Question Video: Determining the Directions of Light Rays That Pass through a Convex Lens | Nagwa Question Video: Determining the Directions of Light Rays That Pass through a Convex Lens | Nagwa

Question Video: Determining the Directions of Light Rays That Pass through a Convex Lens Science • Third Year of Preparatory School

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The diagram shows five light rays that will pass through a thin convex lens. The center of the lens is indicated by a black dot. How many light rays will not change direction as they pass through the lens?

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Video Transcript

The diagram shows five light rays that will pass through a thin convex lens. The center of the lens is indicated by a black dot. How many light rays will not change direction as they pass through the lens?

To answer this question, we must remember two basic and very important rules: first, that any light ray that passes through the center of a convex lens does not change direction; next, that any light ray that is parallel to, but not along, the optical axis will change direction when it passes through a convex lens so that the ray passes through the focal point of the lens that is on the opposite side of the lens to the side that light enters the lens from.

Looking now at the diagram, we can see that there is only one ray that is not directed at the center of the lens, which is represented by the black point. This is the ray that is traveling to the right and above the optical axis. This ray will have to change direction to pass through the focal point on the opposite side because the ray is parallel to the optical axis and the lens is convex.

All the other rays are directed toward the center of the convex lens and therefore would not change their path. There are five rays in total. Only one of these rays will change direction. And so four rays will not change direction. Therefore, the answer to the question is four light rays.

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