Question Video: Identifying the Path of a Light Ray That Passes through a Concave Lens Science

The diagram shows five light rays that will pass through a thin concave lens. Which of the light rays, once it has passed through the lens, will travel along the dashed line?

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Video Transcript

The diagram shows five light rays that will pass through a thin concave lens. Which of the light rays, once it has passed through the lens, will travel along the dashed line?

In this question, we’ve been given a diagram of a concave lens. We know it’s concave because it’s thinner in the middle than at the edges. We’ve also been given the directions in which five rays of light travel before entering the lens. We need to figure out which of these five rays will travel along the dashed path after passing through the lens.

To answer this question, let’s recall an important rule for light rays passing through concave lenses. A light ray that is moving parallel to, but not along, the optical axis will be refracted by a concave lens in such a way that the refracted ray, when traced backward, will appear to have emanated from the focal point of the lens. In simple terms, the concave lens will refract, or bend, rays of light passing through it.

But any ray of light initially moving parallel to the optical axis gets refracted in a very specific way. The refracted ray can be traced backward to the same side of the lens as the original ray. But the traced back ray will pass through the focal point there. This rule is helpful to us here, because we see that this dashed line passes through the focal point of the lens, also known as the focus of the lens.

So, in order for a ray of light to be refracted so that it travels along this dashed path, it must have been initially traveling parallel to the optical axis. Since this is the optical axis, we just need to find the ray of light that is parallel to this line.

Looking at our answer options, we see that this is ray 4. Therefore, our final answer is that ray 4 will travel along the dashed line once it has passed through the lens.