Man holding a paper with egg on w mirror

I found this on social media. Although it was posted for fun, this is a valid question: How does the mirror know what’s behind the paper?

Show me how light reflected from the egg reaches the mirror.

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    $\begingroup$ Why is a drawing required to be correct? $\endgroup$
    – Jon Custer
    Commented May 11 at 18:51
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    $\begingroup$ I agree that an actual photo would be preferable $\endgroup$
    – Dale
    Commented May 11 at 19:03
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    $\begingroup$ I think a useful question is: what would you expect to see, if not the egg? $\endgroup$ Commented May 11 at 19:27
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    $\begingroup$ assets.iflscience.com/assets/articleNo/68363/aImg/67037/… notice that you cannot see Mario’s feet in the mirror $\endgroup$
    – Dale
    Commented May 11 at 20:22
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    $\begingroup$ This is a terrible question. How does the mirror know what’s behind the paper? What even is the concept/definition of "knowing" here? There is no physics whatsoever in this sentence. You are essentially asking how a mirror works, which by definition is a surface that reflects light perfectly. All the answers are simply restating this definition. $\endgroup$ Commented May 12 at 13:44

4 Answers 4


Perhaps a diagram would help? Excuse my poor artistry:

enter image description here

Clearly, you can see that there is a trajectory a ray can take which starts at the top of the egg, hits the mirror, and then intersects your eye. However, when you follow the ray which hits your eye backwards, you can see that it leads to the mirror image of the real object. This mirror image is simply the set of rays which terminate on the object and your eye, but which reflect off the mirror in between.


The other answers are quite right, of course, but I thought it might be helpful to know the comic has a second page in which Aki Hayakawa explains how mirrors work. This prompts a rather... crude response from Power. enter image description here


Is it not just a matter of at which angle you are and how is constructed the reflection?

The reflection is just the brain trying to reconstruct the position of the object by continuing the ray of light without knowing that is has been reflected. So if you are looking very sideway, only what you can see will be reflected, as in the drawing here:

enter image description here

Where the image on the left is the true one and the image on the right is the one reconstructed from the light that comes into your eyes.


The interesting thing here is how our eyes perceive light.Light has different wavelength,for this context, consider you give all different colour of light we perceive a specific code, we will call it wavelength, as the colour of the light is spectrum, we have all different wavelengths for it.

Now visible light has wavelengths(in nm) from 380 to 700. 380 corresponds to violet and 700 corresponds to red.

Every object has its own physical property to absorb some of these wavelength and reflect some of these wavelength. In this way, When White light(Approximately containing of VIBGYOR colors) fall onto Vibrant red apple, the apple absorbed VIBGYO color wavelengths and Reflected back Red wavelength light.

Now it is important how Our brain Perceives the reflected light.When a camera lens gets light reflected from different objects, It simply transforms it into 2-D image.The same is the case with our brain, But our brain also has depth perception as a result of 2-eyes,that has its own explanation not relevant here.

What image is formed on retina that gets the light from from your eyes, It is always perceived as if the light was always travelling in a straight line with no reflection by our brain, Because note that the rode and cone cells on retina only sends data of light intensity and wavelength(color) of light received

mirror is a fascinating object. It is silvered glass, which reflects most of the wavelengths of light that strikes on it.

The normal path of light for our vision is

1.(Sun->Apple(absorption of all colors other than red)->eyes)

But when the light adds one more path of reflects in its way,

2.(Sun->Apple(absorption of all colors other than red)->MIRROR->eyes)

then our brain interprets it as if the object apple is at the position of mirror, which means,mirror itself has no relation to apple other than just reflecting its light one more time.

This confusion of our brain of seeing the apple in the position of mirror is called an image.

What adds the perception of reality to mirror image is the interpretation of Slightly different data from both of our eyes, Which gives the 3-D perception that apple is actually at a depth inside the mirror.

So to answer your question, mirror does not know anything, it is our perception of brain of reflected light of apple that makes us think that the apple is actually behind the mirror.

enter image description here


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