Dear Mr. Scientist,
How does insect repellent work? Does spraying it somehow make me invisible to bugs?
—Kyle R.
There are a lot of different chemical signals humans give off like the carbon dioxide we exhale or the lactic acid in our sweat that attract pesky mosquitoes and gnats. Most insect repellents (like DEET, R-326, or citronella) evaporate once applied and create a small cloud of vapor around you. This cloud not only covers up the “scent” you give off, but is also extremely unpleasant to bugs. Since the insects can’t sense you, and what they do sense is very off-putting, they avoid you and find someone else to bite.
Dear Mr. Scientist,
How do those combination eyeglasses and sunglasses with lenses that get darker when you go out in the sun work? —Adam D.
When this technology was first developed, the lenses contained small amounts of silver ions in them. The silver ions were invisible, but once exposed to the UV light from the sun they turned into elemental silver which is visible and caused the lenses to get darker. This process reversed itself when the glasses were no longer exposed to UV light. Nowadays these kinds of glasses contain something called a photochromic dye instead of silver, but they work in a similar way. The dyes are invisible in normal white light, but break apart and rearrange into a visible form in the presence of UV light. Once the UV light is taken away, the dye rearranges itself back into its invisible form.
