Mirror Reflection


Name: Joe Michnick

Date of Lesson: Oct. 2004

Length of Lesson: 90 mins.

Description of Class: Algebra 2 Students

Source of Lesson: http://www.glenbrook.k12.il.us/gbssci/phys/Class/refln/u13l3f.html,

            Stephen Alwon, Lisa Fefferman, Nicole Williams

TEKS addressed:

á      organize, analyze, evaluate, make inferences, and predict trends from data

á      communicate valid conclusions 

The Lesson:

I.          Overview

      The students will explore and discover the focal point of a flat and a convex mirror. They will first use a flat mirror to determine the angle of reflection, and then use their knowledge of a flat mirror to find the focal point of a convex mirror.


II.            The students will be able to:

¤      Use flat mirrors to find the angle in which the image reflects

¤      Know that the angle at which something reflects is equal.

¤      Use a series of flat mirrors to make a convex mirror and find the focal point

¤      Connect their knowledge of a series of flat mirrors to one convex mirror to find the focal point

III.           Resources, materials and supplies needed

¤      Laser pointers

¤      Small flat mirrors

¤      Convex mirrors

¤      Protractors

¤      Ruler

¤      Graph paper

¤      Cardboard setup

IV.          Supplementary materials, handouts

¤      Flat Mirror Worksheet , with diagrams of the different surfaces used so that the students can draw incoming and outgoing rays.

Five-E Organization


Teacher Does

Student Does



  • What is a mirror used for?
  • What does it mean to reflect
  • Where do you stand to look at yourself in the mirror?
  • What happens if you werenÕt standing directly in front of the mirror?
  • I need two volunteers. (Place the students in the angle in which they can see each other and not see themselves.
  • Ask the students what they notice while looking into the mirror?
  • The teacher will talk with the students about their observations.





  • To see a reflection of yourself.
  • To see a copy of an image.
  • Straight in front of it.


  • You wonÕt be able to see yourself.







  • I could not see myself but I could see the other person.






Decision Point Assessment (DPA) Teacher will make sure that no student is dominating the discussion and that everyone is paying attention to the demonstration.




Now what is different about light shining into a mirror?


(do demo on laser pointer in mirror)


If you were standing where the laser pointer was, Could you see yourself?


Where could someone else stand to see the image of you?


So the laser is like the image of you shining into the mirror. J


Now we will get into groups to explore what is happening to the light/images in greater detail.


Pass out Flat mirror simulation worksheet (where is the worksheet?  I donÕt see it attached.  I would definitely recommend making up a worksheet so the students can keep track of what they are doing and what the results are) (per student), one flat mirror (per group), cardboard setup (per group), 1 sheet of graph paper (per group), protractor and ruler(per group).

Ask groups begin working on the simulation together.


The simulation has students placing a flat mirror on the cardboard setup and a graph paper in front of it. One student will shine the laser pointer into the mirror at different angles, while another student dusts chalk, and another student sketches the beams of light on the paper. The students will then measure the angles formed and compare them.




Hoped for student response:









- where the beam of light is that has been reflected.








DPA: The teacher will be walking around and monitoring each group to make sure that they stay on task and to guide them when necessary. Also, the teacher may collect the worksheets if he/she feels necessary to further evaluate the students.




Now that the groups have all completed the flat mirror simulation we will all come back together to discuss.




What did you notice about the reflection of laser beam?


Can someone come up and demonstrate that by drawing a picture?

One of the students will come to the front of the room and draw a picture of these angles and which ones are equal.


Is this true for any angle? Does it matter at all what angle you hold the pointer?


Hoped for student response:



Expected Student Response:



-       The angles were equal.











-       No



DPA Teacher will make sure that not just one person answers all of the questions and check to make sure that these are the answers on their worksheet.



Extend / Elaborate:



WeÕve seen what happens when we shine a light on a flat mirror,  how does the light come back?


Now, look at this setup, we have several mirrors along a curve. What kind of shape is this curve?


What would happen if we shine a light perpendicular to the mirror in the back, how would it bounce off the other ones? Can you predict what would happen?


Have the students perform the experiment: Have them figure out where perpendicular to the flat mirror is first of all, and then figure out how they can be sure they keep the laser pointer straight as they move it back and forth. Three students will be needed per group: one to hold the lp, one to make chalk dust so they can see how the light is coming back, and one to mark the angles coming off of each mirror on a piece of paper.  Are you going to have a worksheet for this investigation?

 Also, you might want to draw a big picture of the multiple flat mirrors on the board to help facilitate discussion.  Or, better yet, have one of the groups put their diagram on the board.

So how does the light behave?




How is that mirror like the setup we just worked with?





Have them do a similar investigation to above with an actual concave mirror, and see that these lines actually do come to a point.



Hoped for response:

It comes back at the same angle



ItÕs a parabola!




At the same angle, but itÕs different for each mirror




Do the experiment ¤












It all comes back to the same point, almost.


It was concave



Hopefully this will lead to a discussion, but they should be able to get to the fact that the curved mirror can be looked at as a lot of little flat mirrors.



Do this, see the focal point of the mirror.


DPA So we found that all of the light on the mirror comes back to one point. Why is this called the Focal point of the mirror?



IÕve seen this lesson in action before, and your plan seems like it should be engaging and keep their attention.  One thing, make sure you have all of the supplies youÕll need.  We wonÕt be bringing anything, so itÕs up to you to make sure what you need is there.  We need to check specifically on laser pointers and curved mirrors.  Also, donÕt forget the flour or talcum powder or whatever you plan on using.  I think UTeach has all of these supplies, youÕll just have to check them out.  Otherwise it looks good.