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Refraction and Sight


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. 

This visual distortion is witnessed if you look at a pencil submerged in a glass half-filled with water. 

1-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. 

2-As you sight at the portion of the pencil that was submerged in the water, 

light travels from water 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.

Q1: Is the fish deeper or shallower than its image appears to be? (Using a sketch explain your reasoning.)  

The fish is deeper than it appears.


 Q2Should you stab above or below the image of the fish to spear it? (Explain your reasoning using a sketch.)

You should stab below the image of the fish. 

Q3: What did you observe?

As the water level rises, the coin becomes visible.

Q4: How did the image of the coin viewed through the water compare to simply looking at the coin? Compare the apparent size of the coin.

The coin appears to be slightly magnified.

Q5: Draw two ray diagrams for your eye, the coin, and the cup, one with and one without water. Does the light bend in either case? 

The light is bent at the interface between the air and water. 

Q6: Use your diagrams to explain what you observed, describe how light must change paths.

Light must be bent away from the normal as it leaves the water in order for your eye to see the coin.

Q7: Now re-address (discuss and amend your responses for) the spearfishing questions.

Dependent on answer to question 1 & 2.

Q8: You are hired by the neighborhood greasy spoon restaurant, and assigned to wash dishes in an industrial sized sink. You see the drain plug on the bottom through the cold, greasy water and it looks like it is just barely within reach. Can you pull it out by hand? Why or why not? (Physical science reasons, please)

You can not reach the drain plug. Light reflected off the drain is bent away from the normal as the ray leaves the water. Therefore the drain's position appears to be shallower than it actually is. It also appears magnified adding to the perception of being closer.