This is an interesting question that gets at some fundamental physics principles about light, reflection and vision. To understand why a mirror can “see” behind a piece of paper held up in front of it, we need to break this scenario down bit by bit and consider what is happening at each step.
First and foremost, we know that human vision works by light entering our eyes. The light reflects off objects around us, and some of that reflected light reaches our eyes, allowing us to see the objects. A similar principle is at work with mirrors. Light bounces off objects in front of the mirror and then some of that reflected light bounces off the mirror to reach our eyes.
So if we hold up a piece of paper in front of a mirror, light bouncing off the paper reaches the mirror. But light is also bouncing off objects behind the piece of paper. Some of this rear light passes around the edges of the paper and reaches the mirror. The mirror then reflects both sources of light – from the paper and from behind it – to our eyes.
How Mirrors Reflect Light
To really understand what’s going on, we need to take a closer look at how mirrors reflect light. Mirrors have a smooth, shiny metallic surface that reflects almost all of the light that hits it. When light rays strike a mirror, they bounce off at the same angle that they hit the mirror surface. This is known as “specular reflection” – the reflection is a mirror image.
The law of reflection states that the angle of incidence (the angle at which a light ray hits the mirror) is equal to the angle of reflection (the angle at which it bounces off). So if a light ray hits a mirror at a 30 degree angle, it will reflect off at a 30 degree angle relative to the mirror’s surface.
This predictable reflection is what enables mirrors to produce clear images. All the light rays bouncing off points on an object in front of the mirror reflect to our eyes, producing an image that accurately reproduces the original object’s appearance, except reversed left-to-right.
Light Rays From Behind the Paper
Now let’s consider what happens when we put a piece of paper in front of a mirror. Light rays strike the paper from the ambient environment. Some of this light is absorbed by the paper, some is transmitted through it, and some is reflected off its surface.
The reflected light bounces off the mirror just like it would if the paper was not there. But meanwhile, light rays from objects behind the paper reach the mirror’s surface unobstructed. These rays also reflect off the mirror according to the law of reflection.
Our eyes receive light rays reflecting off the mirror from both sources – the paper and the objects behind it. As a result, we see an image that includes the paper and a reflection of objects behind it. The mirror allows us to “see” behind the paper.
Why We Can’t See Behind Opaque Objects
This effect of seeing behind obstructions only works with thin, transparent or translucent objects like a piece of paper. If the obstruction was an opaque object like a wooden block, then light from behind it would be blocked.
With an opaque object, light is either absorbed by the object or reflected off its surface – very little light is transmitted through it. So there are no light rays reaching the mirror from behind the opaque obstruction. The mirror can only reflect light from the obstruction itself and objects in front of it.
Paper is thin enough, and translucent enough, that some light passes through it and reaches the mirror. This allows us to get a reflection from objects behind even though the paper is physically blocking the view. A thicker or more opaque obstruction would prevent light passing through, cutting off the view of anything behind it.
The Science of Reflection
The reflection of light follows the law of reflection and depends on having a smooth surface, like a mirror, to specularly reflect light to our eyes. Here are some key principles that explain how mirrors enable seeing behind obstructions:
- Light bounces off objects in all directions.
- When light hits a mirror, it reflects at the same angle it hit the surface.
- Light from behind a translucent object like paper reaches the mirror.
- The mirror reflects this rear light to our eyes along with light off the paper itself.
- Our eyes receive light from both sources, allowing us to see behind the obstruction.
So in summary, mirrors don’t actually “see” anything – they simply reflect light to our eyes. By taking advantage of the way light propagation works, a mirror gives the illusion of seeing behind an obstruction that is transparent enough to allow some rear light through.
Everyday Examples
The classic physics demonstration of seeing behind an obstruction involves a mirror and a piece of paper with writing on one side. When the paper is held up to the mirror with the writing facing the mirror, the writing appears reversed but legible in the reflection. This works because light reflects off the paper to the mirror, and light also passes through the paper to reflect off the mirror from the back side.
