M4 Light Entertainment II

Background

Two of the number of processes light undergoes when it interacts with matter is reflection and refraction. Depending on the property of the matter it interacts with, light either bounces back (undergoes reflection) or it passes through the matter but its speed and direction may change–referred to as refraction. The consequence of these simple processes can be from just unusual to dramatic in real life.

The Discrepant Event: Different Sides of a Rod

For this light entertainment, I took my students to the swimming pool. There, I had them observe the appearance of a pool cleaning rod when dipped inside the pool. Again, instead of having someone film this, I took a series of photos.

rod in pool at angle- side view 2 rod in pool at angle- back view
 Fig. 1. Side view of the rod inserted at an angle into the pool. Fig. 2. Back view of the rod inserted into the swimming pool at an angle.
rod in the pool at angle - front view rod in pool vertical
 Fig. 3. Front view of the rod inserted at an angle into the pool. Fig. 4. Side view of the rod this time inserted straight down into the pool.

And the question is: Why does the swimming-pool cleaning rod appear bent as it does in figures 1 and 2 but not in figures 3 and 4?

Extension: When at a swimming pool, standing at one end of the pool looking across about to jump or dive in, I am sure you have noticed how the far end appears to be shallower. The pool appears to be getting steadily shallower as your gaze travels from the near end to the far end. Why?

Concepts involved:

  • image, normal, rare medium, dense medium, reflection, refraction, angle of incidence, angle of refraction, speed of light, line of sight (Additional advanced concepts: refractive index)

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