Question Video: Determining Changes in the Size of an Image | Nagwa Question Video: Determining Changes in the Size of an Image | Nagwa

Question Video: Determining Changes in the Size of an Image Science • Third Year of Preparatory School

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The figure shows an object and its virtual image that is formed by light rays reflected from a convex mirror. If the object is moved away from the mirror, how does the size of the image change?

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Video Transcript

The following figure shows an object and its virtual image that is formed by light rays reflected from a convex mirror. If the object is moved away from the mirror, how does the size of the image change? Is it (A) the image becomes smaller, (B) the image becomes larger, or (C) the size of the image does not change?

The size of the virtual image can be determined by drawing ray diagrams for these two scenarios. Here we have an object that extends above the optical axis. We will consider just two rays that come off the top of the object. One of these rays travels parallel to the optical axis. The first light ray will be reflected so that if we trace the reflected ray backward, we cross through the focal point. The second one we’ll consider moves straight towards the center of curvature of our mirror. This second ray will be reflected straight backward along the path it came from. The point of intersection of the dashed lines shows where the image of this part of our object forms. The same method can be applied for the bottom of the object, and the virtual image is formed.

Now, let’s move the object further away from the convex mirror and apply the same methods to form the virtual image for this case. The first light ray that travels parallel to the optical axis will be reflected so that if we trace the reflected ray backward, we cross through the focal point. The second ray moves straight towards the center of curvature of our mirror and will be reflected straight backward along the path it came from. Drawing in the incident and reflected rays for the bottom of the object and we can determine the updated virtual image from the points of intersection.

So now we can compare the image sizes from these two scenarios. As can be seen here, when we move the object away from the mirror, the image size becomes smaller. Therefore, the correct answer is option (A).

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