Why was discovery of radioactivity accidental?

When I talk about something accidental leading someone to become famous, it fascinates you...Isn't it? Well, you know I mean, becoming famous in a good manner!....Lets know about one of those incidents.....So, Why was the discovery of radioactivity said to be accidental?

 In 1896, Henri Becquerel was using naturally fluorescent minerals to study the properties of X-rays which had been discovered in 1895 by Wilhelm Roentgen.

He exposed potassium uranyl sulfate to sunlight, and then placed it on photographic plates wrapped in black paper, thinking that the uranium absorbed the Sun's energy, and then emitted it as x-rays. His experiment failed, because the sky was overcast, and there was no sunlight. For some reason, Becquerel decided to develop his photographic plates anyway. 

To his surprise, the images were strong and clear, proving that the uranium emitted rays spontaneously without the help of any external source like the Sun. Becquerel had discovered radioactivity by accident, and now an element that spontaneously emits radiation is said to be radioactive. Radioactive substances such as uranium produce three types of rays known as alpha, beta, and gamma rays. Radio active elements are highly useful in the treatment of diseases like cancer.

You can always let me know what you want me to write about when I come back again!

Do subscribe to get more of these daily!

Any ideas and suggestions will be appreciated!

"Will be back tomorrow with some more interesting facts!"

Do download my app which contains Blog and many other features in it - https://play.google.com/store/apps/details?id=appinventor.ai_praneetht45.Brainiac_Machine