Question Video: Determining the Number of Rays Passing through the Left Focus | Nagwa Question Video: Determining the Number of Rays Passing through the Left Focus | Nagwa

Question Video: Determining the Number of Rays Passing through the Left Focus Science • Third Year of Preparatory School

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The diagram shows many light rays and a thin convex lens. How many rays will pass through the left focus?

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Video Transcript

The diagram shows many light rays and a thin convex lens. How many rays will pass through the left focus?

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 a 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.

Analyzing the diagram, we can see that there is no ray directed toward the center of the lens. To find rays that would pass through the left focus, we must instead look for the rays that are parallel to the optical axis. We know that the optical axis passes through the two focal points. In the diagram in this question, the optical axis is therefore a horizontal line. This means that we should look for horizontal light rays on the right-hand side of the lens.

We can see that there are three rays that fit this description. One of the rays is traveling to the right, however. So that ray will not pass through the left focus. Therefore, the answer to the question is two rays will pass through the left focus.

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