Question Video: Determining the Position of a Focus for a Convex Lens | Nagwa Question Video: Determining the Position of a Focus for a Convex Lens | Nagwa

# Question Video: Determining the Position of a Focus for a Convex Lens Science • Third Year of Preparatory School

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The diagram shows a thin convex lens. The lens is symmetrical. The point marked P is one of the foci of the lens. Using the grid, determine which of the points (I, II, III, or IV) is the other focus of the lens.

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### Video Transcript

The diagram shows a thin convex lens. The lens is symmetrical. The point marked P is one of the foci of the lens. Using the grid, determine which of the points I, II, III, or IV is the other focus of the lens.

Here, we are given a diagram showing a grid with a thin convex lens, a focal point labeled P, and four other points on the other side of the lens. We are asked to use the grid to determine which point is the other focal point of the lens. We are also told in the question that the lens we have here is symmetrical. That means that if we cut it in half along the vertical or horizontal axis, we would have two halves that are identical.

If we look at the shape of a convex lens viewed from the side, we can see that it is made up of the overlap of two circles put on top of each other, like seen here. The shape of the convex lens allows it to focus rays of light to a single point called the focal point. We can see how this works by looking at light rays traveling parallel to the optical axis through a lens and noticing that they all converge to a single point on the other side of the lens. This is the focal point of our lens. And because convex lenses are symmetrical, this will also happen if we send light rays through the other side in the same manner. Only this time the light rays will converge to the focal point on the other side. Notice how these two points are the same distance away from the lens.

Now, back to our question and diagram, take a look at the lens and point P. And let’s measure the distance from the center of the lens to that point. We can see that point P is five squares away from the center of the lens. This means that we would expect the focal point on the other side to be the same distance away. So let’s find which point corresponds to it.

Starting at the center of the lens and moving five squares to the left, we find that point II is that distance away. Therefore, point II is the point where the other focus would be. So point II is the correct answer.

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