Focal Length of a concave mirror
image

Activity with a spoon:

  • Pick up the spoon by its handle and hold it. So you are looking at the curved-in or curved-out side. You are looking into a concave mirror. Note whether your image appears larger or smaller.
  • With the spoon held at a distance from your face, move a pencil point slowly from your face toward the spoon's bowl.
  • Note how the appearance of the pencil point changes as it moves closer to the bowl of the spoon.
  • Turn the spoon around to look at the bowl's back side. You are looking at a convex mirror. Note your appearance on the spoon. Repeat the same process with the pencil point as you did with the concave side of the spoon.
Q

How did your appearance differ when you looked at the concave and convex sides of the spoon?

Ans

The image is inverted on the concave side and erect on the convex side.

Q

How did the appearance of the pencil point, change as you moved it toward the spoon? Which side was inverted at one point, and the image was erect at another point? Which side gives a magnified image?

Ans

As you moved the pencil toward the concave side of the spoon, the image started getting inverted, but eventually flipped to an erect position and was enlarged. On the spoon's convex side, the pencil point's image remained erect as you moved forward, but it did not enlarge as you got closer to the spoon.

Q

How to find out the Focal Length of a Concave Mirror quickly and approximately?

Ans

The fact that "when the object is at a considerable distance (or at infinity) from a concave mirror, then its image is formed at the focus" can be used to find out the focal length of a concave mirror quickly but approximately. We focus a distant object like a tree or window on a screen (or a wall) by using a concave mirror whose focal length is to be determined. The image of a tree or window will be formed at the focus of the concave mirror. That is, the distance of the image (or screen) from the concave mirror will equal the focal length of the concave mirror. This distance can be measured with a scale. It will give us the approximate focal length of the concave mirror.