constantrandomness asked: regarding viewsofalien's question: you wrote that "Visible light tends to get scattered and blocked by crystallinity, i.e. regularly repeating atoms" -- what about diamonds, quartz... all kinds of jewels/gems? they are all crystalline and highly transparent.
Excellent point username: constantrandomness (and totally non-random!).
Our answer was pretty incomplete, because crystallinity will only scatter light if it has some order that is about as big as the wavelength of the light in question. Other factors cause opacity, such as the number of interfaces the light has to cross. In this example from a while back, the Clear Science staff used this to explain why snow is opaque. But instead of individual snowflakes, the grain boundaries in crystalline substances can also act this way. To add more complication, substances can also absorb light instead of transmitting it.
So it’s really an semi-unsatisfying answer: “anything visible light can’t go through is opaque.” And crystallinity and interfaces can play a part in that.
viewsofalien asked: What makes different things transparent, translucent and opaque?
Well, username: viewsofalien, it all has to do with visible light since that’s what our eyes “see.” Does an object block visible light, smear it, or let it straight through? That will determine it. Visible light tends to get scattered and blocked by crystallinity, i.e. regularly repeating atoms. “Amorphous” materials like glass tend to let light go right through!
emgremlyn asked: Hi I am a new science tumblr and I need some one to notice my blog. I love your blog. If you could help my blog get people to notice me it would mean a lot. Thank you soooo much.
Hi username: emgremlyn, why not. The Clear Science staff liked the article you posted about 3D printing organs.