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Q & A: The Principal Focus

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Most recent answer: 03/22/2011
What is the "principal focus" when dealing with concave mirrors?
- C. Goodsell
Dodge City High School, Kansas
Fortunately, this is one of those questions that sounds harder than it actually is...

The "principal focus" for a concave mirror is exactly the same thing as the focal point, which you may have heard of before. If you were to take a concave mirror and shine a whole lot of rays of light straight at it (all parallel to each other), then all of those beams would be reflected by the mirror, and they’d all converge at one point. That point is what’s called the focal point (or the principal focus). Here’s a picture to better show you what I mean:

Concave mirror with a lot of rays of light shining straight at it

On this picture, the focal point (the principal focus) is labeled with the F.

This picture came from the page on concave mirrors.


(published on 10/22/2007)

Follow-Up #1: Focus of a Lens

what do u mean by focus of concave lens ?
- vasavi (age 15)
Hi Vasavi,

A focus of a concave lens is very similar to a focal point of a concave mirror. Imagine many light rays pointed towards a lens, all parallel to each other. When the light rays reach the lens, they will either all refract away from each other (a diverging, concave lens) or towards each other (a converging, convex lens). In the convex case, the point the rays all converge at is called the focus, and the distance from this point to the lens is the focal length. For a concave lens, this is a little trickier (but only a little!), since the rays diverge. Imagine tracing the rays backwards, in exactly opposite the direction they travel after going through the lens. Then they would converge, on the same side of the lens as the original, parallel rays. This point, where the "virtual" rays converge, is the focal point. An image formed here would be called a "virtual" image for this reason.

This figure (from may help clear things up for you; it illustrates the scenario I just described.
Thanks for the question!
Ben M.

(published on 03/22/2011)

Follow-Up #2: Real and Virtual Images

How can you tell the difference between real and virtual images displayed by concave and convex mirrors?
- Ali (age 13)
Mirrors work by changing the direction that light is moving. In a concave mirror, the light gets reflected towards the center. In a convex mirror, the light moves away. To make an image, we can trace a few rays that the light makes and see what happens to them.

Let's start with a concave mirror. If the object is far away, the light rays come in, and then bounce and come back together. See the picture below. This is an example of a real image.

(Whoops, link lost, but see follow-up for picture.)

If the object is very close to the concave mirror or you use a convex mirror, the light doesn't come back together. However, if you look at the light that comes off of the mirror and trace it back while pretending the mirror isn't there, they will come back to a point. This is called a virtual image.

(published on 10/22/2007)

Follow-up on this answer.