Your Mirror Image Isn’t an Illusion! The Hidden Physics of Light Behind Virtual Images
I’m Science Trainer Ken Kuwako. Every day is an experiment.
The “virtual images” we see when we look into mirrors or through lenses.
Are they just illusions? Or is our brain somehow fooling us? In fact, this phenomenon, which is often misunderstood, turns out to be surprisingly deep when you try to explain it scientifically and accurately.
The Trap Hidden in the Term “Virtual Image”
When explaining virtual images, I sometimes find myself saying things like, “You can see something that isn’t really there” or “It’s all in your mind.” But the more I thought about it, the more I felt that this explanation wasn’t quite right. So I started wondering: what is the most accurate way to explain it?
If you look up “virtual image” in Kotobank, you’ll find the following explanation.
In general, we see an object because light rays coming from that object enter our eyes and form an image of the object on the retina. However, even when no physical object is actually present, if light enters the eye in the same way as it would if an object were present, we can see that nonexistent object. However, it is not correct to think of this as a hallucination or optical illusion. In principle, there is no way to distinguish between light rays coming from a real object and light rays associated with a virtual image. The face we see in a mirror and the car following us that we see in a rearview mirror are everyday examples of virtual images. — Kazuo Miyake Kotobank
The key point in this explanation is the sentence: “A virtual image is not an optical illusion.”
Using words like “assumption” or “illusion” can make it sound as though our brains are simply creating a phantom out of thin air. But that’s not what is happening. The light entering our eyes is completely real. The only thing that differs is where that light appears to be coming from compared with the actual position of the object.
Seeing an object in the first place means interpreting the light that reaches the retina (which is converted into electrical signals, sent to the brain, and used by the brain to create an image). This is true whether we are looking at a real image or a virtual image. A digital camera works in much the same way. So it isn’t really a matter of “being fooled,” is it?
And this is where things get really interesting. Our eyes and digital cameras share essentially the same basic role: they are “light sensors” that receive light and convert it into an image. Just as a camera sensor doesn’t take a picture based on some preconceived notion, our eyes simply process the incoming light according to the information they receive. In that sense, seeing a virtual image isn’t a mistake made by the brain. It is the result of a highly precise optical system—the eye—correctly processing the information available to it.
Real Images vs. Virtual Images: What’s the Crucial Difference?
When it comes to the difference between real and virtual images, a real image forms where light rays actually converge. If you place a screen at that location, the light scatters from the screen, allowing you to see the image from different directions.
The image projected onto a movie-theater screen is a classic example of a real image. The light passing through the lens actually converges at the screen, which acts as a physical “receiver” for the image. Anyone can see the same image, and it remains in place even when you change your viewing angle.
With a virtual image, on the other hand, the light rays do not actually converge at the apparent location of the image. There is no physical light sitting behind the mirror or on the other side of the lens.
The figure comes from the virtual-image simulation at See. Move. Think.
URL：https://理科sim.jp/
This virtual-image simulation is pretty fascinating.
If you place a screen at the apparent location of the image, nothing will be projected onto it. What we are seeing is simply positional information created by the way light travels—the light appears to be coming from that location, even though it isn’t actually coming from there.
That distinction—whether the light rays actually converge at the image location—is the crucial difference between a real image and a virtual image.
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