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This is something that completely blew my mind. Last week I was playing randomly with a mirror and my pen and then I noticed that the image of the pen was also having its shadow on a nearby wall.

How is that even possible? For something to form a shadow it must physically block the path of light but a virtual image is "virtual". So how can it block the path of light?

Here's a picture of that:

enter image description here

To add :- There is only one light bulb in the room.

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    $\begingroup$ Virtual images are almost as good as the real ones. This is why we see them. $\endgroup$
    – fraxinus
    Commented Apr 29, 2023 at 20:51
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    $\begingroup$ This is a great brain-teaser. $\endgroup$
    – garyp
    Commented May 1, 2023 at 11:58

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To add to Solomon's answer, here is a diagram that shows how you could get two shadows. Shadow 1 is cast by the light that first hits the object before it would have hit the mirror, shadow 2 is cast by the light that has first hit the mirror and then hits the object. Interestingly, you could argue that shadow 2 is cast from the image of the light source.

enter image description here

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    $\begingroup$ +1 When I read the question, I thought that a picture like this would be key to an answer. But I also think just one or two sentences would flesh this out better to a complete answer on its own (without referencing another answer). $\endgroup$ Commented Apr 29, 2023 at 22:44
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    $\begingroup$ "Interestingly, you could argue that shadow 2 is cast from the image of the light source." – Indeed, you could argue that both shadows are cast from the image of the light source. If you add the image of the light source and the image of the pen to the diagram, then it looks like the image of the pen is casting shadow 1, and the real pen is casting shadow 2, both from the same virtual light source. $\endgroup$ Commented May 1, 2023 at 15:51
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It isn't an image of the pen that blocks the light, it's the actual pen.

The light colored, oval spot on the wall... That's where light rays that hit the mirror are reflected onto the wall. Some of those light rays are stopped by the pen before they ever hit the mirror. That explains one of the two shadows. Other light rays are stopped by the pen after they leave the mirror. That's the other shadow.

If you want to visualize it, just look at the photo you posted. The camera can see part of the wall reflected in the mirror, but there are two places where the reflected image of the wall is blocked from the camera's view by the pen. If you imagine moving the light source to the location of the camera's lens, it should be obvious that it would cast two shadows.

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I suppose main point is that the bundle of light rays which forms the image of pen which comes in our eye, is not the one which is reflected onto the wall. However, both come from the source.

Psychologically speaking, the image of the source on both screens look similar, and one may be think that the one on the wall is related to the one on the mirror. However, by reflection law and such, this is not right.

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    $\begingroup$ Snell's Law, which relates to refraction, is not relevant. $\endgroup$
    – Peter
    Commented May 1, 2023 at 1:20
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    $\begingroup$ That's weird. In Indian syllabus, we often refer to Snell's law of reflection and refraction. I'll change it still $\endgroup$ Commented May 1, 2023 at 6:26

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