Physics lab assignments

macaulay.cuny.edu

Kent State University

Act IV Lab 11

Exploring reflections

The idea: You see reflections in mirrors so often that you probably think you understand them. And you probably do – up to a point. But there are some interesting consequences and complications that you will explore in this fun and easy lab.

What you’ll learn: By the end of this lab you will understand 1) how to discriminate real images from virtual images, 2) more about geometrical optics with flat mirrors, and 3) how real images are formed from concave reflecting surfaces.

What you’ll need: Post-It notes or a roll of masking tape Ruler

Scissors or X-Acto knife Flat Albert image from the lab packet Wall mirror

Large, shiny spoon Willing assistant

What you’ll do: 1) Use scissors or an X-Acto type knife to cut an eye hole in the cartoon

image of Albert Einstein in your lab packet.

2) Grab some Post-It notes and a friend and go to a room with a normal-sized wall-mounted mirror. Any bedroom or bathroom mirror will d0. Give the Post-its to your friend to hold.

3) Stand about six feet or so away from the mirror, hold the Einstein image in front of your face, and with one eye look through the eye hole so that you see his cartoon image in the mirror. Ask your friend to stick a Post-It note on the mirror where you say to put it, so that it looks to

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you like Einstein is standing on the note. It’s fun to watch this, as people will almost always assume you will tell them to stick the note down low on the mirror. In fact, the note will have to move up surprisingly high. Give the person directions, such as “A little higher, over to the left,” and so on, until it looks to you that Flat Albert is standing right on the note.

Do the same with another Post-It note on the mirror so that it seems to be resting right on top of Flat Albert’s head, as if the two notes were clamps that were holding him in place.

4) Use a ruler to measure the distance between the Post-Its on the mirror. It’s always better in physics to use centimeters! Record that distance, which is the height of Albert’s image, on the Report Sheet.

5) Now use the ruler again to measure on the piece of cardstock the actual height of Flat Albert, from his toes to the top of his pointy hair. Record his height on the Report Sheet as well.

How do those last two measurements compare – Flat Albert’s actual height, and the distance between the Post-It notes on the mirror? Record your observation on the Report Sheet.

6) Go back to where you were standing and look through Albert’s eye hole again. Take a few steps forward and backward, as far as you can move in the room, and observe Albert’s image and the Post-Its on the mirror. As you move, does Albert still fit between the notes? Record your observation on the Report Sheet.

What do you conclude, in general, about the size of a mirror needed to see your whole self? Record your conclusion on the Report Sheet.

Most everyone, in my experience, assumes that the size of the mirror you need to see your whole self depends on your distance from the mirror. That misconception stems from the fact that we are not two- dimensional objects, and that our eyes are in front of the front-to back midline of our bodies.

If you look at yourself very closely, as in a small mirror in a compact, you see only a small part of yourself. If you pull it back to arm’s length, you will see quite a bit more of yourself. But if someone held the mirror for you and you backed up to two arm-lengths away, the amount of yourself

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you could see would hardly change at all. This is another example of where our routine experience fails us if we apply that experience to more extreme circumstances.

7) You notice, of course that as you look at yourself in a mirror, you are right side up. That means that your image is virtual. It may or may not be virtuous; that’s not for me to say! But seriously, a virtual image is one that cannot be projected onto a screen or surface, and it depends on one’s point of view. After all, while you were looking at yourself in the mirror, you had to direct your assistant where to move to place the notes, because he or she could not see that same image that you did. A virtual image is not on the mirror, but appears to be behind it.

We can explore virtual images further and their opposite, real images, using something as simple and common as a spoon. (I hope that you have noticed this on your own way before this.) Real images are always inverted and can be projected.

Look at yourself in the back of a shiny spoon. Write a few words on the Report Sheet to describe your image, and whether it is virtual or real.

8) Now flip the spoon over and look its bowl. Describe briefly on the Report Sheet what you notice about that image when you look into the bowl of a spoon. Is it virtual or real?

Web extension

Look up information on the web about Cassegranian or catadioptric telescopes – both terms mean approximately the same thing. Tell me how they use both concave and convex mirrors.

Be sure to include the basic web addresses where you found your answers.

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