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== Age ==
{| class="wikitable" style="color:black; background-color:#ddd; margin-left: auto; float:right"
 
| [[Biology]], [[Physics]]:
Elementary School, Middle School, High School
| Parabolic Mirrors, Real and Virtual Images
 
|-
== Format ==
| Grade Range:
 
| [[Elementary School]], [[Middle School]], [[High School]]
Hands-on
|-
| Format:
| [[Hands-on]]
|}


Mirror Mirage is an easy to setup, easy to present demonstration that can be explained in varying levels of depth for the age group. This is a hands-on demo that younger students like to try and figure out, and older students will have a variety of questions about.
== Materials ==
== Materials ==


    Mirage toy, consisting of two parabolic mirrors
* Mirror Mirage
    An object to stick in the mirage toy such as a quarter or dice.  
* Small Objects (Pennies, Small Toys, etc.)


== Safety Precautions ==
== Safety Precautions ==


Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.
Please read the General Safety Precautions section of the [[Demonstration Safety]] page before performing this demonstration.
 
== Preparation ==
 
Simply place the parabolic mirror with the hole in the center on top of the parabolic mirror with no hole.


== Demonstration ==
== Demonstration ==


Insert an object at the focus of the bottom mirror (simply at its center). A real image of the object will then appear at the focus of the second mirror, easily visible from an angle.
# Set out the Mirror Mirage and put an object inside. Let students come up and try to figure out the demonstration. Ask the students questions to guide them to a hypothesis, with questions such as:
 
#* Where do you see the mirage? Where is the actual object?
== What to Say ==
#* Where do you have to stand to see the mirage? Where do you have to stand to have the mirage disappear?
 
#* How do you think it works? What ideas do you have on why this works?
This is a very simple demonstration, but it teaches several interesting concepts in geometrical optics.
 
Reproduce the diagram to help students understand that:
 
Light naturally spreads out spherically (in all directions); for instance, light from the object you place in the mirror will spread out in all directions. In each direction, we say that a "ray" of light is travelling in a straight line. For older audiences, you may want to introduce the photon concept.
 
Rays of light reflect off of shiny surfaces such as mirrors. Reflected rays bounce off the surface at the same angle at which they hit it. In other words, a ray of light that hits a surface head on will be reflected right back and a ray that hits the surface at a large angle will bounce off at a large angle.
 
Parabolic mirrors have a special shape. Objects at their center are in their "focus." Light from these objects heads out in all directions, but, in every direction, they hit the mirror at such an angle that they bounce off going straight up in parallel.
 
Using two parabolic mirrors, we can refocus those parallel rays! The object is placed at the focus of the top mirror. Parallel rays are then sent to the bottom mirror, which focuses those rays at the hole in the top mirror.
 
Some questions you could ask:
 
You can easily see the real image at an angle. How come you don't see two objects (the object and its image) when you look straight down? The reason is that no rays reflected by the mirror will reach your eye when it is positioned directly above the object. Only rays emitted by the object itself can go straight up to your eye! Reproduce the diagrams to help students understand the path the light travels in.
 
== Why It Is ==
 
Light is always reflected from a surface at the same angle at which it is incident. In other words, a ray of light that hits a mirror at 30 degrees relative to the normal (the normal is a line perpendicular to the mirror) will be reflect at 30 degrees on the other side of the normal.


A parabolic mirror is in the shape of a parabola rotated around an axis. This shape is special in that rays emitted at its center (its focus) will always reach the side of the mirror at such an angle that they are reflected straight away from the mirror. In other words, all the light from an object at the focus of the mirror will exit the mirror collimated - the rays will be parallel.
== Why This Works ==


The mirage mirror toy then uses a second parabolic mirror to converge those parallel rays back into a real image at the focus of the second mirror. The object is placed at the focus of the top mirror (which is located at the center of the bottom mirror). The focus of the bottom mirror is where the image is formed: at the hole in the top mirror. The image is real, upright, and at a magnificiation of 1, so it looks exactly like the original object!
===Short Explanation===