We encounter this effect in everyday situations too, although we often don’t notice it. For example, if you have a glass shower door, you can see yourself behind it in the bathroom mirror. Or if you are sitting in a car with the side rear-view mirrors, you can see the road behind you even while large blind spots are blocked from the driver’s view.
Any time you look in a mirror and notice you can see objects or spaces behind a see-through barrier, it is thanks to the same physical principles of reflection at work.
A Tricky Physics Problem
The classic mirror puzzle of seeing behind an obstruction has stumped many noted scientists over the years. René Descartes reportedly hypothesized that light could bend around large obstacles to reach the mirror from behind. But this was incorrect – light only reflects at predictable angles according to the law of reflection.
In 1660, the astronomer Johannes Kepler recognized that it was the transparency of the obstruction that allowed seeing behind it in a mirror. But he incorrectly thought this was due to light bending through the paper. It took some time before the real mechanism was properly explained in terms of simple reflection.
This puzzle shows that our everyday intuition about how light and vision work can sometimes break down. When we take a closer scientific look by considering how light propagates and reflects, we gain a deeper understanding of the optical illusions that can fool our visual perception.
The Properties of Light
The fact that mirrors can reflect light from behind transparent obstructions points to some key properties of light that are worth reviewing:
- Light travels in straight lines until it hits an object or optical interface.
- Light can be absorbed, transmitted through, or reflected by an object.
- Reflection obeys the law of reflection – angle of incidence equals angle of reflection.
- Smooth surfaces like mirrors result in specular reflection – direct bounce light.
- Light scattering off rough surfaces results in diffuse reflection in many directions.
Understanding these core principles of light propagation, interaction and reflection provide the foundation for explaining mirror illusions as well as many other optical effects we encounter. Light is a complex phenomenon, but its behavior follows fundamental rules that can be modeled and predicted.
Everyday Optical Effects
The mirror trick of seeing behind an obstruction is just one of many optical illusions and visual effects that we experience regularly. Here are some other common examples:
- Reflections in windows and eyeglasses – These smooth surfaces produce reflections from light bouncing off them just like a mirror.
- Seeing around corners with mirrors – Mirrors placed at 45 degree angles allow seeing around 90 degree corners by reflecting light to our eyes.
- Objects under water appearing shifted – The different refractive indices of air and water cause light to bend, shifting images of underwater objects.
- Heat haze or mirages – Temperature variations in the air bend light rays, producing optical illusions like “puddles” on hot roads.
In all these cases, light is being reflected, refracted or diffracted in complex ways that can trick our visual perception. Physics can provide an explanation of the mechanisms behind such optical effects in our everyday world.
Applications and Technologies
Understanding how mirrors work has enabled many practical applications and technologies including:
- Telescopes – Using curved mirrors to collect and focus light for astronomical observations.
- Microscopes – Reflecting light from specimens through compound lens systems.
- Lasers – Bouncing light between mirrors to amplify it in a coherent beam.
- Periscopes – Using mirrors to see around obstructions and corners.
- Rear-view mirrors – Allowing drivers to see behind them.
- Mirror mazes – Creating confusing reflected optical illusions.
Without reflections from smooth mirror surfaces, many optical and imaging systems would not be possible. Applications from simple bathroom mirrors to sophisticated telescopes all rely on the same principles of reflective optics.
Conclusion
In conclusion, mirrors give the appearance of seeing behind obstructions because they reflect light reaching them from both the obstruction and objects behind it. The transparency of thin objects like paper allows enough rear light to pass through and reflect off the mirror. Opaque objects block this rear light, preventing seeing behind them in a mirror.
This classic physics puzzle reveals phenomena about light propagation, reflection and visual perception. As with many optical illusions and effects, taking a scientific approach helps explain the mechanisms involved and provides deeper insight. So next time you look in a mirror and notice reflections from behind partially transparent surfaces, you’ll know it’s thanks to the intrinsic properties of light itself.
