Optics Lesson

Seasonal changes occur very slowly. I enjoy watching for them as the year passes. Last fall, I did a simple set of observations to record how much lower the path of the Sun was tracking as the weeks went by. That post is here.

An unplanned seasonal change got my attention in recent weeks I looked out the north window toward out neighbor’s house at about 3 pm on 25 January. It was a very cold and sunny day. It was obvious from the direction of the shadows that the Sun was in the upper left, over my shoulder, out of view. Something bright caught my eye on the snow in the foreground.

I used image editing software and drew some rays coming from the Sun down toward the window on his house. Those rays apparently were hitting a slightly concave surface of glass which reflected them in a converging pattern at the snow surface making the bright spot.

I checked several times since that day and noticed the same effect. Some times it was a little earlier or a little later in the afternoon. Each time there was a bright spot but reflected onto a slightly different place.

The Sun’s path has moved higher in the sky in the 2 weeks since that first image. I got another image on 7 February within 1 minute of the previous one on 25 January. I superimposed the new bright spot onto the old image and circled it in orange. That new spot of light is a little closer to the house.

One of the rules of optics is the law of reflection. It states that the angle of incidence is equal to the angle of reflection as measured from a line perpendicular, or normal, to the surface. This diagram assumes we are at the side of the window looking along the surface. The incident ray on 7 February approached the window at a larger angle to the normal line than on 25 January. This caused the reflection angle to also be larger.

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