How can invisibility cloaks be seen?

Question

When Harry receives the Invisibility Cloak for Christmas, it is described as fluid, shining and silvery gray (emphasis mine):

This only left one parcel. Harry picked it up and felt it. It was very light. He unwrapped it.
Something fluid and silvery gray went slithering to the floor where it lay in gleaming folds. Ron gasped.
"I've heard of those," he said in a hushed voice, dropping the box of Every Flavor Beans he'd gotten from Hermione. "If that's what I think it is — they're really rare, and really valuable."
"What is it?"
Harry picked the shining, silvery cloth off the floor. It was strange to the touch, like water woven into material.
"It's an invisibility cloak," said Ron, a look of awe on his face. "I'm sure it is — try it on."
_{-Harry Potter and the Philosopher's Stone, Chapter 12, The Mirror of Erised}

This leads me to the question, why is an Invisibility Cloak seen? Shouldn't it be by definition invisible? Or does it work in the way that it's only invisible when someone wears it but not at other times?

Answer 1

It looks like silver fabric when not being worn, invisible when it is.

In occasions other than when it’s first introduced, when the appearance of the Invisibility Cloak is described, it’s described as silvery fabric.

“Harry had inherited just one thing from his father: a long and silvery Invisibility Cloak. It was their only chance of sneaking out of the school to visit Hagrid without anyone knowing about it.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 14 (Cornelius Fudge)}}

It’s repeated too many times to be a simple mistake, so we can presume it’s true. When no one is wearing it, it looks like silvery fabric - so it is visible when it’s not being worn.

“She hasn’t gone to get it?’ Ron said, staring after her.

She had. Hermione returned a quarter of an hour later with the silvery Cloak folded carefully under her robes.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 16 (Professor Trelawney’s Prediction)}}

However, it does become invisible as soon as someone puts it on. When Harry tries it out, it does make everything but his head invisible immediately.

“It’s an Invisibility Cloak,’ said Ron, a look of awe on his face. ‘I’m sure it is – try it on.’

Harry threw the Cloak around his shoulders and Ron gave a yell.

‘It is! Look down!’

Harry looked down at his feet, but they had gone. He dashed to the mirror. Sure enough, his reflection looked back at him, just his head suspended in mid-air, his body completely invisible. He pulled the Cloak over his head and his reflection vanished completely.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 12 (The Mirror of Erised)}}

Therefore, it seems like it is visible when not being worn. This is logical - there’s no reason for it to be invisible when it’s not being worn. In fact, if it was invisible all the time, it would be a lot harder for its owner to find it if they’re not sure where they left it.