# Day 139: How Bright is the Sun?

College-Prep Physics: A wax photometer (two parafin blocks taped together with foil in between) is a neat little device which can show you the relative light intensity of two light sources. When the light intensity is equal on both sides of the photometer, the wax blocks are illuminated equally:

When one of the lights is moved twice as far away, the wax blocks are of unequal brightness:

How many light bulbs would need to be added to the left side so the light intensity on both sides of the block would be equal again? Based on yesterday’s investigation, we saw that when distance changes by factor n, the light intensity changes by factor 1/n2. So, in this case, we’d need four bulbs on the left side:

What was amazing was that a student then asked, “Could we do this comparison with the sun?” … which is exactly what I had planned 🙂 . Groups of students took turns going outside and repeating the experiment with the wax phototometer and a 200 watt bulb in order to calculate how many 200 watt lightbulbs would be as bright as the sun (first picture).

Conceptual Physics: Students began drafting possible circuit designs to modify the bike light generator so that: 1) a battery lights the bulb when the rider is not pedaling and 2) that battery gets recharged while the rider is pedaling.