# Refraction of Light in the water and other substances

Refraction is the phenomenon in which direction of light changes while entering into different medium. Light refracts in the different depending on a refract index.

## Water refraction

When light transits into the water, the speed of light reduces, causing it to change direction slightly. This change of direction is called refraction. When light penetrates a dense substance with a higher refractive index, it ‘bends’ more towards the normal line.

### The Broken Pencil

As light travels through a given medium, it travels in a straight line. However, when light passes from one medium into a second medium, the light path bends. Refraction takes place. The refraction occurs only at the boundary. Once the light has crossed the boundary between the two media, it continues to travel in a straight line. Only now, the direction of that line is different than it was in the former medium. If when sighting at an object, light from that object changes media on the way to your eye, a visual distortion is likely to occur. This visual distortion is witnessed if you look at a pencil submerged in a glass half-filled with water. As you sight through the side of the glass at the portion of the pencil located above the water’s surface, light travels directly from the pencil to your eye. Since this light does not change medium, it will not refract. (Actually, there is a change of medium from air to glass and back into the air. Because the glass is so thin and because the light starts and finished in air, the refraction into and out of the glass causes little deviation in the light’s original direction.) As you sight at the portion of the pencil that was submerged in the water, light travels from water to air (or from water to glass to air). This light ray changes medium and subsequently undergoes refraction. As a result, the image of the pencil appears to be broken. Furthermore, the portion of the pencil that is submerged in water appears to be wider than the portion of the pencil that is not submerged. These visual distortions are explained by the refraction of light.

In this case, the light rays that undergo a deviation from their original path are those that travel from the submerged portion of the pencil, through the water, across the boundary, into the air, and ultimately to the eye. At the boundary, this ray refracts. The eye-brain interaction cannot account for the refraction of light. As was emphasized in Unit 13, the brain judges the image location to be the location where light rays appear to originate from.

This image location is the location where either reflected or refracted rays intersect. The eye and brain assume that light travels in a straight line and then extends all incoming rays of light backward until they intersect. Light rays from the submerged portion of the pencil will intersect in a different location than light rays from the portion of the pencil that extends above the surface of the water. For this reason, the submerged portion of the pencil appears to be in a different location than the portion of the pencil that extends above the water. The diagram at the right shows a God’s-eye view of the light path from the submerged portion of the pencil to each of your two eyes. Only the left and right extremities (edges) of the pencil are considered. The blue lines depict the path of light to your right eye and the red lines depict the path of light to your left eye. Observe that the light path has bent at the boundary. Dashed lines represent the extensions of the lines of sight back into the water. Observe that these extension lines intersect at a given point; the point represents the image of the left and the right edge of the pencil. Finally, observe that the image of the pencil is wider than the actual pencil. A ray model of light that considers the refraction of light at boundaries adequately explains the broken pencil observations.

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