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.
