Finding the focal length

Light from something far away arrives as near-parallel rays, and a concave mirror brings parallel rays to its focus. That gives a quick way to measure its focal length.

Activity 5.1

You will need: a concave mirror, a white screen

Open the window of a room.
Stand inside, holding a concave mirror facing the open window.
Hold a screen such as white paper in front of the mirror, and adjust until an image of a distant scene outside forms on it.
When a very clear, small, inverted image forms, like a photograph, measure the distance between the screen and the mirror.

Figure to be added

Figure 5.16 — a concave mirror on a stand facing an open window, and a small image of a tree outside formed on a screen in front of it; the focal length is marked between them.

The observation

Because the image can be caught on a screen, it is a real image. Rays from a distant object can be treated as parallel, so the distance from the mirror to this image is approximately the focal length.

Real image

Formed where rays actually meet; can be caught on a screen.

Virtual image

Rays only seem to come from it; seen only by looking into the mirror.