Detailed Instructions: In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.
Q: Article: Today, I'm going to talk about the invention of the camera and photography. The camera is often thought to be a modern invention. But as early as 1727, a German physicist discovered that light darkened silver salt, a chemical compound. Using as a camera, a big box with a small hole to let the light in, he made temporary images on the salt. Silver salt is still the base of film today. Then a French scientist made the first permanent picture by using a special piece of metal sensitized(...)with silver salt. A photograph he made in 1826 still exists. The painter Daguerre improved on the process by placing common salt, the kind we eat, on the metal. This was in 1839, the official date of the beginning of photography. But the problem was the printing of the photographs. And it was not until other scientists developed the kind of paper we now use that good printing was possible and photography became truly common. In the 1860s, Mathew Brady was able to take his famous pictures of the American Civil War, thus making portrait poses very popular. In the 20thcentury, George Eastmen of theprefix = st1 /United Statessimplified film-developing film, and Dr. Edwin Land invented the so-called instant camera with self-developing film. If we say that the photography came into existence in 1839, it follows that it has taken more than one hundred years for the camera to reach the present condition of technical refinement .
Answer: The discovery that light darkens silver salt.
A:
What was the basis of photography?