How does vision work?

To see, enough light must go into the eye. In general, the light that propagates from the source to the object and then from the object to the source is invisible.

The objects that are seen in this way are secondary sources. In contrast, the light bulb that illuminates the scene is a primary source.

The eye and the brain are both involved in the vision process.

Click on [next-image] to progress through the animation.

• To apply the linear ray model of light.
• To understand that light has to enter the eye, and that the object being observed must itself be receiving sufficient light from a primary source.
• To show that the eyes are only light  receptors. The brain is the interpreter that enables vision to occur.
• To show that light is everywhere around us. Only the tiny part of it penetrating our eyes participates in  vision.

In 1781, the academy of sciences defined light as follows:

"Light:  A very loose fluid, that affects our eye with a strong feeling that we call clarity making objects visible. This fluid resides, like an intermediate between the visible object and the organ which receives its impression and it occupies by itself and by its action the interval between them. What makes the light, making visible objects is a matter, whose action can be stronger or weaker depending on the circumstances."

Light travels in a straight line at a very high speed. Light is invisible. For this reason, it is convenient to represent it in the form of 'rays'. A ray of light is a straight line segment with an arrow indicating the direction of propagation. An object illuminated by a ray of light reflects that light becoming a secondary light source.

The eye is an organ that perceives light and with the help of the brain reconstructs an image. To see an object, light from the object must enter the eye. In the animation, the red brick located behind the toy is invisible to the child because no ray of light from the red brick reaches the eye of the child.

So, light is all around us, but only the rays that fall into our eyes give us an image.