BTW aluminium is not transparent. What you see is not aluminium, but an aluminium crystal. Just like chlorine is an extremely dangerous gas, but sodium chloride is table salt.
This. Because metals can't be transparent. They just can't. Kinda by definition. They're electric conductors, meaning their electrons respond to incoming light and cancel the inner field pretty much immediately (reflecting it to conserve energy). A metal can be transparent only if it's thinner than its skin depth (typically less than one micrometer), in other words if there's barely any metal at all.
saw this funfact: Scotty helps Dr. Nichols "invent" transparent aluminium, which in real-life, became possible 23 years later, in 2009. It was developed in part by Professor Justin Wark of Oxford University's Department of Physics.
Marvelous