The answer to this question may help you to focus on the following passage:

What did the Impressionists contribute
to the art world?

(Excerpted from Art Through the Ages, Chapter 21:
The Nineteenth Century. p.923-24.)

The Impressionists sought to create the illusion of forms bathed in light and atmosphere. This goal required an intensive study of outdoor light as the source of our experience of color, which revealed the important truth that local color, the actual color of an object, is usually modified by the quality of the light in which it is seen, by reflections from other objects, and by the effects produced by juxtaposed colors. The juxtaposition of colors on a canvas for the eye to fuse at a distance produces a more intense hue than the mixing of the same colors on the palette. The Impressionists achieved remarkably brilliant effects with their characteristically short, choppy brush strokes, which so accurately caught the vibrating quality of light.

Of the Impressionists, Claude Monet carried the color method furthest. Monet called color his daylong obsession, joy and torment. He responded to lighting and atmospheric conditions of the natural scenes he painted from in terms of color, which he applied to the canvas in thick, dabbing strokes that caused the surface of the painting to shimmer. Monet's contribution was especially evident in several series of paintings on the same subject. He painted sixteen views of Waterloo Bridge in London and some forty views of Rouen Cathedral. In each canvas of this second series, the cathedral was observed from the same point of view but at different times of the day or under various climate conditions. Monet, with a scientific precision created an unparalleled and unexcelled record of the passing of time as seen in the movement of light over identical forms. Later critics accused Monet and his companions of destroying form and order for the sake of feeling atmospheric effects, but we may feel that light is properly the form of Monet's finest paintings, rather than accept the narrower definition that recognizes formal properties only in firm, geometric shapes.

The Impressionist artists ignored much that was prized by the Realists, the world of graduated tones of lights and darks; rather, the Impressionists recorded their own sensations of color, and the outlines and solidities of the world as interpreted by common sense melt away.