Mirror Mirror

Mirror Mirror

Name: Allyson Berglund

Date of Lesson: March 2005

Length of Lesson: 90 mins.

Description of Class: Algebra 2 Students

Source of Lesson:

http://stardate.org/pdfs/teachers/plans/mirror_mirror_S.pdf

http://www.glenbrook.k12.il.us/gbssci/phys/Class/refln/u13l3f.html

Stephen Alwon, Lisa Fefferman, Nicole Williams

TEKS addressed:

(2) Scientific processes. The student uses scientific methods during field and laboratory investigations. The student is expected to:

(A) plan and implement investigative procedures including asking questions, formulating testable hypotheses, and selecting equipment and technology;

(B) collect data and make measurements with precision;

(D) communicate valid conclusions.

(5) Science concepts. The student knows the effects of waves on everyday life. The student is expected to:

(B) demonstrate wave interactions including interference, polarization, reflection, refraction, and resonance within various materials;

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. Performance or learner outcomes

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

· Pen and pencils

· Laser pointers

· 3 binder clips

· Small flat mirrors

· Convex mirrors

· Protractors

· Ruler

· Graph paper

· Flour

· Chalk eraser

IV. Supplementary materials, handouts

· Flat Mirror Worksheet

Five-E Organization

Teacher Does

Student Does

Engage:

What mirrors have you encountered today?
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 weren’t standing directly in front of the mirror?
I need two volunteers. (Place the students in the angle in front of a mirror 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.

bathroom mirror, rear view mirror, side mirror on car
To see a reflection of yourself.
To see a copy of an image.
Straight in front of it.

You won’t be able to see yourself.

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

Evaluate

Teacher will make sure that no student is dominating the discussion and that everyone is paying attention to the demonstration. Make sure every student realizes that you can look into a mirror and see reflections, but not see yourself.

Explore:

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.

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

Pass out Flat mirror simulation worksheet (per student), one flat mirror (per group, 1 sheet of graph paper (per group), protractor and ruler(per group) flour and chalk eraser. Ask groups begin working on the simulation together.

1. Using a pen, draw a straight line on your graph paper. Label it “mirror”.

2. Mark a central point on the line where the graph paper’s grid intersects your line, and label it “reflection point”.

3. Using your pen and protractor, draw the normal line. The normal should be

perpendicular to the “mirror” line. Label the normal line “normal”.

4. Using the protractor and pencil, draw an incoming angle to your “reflection point”. You may choose an angle between 20 and 50 degrees from the normal. Label this new line “incident ray”.

5. Using the protractor, measure the angle between the “incident ray” and the “normal”. Write the angle measurement on your graph paper.

6. What do you think will happen when the laser’s incident ray strikes the mirror? Draw a dotted line to represent your “hypothesis ray”.

7. Place the flat mirror so the front of the mirror is on the “mirror” line.

8. Turn the laser pointer on. Never look directly at the laser beam or allow it to shine in someone's eyes. Use the binder clip to maintain the laser pointer in the ON position. If you need help seeing the laser beam, use flour and chalk erasers. Make sure to bring the flour or chalk or talcum powder. Make sure that the laser beam follows the “incident”

line, and clearly shows a reflected beam on the other side of the normal line.

9. Mark the reflected laser ray in several places (at least 3) along the laser’s reflected beam. Do not disturb the laser pointer’s position. Turn the laser pointer off when you are done.

10. Using your protractor, line up the reflected ray’s marks, and draw a straight line that intersects the “mirror” line. Measure the angle between the reflected ray and the normal. What do you notice?

Hoped for student response:

-nothing

-no

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

Evaluate

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 (s)he feels necessary to further evaluate the students.

Explain:

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

Questions:

1. What did you notice about the reflection of laser beam?

2. Did the law of reflection work with the flat mirror? Explain.

3. What do you think happened to the incident laser beam inside the glass based on your observation data?

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.

Yes

The light is reflected back at the angle (explain their drawing)

Yes

No, focus point will always be the same

Evaluate

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:

Questions:

We’ve 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 (we need to locate the laser pointers on Monday and check them out to you and the others teaching this lesson – remind me) straight as they move it back and forth. Three students will be needed per group: one to move the lp (the laser pointer will be secured with clips), 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.

So how does the light behave?

You might want to draw a large diagram of the different mirrors and the line of reflection for each of the mirrors to help with the discussion. Or you could have one group draw their results for the class to look at.

What did the big mirror that you put in the telescope look like?

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

It’s a parabola!

At the same angle, but it’s different for each mirror

Do the experiment ß

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 ß, see the focal point of the mirror.

Evaluate

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?

Flat Mirror Worksheet

Under the directions draw a picture replicating what you see.