We’l be playing with optics for the next couple of weeks which is always a lot of fun. The first lab involved using a flat mirror and a pin stuck in clay as our object. The students then put their eye near the edge of the paper (at the bottom of the photo) and used a ruler to draw the reflected ray from the image in the mirror (Ar, Br, Cr) and they did that for 3 different positions of their head. They then removed the mirror and used a ruler to continue the lines behind the mirror. These lines are draw as dashed lines because they are virtual rays, the light does NOT really pass behind the mirror, our brain just processes the reflected rays as coming from behind the mirror. The students also drew lines (Ai, Bi, Ci) from the object to the points on the mirror that the reflected rays crossed, these are the incident rays of light. Incident and reflected angles were measured for ray and found to be nearly identical with our error.
We also did experiments with concave and convex mirrors. Students placed a concave mirror so that it produced an image of an object that was very far away so that the incoming light rays would enter the mirror ~parallel. We used the back door and got a fairly bright image on a piece of paper as shown in the photo below. You can also see that the image is upside down (inverted) and smaller than the object. Because the image can be projected on paper its called a REAL image, the light rays actually pass through that spot and hence light up the paper. Virtual images like in the flat mirror can NOT be projected on paper. Once the image on the paper was in focus, done by moving the paper closer or further away from the mirror, the students measured the distance between the mirror and the paper and recorded that as the focal length.
The lab handout asked the students to draw ray diagrams in their lab books for each mirror and for different positions of the object, very far away, very close (object distance smaller than the focal length of the mirror) and with the object just outside the focal length. Here’s what a ray diagram looks like for the case above, when the object is very far away.
At the beginning of class we went over the ray tracing rules and did some practice sheets. Basically, the big arrow on the left is the object and you draw a line parallel to the optic axis (the big line going across the page and thru the center of the mirror), when it reflects from the mirror it must go through the focal length, F. Likewise, a ray coming from the object that goes through F reflects off the mirror and is parallel to the optic axis. Where those 2 rays cross is the location of the image. Just like in the photograph of the real set up above, we see the ray diagram shows an inverted, smaller real image.
The mirrors I used for this lab were a set of 6 mirrors with different focal lengths. I believe they are the same as this set at HomeScienceTools. If you don’t have a set of mirrors you can play around with a spoon – when you look at the part of the spoon you place food on, you’re looking at a concave mirror and you can see how the image changes as you move it farther away from your face. The other side of the spoon is a convex mirror.
The lab handout can be found by googling “AP physics mirror lab”, the first link is the lab I used and when you click on it the document will download. I tried putting a link here but its not working.