Final answer:
Becquerel expected weak images because he believed fluorescent minerals required direct sunlight to produce X-rays, but his discovery of radioactivity showed that uranium salts emitted radiation without sunlight.
Step-by-step explanation:
Henri Becquerel expected to see only weak images on the photographic plate containing the uranium salt because he thought fluorescent minerals needed direct sunlight to produce X-rays. Becquerel's experiments initially aimed to see if his fluorescent minerals would emit X-rays when exposed to sunlight, similar to Röntgen's discovery of X-rays. He was very surprised to find that one of his minerals, a uranium salt, had fogged the photographic plate, even without exposure to sunlight, leading him to discover that the uranium was emitting radiation on its own. This phenomenon was later understood as nuclear radioactivity, originating from the atomic nucleus and not reliant on an external energy source like sunlight.