Question Video: Recalling the Definition of the Focal Length of a Concave Mirror | Nagwa Question Video: Recalling the Definition of the Focal Length of a Concave Mirror | Nagwa

Question Video: Recalling the Definition of the Focal Length of a Concave Mirror Science • Third Year of Preparatory School

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Fill in the blank: The focal length of a concave mirror is the distance between the point on the surface that the optical axis passes through and the _.

02:30

Video Transcript

Fill in the blank. The focal length of a concave mirror is the distance between the point on the surface that the optical axis passes through and the blank. (A) Observer, (B) focal point, or (C) center of curvature.

In this question, we are asked what the focal length of a concave mirror is. First, let’s draw a diagram to remind ourselves of how a concave mirror works. The mirror itself looks like this. We can draw in the optical axis, which is essentially the line of symmetry of the mirror. We can then label the center of curvature of the mirror. This is the point that is the same distance away from every point on the mirror.

Now, if some rays of light parallel to the optical axis were incident on this mirror, they’d be reflected like this. Notice that the reflected rays converge and pass through this point here. This is called the focal point of the mirror. It’s where rays which are parallel to the optical axis will be focused to. We can also draw in an observer over here, who can see light rays which are reflected from the mirror.

Now, to answer this question, we’ll need to recall the definition of focal length. The focal length of a mirror is the distance between the mirror and the focal point, measured along the optical axis. So, to complete the sentence given in the question, we’d need to fill in the blank with “focal point.” That’s option (B). But let’s quickly think about the other answer options just to make sure that we’re right.

Option (C) is center of curvature. However, the distance between the point on the surface of the mirror that the optical axis passes through and the center of curvature, that’s this distance on our diagram, is called the radius of curvature. We can see that this is not the same thing as the focal length. So we can confidently say that option (C) is not the correct answer.

Option (A) is observer. We can also rule this one out since focal length is a property of a given mirror. So it can’t depend on the position of an observer. The distance between the center of the mirror and the observer doesn’t really have a special name.

The correct answer then is option (B). Our completed sentence reads “The focal length of a concave mirror is the distance between the point on the surface that the optical axis passes through and the focal point.”

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