peak you get a frozen casing, in 1/20 of a second that appears to happen 6 times (probably a bit more as there's burning powder in the chamber), so the average means you're also getting peak exposures when nothing is there. Very similar to stroboscopic mode on flashes, or ghosting during long exposures. I'm a bit tired to do the math and explain the rest.

This is the answer.

Basically, the flouro lights flicker on and off really fast. it's like using a flash in stroboscopic mode.

Ok, it's been 24 hours so I'll let the first cat out of the bag.

Yup, it's because of the fluorescent lights above the range. That's the answer for the 1st question.

However, I haven't seen a complete answer for the second part yet. The first hint is that in the US, the frequency of the electricity is 60 Hz. But 1/60th of a second is 3 times faster than 1/20 of a second, thus one would think there should only be 3 ghost images.

I'll post the answer in another 24 hours, but from the answers already given, I bet more than a few people will get it before then.

Because there is alternating current you get flashes as the current travels both directions so you get lights cyling on at the effective rate of 120 flashes per second. That gives you double the number of images. A complete cycle has current on one direction then current on the other direction so the light flickers on as the power comes on going both directions.

It also has to do with how long the exposure time was though. The shutter wasn't open for a full second. Six ghosts should happen in a fifth of a second.

Yup, you got it. The number of images are effectively double the electrical frequency. Similarly, in countries where the electricity runs at 50 Hz, you'll see the ghosts at 100 Hz.