Lasers emit coherent beams of light. The shields and barriers conjured by unicorns and alicorns may come in many different colors, but they are not opaque, which means at least some wavelengths of light should pass through them. I don't know what magic shields are made of (besides magic), but if a pony like Twilight or Starlight had created a protective barrier, and the beam from a powerful laser weapon in either the visible or infrared spectrum was fired at them, would they be safe or toast?

Going by our laws of physics if you can see through it that means visible light can get through, how much light/energy depends on just how opaque the shield is

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This is true BUT it depends on the power of the weapon along with shield strength say...it is a filly unicorn who just learned the spell and then add in a person with a lasrifle (Warhammer 40k weapon as strong as a .50 sniper rifle) better tell the filly's parents to ID the body, if its a group of elite guard ponies who knows the spell very well and is trained very will in the spell than its most likely the lasrifle will bounce off but bring in a lascannon (A much stronger weapon that can stop a terminator marine ((who has armor that withstand super heated plasma and radiation along with able to stay in space for long periods of time)) in their tracks) their going to be BBQ.

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Too many variables. Could you design a shield that only lets in so much light before it darkens? Possibly. By default, if you can see through it, lasers are going to pass right through it without hindrance and cook the pony inside.

Lasers travel at the speed of light; thus it doesn't matter whether or not magical shields are effective, when ponies do not have reflexes fast enough to erect them to counter said lasers.

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Who knows? We don't really know how magic shields actually work. Realistically the light would make it through since the shields are usually translucent, but the beam may be weakened to some degree or another. Or not? Going by cartoon logic, energy shields tend to block things like lasers even if it doesn't make sense. It's kinda impossible to tell.

Really, there's not much we can do with this as a thought experiment due to magic being, you know, not real. And if you're trying to research the subject for a story, you may as well go with whatever outcome is most appropriate for your plot.

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That is A) irrelevant to the discussion since the OP specifically asked if a laser could penetrate a magic shield and B) ignoring the fact that the pony can erect the shield before the laser is fired, for example as a reaction to having a weapon pointed at them.

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First, it is relevant to the conversation due to the fact that a shield cannot protect a pony who is already struck (and possibly killed).

Second, that relies on variables of the pony knowing what a laser is and being able to see the holder of said laser.

Assuming the shield is already up by the time laser is fired, it all boils down to one question: Is energy loss from light passing throw the shield linear or not?

If it is linear (so basically, the shield deflects half the photons and lets the rest through), then powerful enough laser will leak through the shield in sufficient quantities to do some damage. To protect from that kind of weapon the shield would have to be entirely opaque.

If it is non-linear (the more energy passes through, the greater ratio of it is deflected), then you can have a shield that is see-through but can protect from laser weaponry. But then another question arises: how fast can the shield react? If it's reaction time is measureable, then pulse lasers will likely pass through, because most of the energy will pass by the time shield adapts its properties. If it's not, no sell on that either.

All of this discounts things like relative caster's and weapon's power.

You know, when reading TCB stories I always wondered if a solar-powered weapon like one in "Die Another Day" would be able to penetrate the barrier - after all, its just sunlight...

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One thing I didn't really see anyone else mention is that reflection/absortion/transmission properties are not constant for all light wavelengths. This means that they could only be translucent in the visible spectrum and highly reflective or absortive in other frequencies used by lasers. So my guess is that they'd perform worse with a viable spectrum laser than a non visible.
Although we don't really know how magic actually works, which is a huge problem in this kind of thinking. We don't know what other kind of properties are built in. We don't even know if it's relying on classical physics and materials properties it could be relying on quantum effects, or stuff we don't even know about.

This logic only works assuming the lasers and shields they emit are actual made of light and not, you know, magical energy? No need to look so deep into it.

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So you know those earmuffs you sometimes see at gun ranges that block harmful sounds outright but then transmits the sounds with a cap on how loud it goes to your ears? I picture it as that but with light. Your not seeing them directly, you're seeing a projection of what you would see but without the light going above safe levels. Boom, transparent laser shield explained.

Wanna see me do invisibility?

So the light doesn't go around all of you, it still lets it hit your eyes, but then it scatters and shifts the light that bounced off your eyes afterwards like a one way mirror but more like a one way display of what you should see over them. Boom, invisibility without blindness.

That said, the alternatives I've seen like telekinetic radar and such are so creative and I love them!

This reminds me of a similar thought from a crossover fic called Renegade.
C&C's Ion Cannon vs Mass Effect's Kinetic Barrier:

Contrary to popular belief, kinetic barriers do not stop ion cannon beams. However, the barrier does greatly diffuse the ion beam, resulting in a massive loss of coherence upon contact with the kinetic barrier. However, enough energy bleeds though the barrier to inflict significant damage; against soft targets it can inflict an incapacitating or debilitating injury, and against vehicles or ships the diffused beam can still destroy or damage subsystems or inflict mobility kills.

I imagine it would be the same here.
It hits you. Your shield stops some of it. Worst case, still dies.