Question Video: Finding the Distance between the Surface and the Focal Point of a Spherical Mirror | Nagwa Question Video: Finding the Distance between the Surface and the Focal Point of a Spherical Mirror | Nagwa

# Question Video: Finding the Distance between the Surface and the Focal Point of a Spherical Mirror Science

A spherical mirror has a focal length of 860 mm. What is the distance, in meters, between the point on the surface of the mirror that the optical axis passes through and the focal point?

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### Video Transcript

A spherical mirror has a focal length of 860 millimeters. What is the distance, in meters, between the point on the surface of the mirror that the optical axis passes through and the focal point? (A) 860 meters, (B) 0.43 meters, (C) 0.86 meters, (D) 430 meters, (E) 1.72 meters.

In this question, we are considering a spherical mirror, so we’ll start by drawing a diagram of one. We’ve been told that the focal length of this spherical mirror is 860 millimeters. Let’s recall that the focal length of a spherical mirror is the distance between the focal point and the nearest point to it on the surface of the mirror. In other words, this is the distance between the focal point and the point on the mirror surface that the optical axis passes through. Now this might sound familiar.

The question actually asks us to find the distance between the focal point and the point on the mirror surface that the optical axis passes through. We’ve just seen that this distance is the focal length. We can imagine then that the answer to the question must be 860 millimeters. However, we need to be careful here. The question asks us for this distance in meters, and not in millimeters. To convert this distance to meters, let’s recall that one millimeter is the same as 0.001 meters. So 860 millimeters is simply 860 multiplied by 0.001 meters. This ends up being 0.860 meters, or 0.86 meters. From our answer options, we can see this matches option (C). Hence, we choose this as our final answer.

The distance between the point on the surface of the mirror that the optical axis passes through and the focal point is 0.86 meters.

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