I am watching the movie right now and after the Patronus leads Harry onto the lake, he can clearly make out the sword through the ice. That suggests the ice is very thin (as to not refract the light too much) and yet it is strong enough to carry him and require magic to break.

Don't recall how it was in the book, but from my experience with ice, it's very hard to see through, as the surface is typically uneven and sometimes full of bubbles etc which distorts the view through it, sometimes so much that the ice just becomes a white diffuse block, for lack of better words.

  • The ice was there after the sword was put in by Prof. Snape, so he may also have magically adjusted it's transparency – user13267 Jan 13 '17 at 3:29
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    This is nothing to do with SciFi, but lake ice comes in clear and white forms. Clear ice is easily transparent enough to see through the few inches required to support a person. – DJClayworth Jan 13 '17 at 3:47
  • Yeah, I went ice skating on a lake last weekend, and the ice was several inches thick and still mostly completely transparent. – tobiasvl Jan 13 '17 at 9:15
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    @close-voters: our policy on science questions says, "Any question that is asking for an answer within the context of a fictional universe, even if that question requires real-world science information, is on-topic." – Rand al'Thor Jan 13 '17 at 12:16
  • There was a glowing Patronus leading him to it! – Möoz Jan 24 '17 at 3:11

Harry was using his magical s̶u̶p̶e̶r̶t̶o̶r̶c̶h̶ wand.

From the book:

"Lumos!" he whispered, and the wand-tip ignited.
Something gleamed in the light of the wand and Harry spun about, but all that was there was a small, frozen pool, its cracked, black surface glittering as he raised the wand higher to examine it.
He moved forwards rather cautiously and looked down. The ice reflected his distorted shadow, and the beam of wandlight, but deep below the thick, misty grey carapace something else glinted. A great sliver cross...
(Deathly Hallows, Chapter 19, The Silver Doe).

Note that it can't be that the ice was thin or transparent. It's explicitly described as being thick and misty. Harry wouldn't have been able to see through it if he hadn't had his wand with him at the time. Yet, on the basis that magic > science the wandlight was able to go through the pool. The ice is thick enough to hold his weight but not so thick that Harry couldn't see through it using magic.

There is a difference between the book and the film. In the book Harry doesn't purposefully look into the pool. His attention was drawn to it only because the shiny sword reflected his wandlight. Then he goes to investigate further. In the book there's no doubt that Harry used his wand to see the sword clearly.

His heart skipped into his mouth: he dropped to his knees at the pool's edge and angled the wand so as to flood the bottom of the pool with as much light as possible. A glint of deep red...it was a sword with glittering rubies in its hilt.
(Deathly Hallows, Chapter 19, The Silver Doe).

Even with his wand alight, Harry has to be quite deliberate in how he angles it to see down to the bottom of the pool. So there's no doubt that the wand was what helped him see the sword (and that he couldn't have seen it without it).

In the films, however, the doe Patronus leads Harry straight to the pool and vanishes beneath the surface. Harry scrapes the frost off the pool over the spot where the Patronus vanished and sees the sword beneath it. He also has his wand lit in the film as you can (just about) see from this screenshot.


Click here for a video of the scene on youtube.

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It is possible for the ice on a frozen lake to be clear

Walking On Water: Physics of Clear Ice
Walking on beautiful clean ice in Slovakian Mountains - YouTube

This is corroborated by multiple sources online:

So either this was the reason, or Prof. Snape magicked it to make sure it was clear, as he needed Harry to be able to find the sword

Snape's magic is the most likely possibility, as the sword hadn't been in the lake long enough for the water to have frozen around it. From Snape's memory we see that it was in Dumbledore's office until they found out that Harry and company were hiding out in the Forest of Dean

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