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Do Mirrors Have A Delay?

Mirrors form an image due to the reflection of light from an object to the mirror. Light takes time to travel but surprisingly our reflection is never delayed, at least that is what we believe.

Light travels at a finite speed yet the delay in the formation of reflection in the mirror is not significantly measurable. Light experiences a retarded effect not only in the case of mirrors but in every interaction with matter. So technically mirrors are not the sole reason for the delay. Nonetheless, mirrors are the subject of discussion over here. So let’s just look at the delay phenomenon in relation to mirrors.

Do we ever notice the delay?

Mirrors reflect light and the photons interact with the surface of the mirror. These photons first get absorbed in the mirror and then, they are emitted again for reflection. This absorption and re emission takes time, negligible though. This time taken for absorption and re emissions is the cause of delay in mirrors. But do we ever notice this delay?

The reflection is never in real time

As a matter of fact, the images we see in the mirror are never in real time. There’s always a delay of a few nanoseconds but it is unnoticeable by the human eye. Reflection of light is studied in terms of incident rays and reflected rays. The incident ray of light falls on the mirror, excites the free electron, re-emits the rays which are known as reflected rays. This whole process happens more quickly than the blink of an eye which is why the delay of a mirror is never noticeable from naked eye.

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Specular Reflection of mirrors

Reflection is mainly of two types; diffuse reflection and specular reflection. The reflection of a mirror is the best example of specular reflection. The process of specular reflection works as such that the delay is totally negligible. However, in case of diffuse reflection the delay can take minutes or even days but that’s not exactly the point of discussion here. Because mirrors do not show diffuse reflection.

Fluorescence materials show negligible amount of delay because photons are absorbed in atoms or molecules and then, are immediately re emitted. However, phosphorescent materials show a very noticeable amount of delay. The delay can even last for minutes. And mirrors fall in the category of fluorescent material, thereby the delay time is very trivial.

Distance between w the object and the mirror

Okay now while we have established the fact that mirrors do have a delay then, how can we measure the time for this delay? Or the time taken for the reflection in a mirror?

Technically there’s a formula for time measurement and that is Time= Distance/ Speed

So we can say that if we know the distance between the object and the mirror we can calculate the time taken for reflection because we know that light travels at 300000 km/s. But obviously, this time will be imperceptible to the human eye.

A Fun Mirror Delay Experiment

Delay mirrors/ Reflector Telescopes

While the delay is not noticeable is our regular mirrors, there are certain types of mirrors used for scientific purposes that show a very clear and noticeable amount of delay. They are often used in astronomy.

Some telescopes leverage on the mirror delay phenomenon. The image formation in such mirrors takes several minutes to show a change that occurred in the object.

While it is true that mirrors do have a delay it can be clearly said that this delay is not visible to the human eye.

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