== Real Life Examples ==
When we see an image in a flat mirror, the image appears to be "behind", or inside, the mirror. This means that the image we are seeing is a ''Virtual Image'', since it can only be seen inside the mirror and not outside of it. A virtual image is always flipped around, which is why your mirror image will put up it's right hand when you raise your left hand. The Mirror Mirage is making an image appear on top of it. This image on top of the mirror mirage is a ''Real Image'', or an image that is seen outside of the mirror, and not inside of it. Real Images are not flipped around, which is why a penny can be read normally in a real image. The type of image you see in a mirror changes because of the type of mirror used.
* ''Flat Mirrors'' reflect light at the same angle the light hits it. If you drew a light ray going towards a flat mirror, it will bounce off the mirror at the same angle it hits it. You can then draw a dotted line behind the mirror that goes straight into the mirror from the original light ray, which is where you see the image behind the mirror.
* ''Parabolic Mirrors'' reflect light differently if the light ray is either at an angle or straight. If the light ray hits a parabolic mirror at an angle, it will bounce off straight. If the light hits it straight, it will bounce off at an angle towards a center point, no matter where it strikes the mirror.
The Mirror Mirage is made with two parabolic mirrors with a small object inside. When light hits the object inside, it bounces off at an angle and strikes the top mirror. This reflects the angled light, making it bounce off straight at the lower mirror. The reflected light then strikes the lower mirror and bounces off toward the center of the upper mirror. Since there is a hole in the center of the upper mirror, the reflected light will all hit the same spot, creating an image that you can see!


The property of parabolic mirrors that they produce collimated rays from an object at their focus is useful in many applications:
===Full Explanation===


Car headlights - your headlights appear brighter if you can collimate them and focus the light in one direction, rather than spreading out in all directions.
When we see an image in a flat (plane) mirror or an outer-shell (convex) mirror, we are looking at a ''Virtual Image''. Virtual images are images that do not exist in a real locations; that is, you cannot put a screen where the image should appear and have the image on the screen. Virtual images are always "within" the mirror, and reflect the direct image of the object in front of them. This is why your mirror image will lift it's left hand if you lift your right, or move it's right foot if you move your left.
* With a plane mirror, the image that we see is within the mirror itself. If you do the ray tracing for a plane mirror, you would find that the image is always behind the mirror, and that the light striking the mirror always bounces off at the same angle it struck the mirror at.
* With a convex mirror, we see a shrunken image across the mirror's surface. If you do the ray tracing for a convex mirror, you would find that the image appears behind the mirror. This is because the rays from an object in front of a convex mirror will always bounce off the surface angled towards the mirror's focal point, which is ''behind'' the mirror.


Directional speakers - these use special "mirrors" that reflect sound waves out in only one direction (as parallel rays).  
The Mirror Mirage uses two inner-shell (concave) mirrors to create an image above the hole in the upper mirror. This image is a ''Real Image'', or an image that you could project onto a screen and is visible from outside the mirror. A concave mirror reflects light through its focal point, since it has a focal point ''in front'' of the mirror. However, if the object is sitting within the focal point of a concave mirror, then the mirror will reflect the light rays away from the object. The Mirror Mirage uses a certain type of concave mirror called a ''parabolic'' mirror. Parabolic mirrors reflect all incoming light rays towards their focal point. Any angled light rays that strike them will reflect off in straight lines, parallel to each other. These properties of parabolic mirrors are what create the real image we see.
* When incoming light rays bounce off the object inside the mirror mirage, they become angled light rays.
* When the angled light rays strike the upper mirror, they bounce off and become straight, or parallel light rays.
* When the parallel light rays strike the lower mirror, they bounce off and become angled light rays towards the lower mirror's focal point.
* When the angled light rays reach the focal point for the lower mirror, which is above the hole in the top of the upper mirror, they create a real image of the object inside!


The inverse also holds - rays entering the parabolic mirror in parallel (as from an object very far away) converge at the mirror's focus. This is useful in a variety of applications:
== Additional Information ==


Satellite dishes
* This demonstration pairs well with [[Blind Spot]] and [[Ray Tracers]]
Directional microphones - these only focus sound coming from one direction, so you can filter out ambient noise.
Astronomical telescopes - mirrors have a big advantage over lenses that they do not suffer from chromatic aberattion.

