How Does a Photon Become a Film Photo?

[♪ INTRO] For most of the time we’ve had photography, you couldn’t just snap a picture on your phone. You had to use specially made film. And taking the perfect photograph meant you
had to master some chemistry, from capturing light,
to developing an image that you can see. When photons enter a film camera, these packets
of light energy fly through the lens and hit a piece of plastic covered in what’s called
an emulsion. And for you chemists out there, that might
be kind of a misleading name. In chemistry, an emulsion is a mixture of
two or more liquids that normally can’t be combined, like the oils and vinegar in
a tasty vinaigrette. The stuff coating a film strip is technically
closer to a suspension, which has tiny solid bits floating around in a fluid. Typically, it’s silver halide crystals floating
around in gelatin, which solidifies on the film. Silver halides are just atoms of silver bound
to atoms of any element known as a halogen, which includes things like
bromine, chlorine, and iodine. Another name for a halide is a salt. Like, ordinary table salt is a halide made
from sodium and chlorine. Halogens can react with metals
to form halide crystals, in which the atoms become arranged
in a very orderly grid called a lattice. And all those atoms are ionized:
they either lose an electron or gain one to give them a charge. Until they’re struck by light. When a photon hits a crystal of silver bromide
on a film strip, for instance, it knocks an electron off a negatively-charged bromide
ion to create an atom of bromine. Then, that electron moves around the crystal
lattice until it settles into a defect. And eventually it can combine
with a nearby silver ion to turn it into a neutrally-charged silver atom. As more light hits the emulsion,
this happens over and over again, and creates small pockets of metallic silver. There’s still plenty of silver bromide left
over, though. The photons don’t change every halide. Only some of the silver atoms need to be changed
in order for the film to be considered exposed. You still can’t see anything on the film
at this point, though. In order to make that image appear clearly,
the film needs to be developed. With another chemical reaction! Chemical developers vary depending on the
kind of film you’re using. But the basic idea is that they react with
the remaining silver halide crystals to create an image you can see. The important thing is that developers include
reducing agents: chemicals that generally donate electrons to other compounds during
a chemical reaction. In this case, the recipients of those electrons
are the remaining silver ions. The silver ions in the exposed halide crystals, the ones with metallic silver in them, will have more reactions with the developer,
creating more metallic silver in those grains. Now, you might assume
that the metallic silver will look like shiny spoons in a fancy dining set. But in order to have that mirror-like surface,
the silver atoms have to be arranged in a rigid, uniform pattern so they all reflect
light the same way. When you look closely at a photograph, though, you can see that the film grains are bumpy and uneven. On a piece of film, metallic silver actually
looks dark. If more photons hit a silver halide crystal,
more reactions with the developer can happen, and that spot on the film strip becomes darker. Now, generally, when you take a picture, your image will have
brighter areas and darker areas. There’s more light coming from the sky
into your camera lens than from a shadow under a tree. And the more light that hits the film, the more metallic silver you get
after it’s developed. So that part of the image will look darker. Basically, shadows will look bright, and highlights
will look dark. Because the light and dark information has
been reversed. We call this a negative image. Photographers use negatives to make copies
of their pictures. They shine light through the negative onto
another piece of photographic paper, to make the exposure reaction happen again. But this time, the dark parts of the negative
block light from reaching the copy. So when the copy is developed, the areas where
the highlights were in real life show up brighter on the print. And vice versa with the shadows. This new photo is a positive image, like what
you originally saw! So if you get your hands on a film camera,
give it a shot. It’s not the same as taking a selfie with
your phone, but with a little chemistry knowledge, film isn’t so mysterious after all. Thanks for watching this episode of SciShow, and a special thank you
to all our patrons on Patreon. If you want to help support
more videos like this one, you can go to And don’t forget to go to and subscribe! [♪ OUTRO]

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