What scientific process can scholars use to view what lies beneath layers of paint on an artwork's surface?

Look at this painting by leonardo da vinci. this painting demonstrates da vinci's use of which of these to enhance the emotion of a painting

Sarah and juan were running in the school track meet. the muscles in their arms and legs them run fast. other muscles that you cannot see were also at work. the most important muscles for this track meet were the muscles in their a) arms. b) eye. c) feet. d) heart.

What types of visual art mediums are heavily based on observation?