# Where is the principle focus of a plain mirror?

• chandubaba
In summary, a plain mirror has a focal point in a plane perpendicular to the mirror's axis. Parallel rays that are incident on the mirror come to focus in the focal plane.
chandubaba
where is the principle focus of a plain mirror?

chandubaba said:
where is the principle focus of a plain mirror?
The principle focus or focal point of a mirror is the point at which light that is incident parallel to the aixs comes together at a point. The focal point is in a plane perpendicular to the axis (parallel to the surface of a plane mirror) called the focal plane. Any parallel rays incident on the mirror surface come to focus in the focal plane. If the incident rays are even the slightest bit converging when they hit the mirror, they will come to focus between the focal plane and the mirror.

What happens to parallel rays when they hit a plane mirror? Where would they come to focus if they were slightly converging? If you start with converging rays, and then imagine making them less and less convergent, what happens to the point where they come into focus?

OR

Suppose you have a mirror that is convergent, so parallel incoming rays come to a focus in the focal plane. Imagine making the mirror flatter (less convergent). What happens to the focal point?

1) Is there such a thing as a plain mirror
2) Does a plain mirror have a focal point
3) where is that focal point

It is meaningless to ask the third question without ascertaining the answer of the first two.
If you decide to ask Q3 you must limit your question to a hypothetical situation and remove some ambiguities in my opinion.

I think that's what Dan was getting at too...

I found the answer in resnick halliday website.its infinity

Do you possibly mean a plane mirror? (I would think that most "plain" mirrors are "plane" mirrors!) Only parabolic mirrors focus light. Mirrors that are not parabolic do not focus light and so do not have a "focal point".
You can, if you wish, think of a plane mirror as focusing "at infinity" but please remember that that is only a figure of speech.

Yes sir,plane mirror.I speculate that plain mirrors can be considered part of spheres with infinite radius and that's why its infinity.I suppose this the property of infinity.Sorry I forgot the URL of that site.pls search in a search engine.

Last edited:

## 1. What is the principle focus of a plain mirror?

The principle focus of a plain mirror is the point at which all incident rays parallel to the principal axis converge after reflection.

## 2. How is the principle focus of a plain mirror determined?

The principle focus of a plain mirror can be determined by drawing a ray diagram and locating the point where all incident parallel rays intersect after reflection.

## 3. Is the principle focus of a plain mirror the same as the focal length?

Yes, in a plain mirror, the principle focus is equal to the focal length, which is half the distance between the mirror and its center.

## 4. Can the principle focus of a plain mirror be located behind the mirror?

No, the principle focus of a plain mirror is always located in front of the mirror, along the principal axis.

## 5. How does the distance between the object and the mirror affect the principle focus?

The distance between the object and the mirror does not affect the principle focus in a plain mirror. It remains at the same location regardless of the position of the object.

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