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Sight-Line Science: Candle in the Mirror

Learn about the perception of light and reflections with this illuminating activity



George Retseck

Key Concepts
Images
Virtual images
Transmission
Reflection
Optical illusions

Introduction
Ever wonder why mirrors can make a room feel so much larger than it really is? Mirrors are so common that we rarely think of how intriguing they really are. They somehow take the three-dimensional world we live in and reproduce it so perfectly that we are subconsciously fooled into perceiving that another 3-D space exists beyond the mirror's surface. We know, of course, that this is just an illusion. But how do our brains get tricked by reflections? A simple activity with a candle will demonstrate some of the ways our brains get fooled.

Background
Visible objects fall into two general categories: luminous objects (such as your computer monitor) that emit their own light and illuminated objects (your keyboard, for instance) that reflect light emitted by other sources. Your eyes see objects by detecting either emitted or reflected light, and under most circumstances that light has to reach your eyes by traveling in a straight line. In other words, you usually need an unobstructed line of sight between yourself and any object you intend to view.

Mirrors and other highly reflective surfaces present a common exception to this rule. Most mirrors are made by layering a reflective metal backing behind a sheet of clear glass. Because the backing is so reflective, it provides an "image" (an apparent reproduction of visible objects). This sort of image can be deceptive because its apparent location doesn't match up with the actual, physical location of the object creating that image. Consider an image of a chair that you see in a mirror versus the real chair that you can see in the actual room if you have a direct line of sight to it.

So-called plane mirrors are even more mind-bending because the images they create are known as virtual images. These images are created when rays of light emitted or reflected by an object seem to converge somewhere behind the mirror. And some surfaces can act as mirrors without the special reflective silver backing. Sound confusing? Don't worry—we'll get to see all of these concepts in action. In this activity we will use a transparent surface to see how it both transmits and reflects light. This will allow us to learn about light by interacting with the real, visible space behind the surface where virtual images appear.

Materials
• Two tea candles
• Matches or a lighter to light the candles—along with adult assistance or supervision for doing so
• CD jewel case with one glossy, transparent surface
• Dark surface (you can create one by laying a black T-shirt flat on a table)
• Identical household items (paper clips, etcetera) in pairs (optional)

Preparation
• Find a room that can be made dark by turning the lights off.
• With the lights still on, prepare your dark surface.
• Remove the album art from the CD case if it has any so that the hinged front cover is now a transparent panel.
• Use caution (and adult assistance/supervision) when lighting the candles and working with lit ones.

Procedure
•   Working on your dark surface with the lights still on, stand the CD case on its bottom edge. Open the CD case at a 90-degree angle.
• Take one unlit tea candle and place it on the side of the CD case's transparent panel nearest you. Make sure you can see its reflection. Where in space does the reflected image appear to be? Why do you think this is?
• Take the other unlit tea candle and place it on the other side of the transparent panel. Adjust the position of the second candle so that it's in the same apparent location as the reflection of your first tea candle.
• Move your head around while looking at the second tea candle through the CD case. Can you see the reflection of the first tea candle? Does the reflection ever not appear to be transposed over the actual location of the second tea candle?
• Now, take a look at the orientation of the CD case and the tea candles from above. Note the position of the two tea candles. What's special about their positions relative to the CD case's transparent panel?
• Nudge the first tea candle away from the CD case slightly. What happens to the reflection?
• Nudge the second tea candle to match the apparent location of the first tea candle's reflection.
• Turn off the lights and light the tea candle located on the side of the transparent panel nearest you. Look at the reflected image. What do you see?
Extra: If you want to turn this into a great illusion for another observer, do it again, just make sure they don't see how you set it up beforehand! Your observer should observe the illusion from the side of the transparent pane with the lit candle. Touch the wick of the unlit candle on the opposite side of the panel with your hand. Your observer may be surprised that you don't burn yourself!
Extra: Without looking at the reflections directly, try to make a diorama with household objects such as paper clips on both sides of the CD case so that the reflections match perfectly. It may be easier if you make it without the lit candles. Hint: The tea candles demonstrated how far away from a mirror two objects need to be to match each other's reflections.

Observations and results
When you lit one of the tea candles and looked through the CD case from the first side, your eyes detected both the light coming directly from the unlit candle and the reflected light emitted by the lit candle's flame. This reflected light creates a virtual image of the flame appearing behind the transparent panel. We interpret virtual images as appearing behind the reflective surface that creates them because our brains naturally interpret light from visible objects as traveling to our eyes in straight lines. Light from the lit candle is reflected by the "mirror," but our brain's tendency is to "unfold" that light , placing a virtual image of the lit candle behind the transparent plastic (and apparently right on top of the actual location of our unlit candle). Light from the lit candle doesn't actually travel to your eye from behind the transparent plastic (as we know the light is only coming from the candle on the front side of the plastic)—but it looks like it does!

This overlapping can be attributed to a property of light known as superposition, meaning that visible light can represent the combination of many sources simultaneously. The light rays coming from the unlit candle and the reflected light rays of the lit candle are superposed on top of one another. In situations where one source of light is much stronger than another, only the stronger source is clearly visible. This helps explain why things that seem bright in the dark (like stars) don't seem that bright or are invisible when the sun is out.

Every surface exhibits some transmission and some reflection. For very transparent substances like glass, only about 5 percent of the light that hits it is reflected. The remaining 95 percent is transmitted—it passes right on through. When you light one of the tea candles in a dark room, some of its light is transmitted through the CD case, and some of it is reflected. Even though the case can only reflect something like 5 percent of the flame's light, that reflected light is more than enough to create an illusion of two flames as opposed to one flame and one unlit wick.

This same basic manipulation of virtual images has been utilized on stage for decades as a technique known as Pepper's ghost . It's how Disneyland creates The Haunted Mansion's ghouls . It's so compelling that the setup of the illusion has changed very little since its introduction in the mid 19th century!

More to Explore
Candle Illusion, from the University of California, Los Angeles, Department of Physics and Astronomy
Candle Illusion, from YouTube
The Candle illusion: Real and Virtual Images, from Education.com
Make a Mind-Bending Illusion!, from Education.com


This activity brought to you in partnership with Education.com
Education.com

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