Question Video: Identifying the Spherical Mirror with the Greatest Focal Length | Nagwa Question Video: Identifying the Spherical Mirror with the Greatest Focal Length | Nagwa

# Question Video: Identifying the Spherical Mirror with the Greatest Focal Length Science • Third Year of Preparatory School

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Below is a series of ray diagrams for convex mirrors. All four mirrors have identical incident light reflecting off their surfaces. This is shown by the yellow lines. Which of the four mirrors has the greatest focal length?

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### Video Transcript

Below is a series of ray diagrams for convex mirrors. All four mirrors have identical incident light reflecting off their surfaces. This is shown by the yellow lines. Which of the four mirrors has the greatest focal length?

To answer this question, we need to understand the definition of the focal length of a spherical mirror. The focal length is the distance between the point where the reflected light rays meet, called the focal point, and the surface of the mirror. However, since these are convex mirrors, the reflected light rays do not meet. Instead, the focal point is the point where they would meet if we traced the direction of the reflected light rays back through the mirror.

Now that we have defined the focal length, we can compare these distances on each of the four ray diagrams and find which distance is the greatest. We can clearly see that mirrors one and two do not have the largest focal length. Hence, these options cannot be the correct answer. Comparing mirrors three and four, we see that mirror three has the larger focal length. Hence, the correct answer is that mirror three has the greatest focal length.

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