Key concepts
Reflection
Mirrors
Light
Infinity
Introduction
Can you imagine a bouncy ball that could bounce back and forth between two walls, infinitely—that is, forever? Wouldn't that be amazing?
What if, instead of a ball, light was bouncing between two walls, which were both covered in mirrors? Do you think that could bounce back and forth forever? Imagine each light bounce added one reflection of an object in the mirror—for example, you! Would it look like there were an infinite number of "yous"? Perhaps you have noticed something like this in a fun house or a room with multiple mirrors. But can we create an infinite number of reflections?
Try this activity, and be amazed by the many images mirrors can create! Before you know it, you might be inspired to create some real works of beauty.
Background
The reason why you see objects is that light hitting these objects reflects back into your eyes (and then is perceived and represented as those objects by your brain). But what happens if light coming from an object hits a flat mirror and is reflected (or bounced back) before it hits your eyes? Your brain, being unaware that the light was reflected, will reconstruct an image with the information it received, assuming the light traveled on a straight path from the object to the eye. The result is what you see in the mirror: an object that looks similar, but appears to be placed behind the mirror. We call this a reflection of the object. It is an optical illusion, a virtual image of a real object.
Mirrors, being shiny, reflect almost all the light hitting their surface. In addition, they have a very smooth surface, causing light to reflect in an orderly way. This allows your brain to reconstruct a clear image. High-quality mirrors reflect light especially well, but even high-quality mirrors absorb and scatter a small fraction of the light hitting them. As a result, a reflection is always a little dimmer and slightly less crisp than the image made with the same light reaching the eye directly.
Now, what image would your brain create if light reflected several times before hitting your eyes? Find out in this activity, and discover ways to create beautiful images!
Materials
- Large wall mirror
- Small mirror with thin or no border
- Optional: trinkets or other small, colorful objects
Preparation
- Collect your small mirror along with any colorful objects you might wish to use.
Procedure
- Stand in front of a large wall mirror, looking into it. Make sure there are about twenty centimeters between you and the mirror.
- Hold a small mirror just under your eyes, so that the reflective surfaces of the mirrors are facing each other.
- Place your index finger between the two mirrors, with your fingernail facing the large wall mirror. Do you see the reflection of your finger in the wall mirror? Do you see the nail, the skin-side of your fingertip or both?
- While looking at your index finger in the large wall mirror, briefly remove the small mirror and then put it back again. Is what you see in the wall mirror different when you hold the small mirror in place? How is it different? Can you explain what you see?
- Tilt the small mirror a little and watch what happens to the reflection(s). Is there a particular position in which you can see very few reflections? What is the fewest number of reflections you see? And is there a position in which you can see a lot of reflections? How many can you see? Do they seem to go on forever?
- Hold your small mirror in a position that allows you to see a lot of reflections. Do you get the impression that reflections are farther and farther away? How are reflections that appear to be far away different from the reflections that appear to be close? Do they get dimmer, less sharp?
- Take a step back from the wall mirror. How does your image change if there is more space between the two mirrors? Now get very close to the wall mirror. Which distance results in more reflections?
- Extra: Hold your small mirror vertically against the large mirror, at a 90-degree angle. It might help to rest the small mirror on a flat surface placed next to the wall mirror. Place your fingertip or a small, colorful object somewhere in the 90-degree angle between the two reflective surfaces. Level your eyes with the small mirror and look toward the vertical line where the two mirrors touch, so that you see the wall mirror and small mirror at the same time. How many reflections do you see this time? Reduce the angle between the two reflective surfaces to close to 60 degrees. How many reflections do you see this time? Repeat with angles of 45 degrees and 30 degrees. Do you see a pattern? Can you explain what you observe?
- Extra: Use the reflection properties you just explored and some other colorful objects, such as beads or other trinkets, to make a work of beauty. Then try to draw your creation and note the symmetry.
- Extra: If you have a kaleidoscope, look through it. Can you explain how it works? How many mirrors do you think your kaleidoscope contains? At what angle do you think the mirrors are placed? If you are allowed, you could try taking the kaleidoscope apart (knowing that you might not be able to return it to its original condition). (You can also use a kaleidoscope-making kit.) If you can take it apart, do so and explore the inside of the kaleidoscope. Then put it back together, changing one item at a time, and watch the result. How does the resulting image change if you cover one mirror with black construction paper? How does it look when you cover more mirrors? What happens when you leave the eyepiece (the piece that you look through) off? Try removing the piece containing the little trinkets and cover the tube with a colorful drawing. Do you see anything? When you lift the tube a little, does an image appear? Why do you think this happens?
Observations and results
Did you get the impression of an infinite series of fingers?
When you looked at your finger in the wall mirror, you saw a reflection created by light bouncing off your finger hitting the mirror surface and reflecting back in your eyes. The finger appeared to be behind the mirror. Did you notice that you saw the side of your finger facing the mirror, the nail side?
When you added the small mirror, you allowed light that was reflected on the wall mirror to hit the small mirror. This created a reflection of a reflection. You could not see this reflection of a reflection, as it was created behind the small mirror while you were looking in the wall mirror. However, you could observe a reflection of it as light from the reflection of a reflection hit the wall mirror and bounced back. A small fraction of this bounced-back light reached your eye, allowing you to see the reflection of a reflection of a reflection. Each bounce of light, back and forth, will add another reflection of a reflection to the count. The reflections in your activity probably looked as if they went on and on an infinite number of times (although given what we know about mirrors scattering and absorbing some of the light each time, we know that the number we could see is actually finite).
Did you notice that the small mirror also reflected the skin-side of your fingertip, so you could see the skin-side of your fingertip as a reflection of a reflection in the wall mirror?
Did your reflections get dimmer and less sharp as they appeared to be farther away? With each reflection, the mirror absorbs and scatters a tiny fraction of the light. After several reflections, the image fades out and you are no longer able to distinguish reflections. Increasing the distance between the mirrors will accelerate this fading, and thus, result in fewer visible reflections.
You might have been able to block reflected light with your finger, preventing any further reflections to occur. In this case, you only saw one reflection showing your fingernail.
When you tried the extra activity, holding one mirror against another at a 90-degree angle or at a smaller angle, you should have seen a circle of images appear. When the angle is an even division of 360 degrees (a full circle) you will get a beautiful pattern of reflections. Kaleidoscopes use both—the seemingly infinite reflections and the angled mirrors—to create beautiful images.
More to explore
The Reflection of Light, from Optics 4 Kids
Sight-Line Science: Candle in the Mirror, from Scientific American
Kaleidoscope Science, from Science Buddies
This activity brought to you in partnership with Science Buddies