Latest revision as of 19:16, 15 August 2016

Biology, Physics: Parabolic Mirrors, Real and Virtual Images
Grade Range: Elementary School, Middle School, High School
Format: Hands-on

Mirror Mirage is an easy to setup, easy to present demonstration that can be explained in varying levels of depth for the age group. This is a hands-on demo that younger students like to try and figure out, and older students will have a variety of questions about.

Materials

  • Mirror Mirage
  • Small Objects (Pennies, Small Toys, etc.)

Safety Precautions

Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.

Demonstration

  1. Set out the Mirror Mirage and put an object inside. Let students come up and try to figure out the demonstration. Ask the students questions to guide them to a hypothesis, with questions such as:
    • Where do you see the mirage? Where is the actual object?
    • Where do you have to stand to see the mirage? Where do you have to stand to have the mirage disappear?
    • How do you think it works? What ideas do you have on why this works?

Why This Works

Short Explanation

When we see an image in a flat mirror, the image appears to be "behind", or inside, the mirror. This means that the image we are seeing is a Virtual Image, since it can only be seen inside the mirror and not outside of it. A virtual image is always flipped around, which is why your mirror image will put up it's right hand when you raise your left hand. The Mirror Mirage is making an image appear on top of it. This image on top of the mirror mirage is a Real Image, or an image that is seen outside of the mirror, and not inside of it. Real Images are not flipped around, which is why a penny can be read normally in a real image. The type of image you see in a mirror changes because of the type of mirror used.

  • Flat Mirrors reflect light at the same angle the light hits it. If you drew a light ray going towards a flat mirror, it will bounce off the mirror at the same angle it hits it. You can then draw a dotted line behind the mirror that goes straight into the mirror from the original light ray, which is where you see the image behind the mirror.
  • Parabolic Mirrors reflect light differently if the light ray is either at an angle or straight. If the light ray hits a parabolic mirror at an angle, it will bounce off straight. If the light hits it straight, it will bounce off at an angle towards a center point, no matter where it strikes the mirror.

The Mirror Mirage is made with two parabolic mirrors with a small object inside. When light hits the object inside, it bounces off at an angle and strikes the top mirror. This reflects the angled light, making it bounce off straight at the lower mirror. The reflected light then strikes the lower mirror and bounces off toward the center of the upper mirror. Since there is a hole in the center of the upper mirror, the reflected light will all hit the same spot, creating an image that you can see!

Full Explanation

When we see an image in a flat (plane) mirror or an outer-shell (convex) mirror, we are looking at a Virtual Image. Virtual images are images that do not exist in a real locations; that is, you cannot put a screen where the image should appear and have the image on the screen. Virtual images are always "within" the mirror, and reflect the direct image of the object in front of them. This is why your mirror image will lift it's left hand if you lift your right, or move it's right foot if you move your left.

  • With a plane mirror, the image that we see is within the mirror itself. If you do the ray tracing for a plane mirror, you would find that the image is always behind the mirror, and that the light striking the mirror always bounces off at the same angle it struck the mirror at.
  • With a convex mirror, we see a shrunken image across the mirror's surface. If you do the ray tracing for a convex mirror, you would find that the image appears behind the mirror. This is because the rays from an object in front of a convex mirror will always bounce off the surface angled towards the mirror's focal point, which is behind the mirror.

The Mirror Mirage uses two inner-shell (concave) mirrors to create an image above the hole in the upper mirror. This image is a Real Image, or an image that you could project onto a screen and is visible from outside the mirror. A concave mirror reflects light through its focal point, since it has a focal point in front of the mirror. However, if the object is sitting within the focal point of a concave mirror, then the mirror will reflect the light rays away from the object. The Mirror Mirage uses a certain type of concave mirror called a parabolic mirror. Parabolic mirrors reflect all incoming light rays towards their focal point. Any angled light rays that strike them will reflect off in straight lines, parallel to each other. These properties of parabolic mirrors are what create the real image we see.

  • When incoming light rays bounce off the object inside the mirror mirage, they become angled light rays.
  • When the angled light rays strike the upper mirror, they bounce off and become straight, or parallel light rays.
  • When the parallel light rays strike the lower mirror, they bounce off and become angled light rays towards the lower mirror's focal point.
  • When the angled light rays reach the focal point for the lower mirror, which is above the hole in the top of the upper mirror, they create a real image of the object inside!

Additional Information

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