 Question Video: Determining the Focal Point of a Convex Mirror | Nagwa Question Video: Determining the Focal Point of a Convex Mirror | Nagwa

# Question Video: Determining the Focal Point of a Convex Mirror Science

Below is a ray diagram for a convex mirror. Which one of the five locations along the optical access represents the focal point of the spherical mirror?

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### Video Transcript

Below is a ray diagram for a convex mirror. Which one of the five locations along the optical access represents the focal point of the spherical mirror?

Since this mirror is convex, that means an observer would be on this side of the mirror. Since the mirror is spherical, that means its surface is part of a larger sphere. In this example, we have five locations, one, two, three, four, five, along what is called the optical axis. That axis follows this line that goes right through the center of the mirror. We want to figure out which of these five locations represents the focal point of the mirror. Notice that the diagram shows these three parallel rays of light running into the mirror. The top ray reflects off in this direction, the middle ray reflects straight back along the path it came, and the bottom ray reflects off the mirror like this.

To find the location of the focal point, we’ll trace these three reflected rays backward. The top ray is traced backward like this, the middle ray like this, and the bottom ray this way. We’ve only traced back far enough to find the point where the rays meet, where they cross. That point is here, and this is the focal point of the mirror. We see this is at the location marked four. Location four is the mirror’s focal point.