Concave Lens Image properties


If you remember our earlier discussion,
you will recall that when a concave lens was kept in front of a paper and it was
kept in sunlight, the parallel rays that were coming from the Sun were unable to
burn the piece of paper. Why? Let us look at the animation and find out. Over here we have considered the Sun
which is an object at infinity, that is very far away, and we have considered the parallel rays
of light that are incident on the concave lens from the Sun. Now , notice after refraction these parallel
rays of light are diverging. They are not meeting at any point on the principal
axis. However, these rays of light are appearing to be diverging from a point
that is on the same side as that of the object. Or in other words, the diverged
refracted rays seem to be coming from a point behind the lens. So, What can we say about the image that
is formed by an object at infinity? We can say that when the object is placed at
infinity, the image is being formed at the focus, that is at, F1. And as you can see, these parallel rays are getting diverged but seem to be
coming from a point. So, the nature of this virtual image is highly diminished. It is highly diminished. Now, let us see what happens if we keep the object
anywhere else, that is, if we bring the object closer. You’ll be interested to learn that if an
object is brought closer to the concave lens irrespective of where it is kept,
that this, anywhere in between the lens at infinity if the object is kept the
image is formed at more or less the same place. Let us consider this situation,
where the object has been placed beyond 2f1. Now we consider two rays, a ray that
is parallel to principal axis, after refraction it appears to be coming from
the focus, that is, F1 and a ray incident at the optical centre. After refraction it is
passing through without any refraction. So, What is happening? These rays are not meeting at any point
instead they seem to be intersecting at a point on the same side as that of
the object. So, What can we say about the image?
We find that the refracted ray and the ray that passed through without any refraction are appearing to meet at this point, If we
extrapolate the refracted ray. So, What can we say about the image?
We can say if the object is placed anywhere except infinity, the image is always formed in
between the focus and the optical centre. And as you can see the images is virtual,
because we are not obtaining it through the intersection of rays in the real world. The size of the image is highly
diminished and it is also upright or erect. Now, if we consider a different scenario, where
the object is placed in between F1 and 2F1. We will find that the image is
formed again in between the optical centre and F1. That is if we considered two
rays, one that is parallel to the principal axis, after refraction, it
appears to be diverging from the focus and we consider another ray, that is incident
at the optical centre which passes through without any deviation or refraction. So,in this case also when the object is
anywhere else except infinity, in this case between 2F1 and F1. The
image that is being obtained is between Focus and Optical centre, and also the image is
virtual because we’re not obtaining the image through the intersection of these rays in the
real world. The image is upright as you can see. And the size of the image is diminished. Now, look at the picture being shown on
the board. This is a very interesting scenario being considered. In the first image you will find that
a human eye has been shown. Now, if you recall from our previous lecture, the
human eye is also an example of a lens. Now, you will be interested to learn
that the human eye is a convex lens. So, What happens to parallel rays of light
falling on the human Eye? Here, we find light rays from a far off object
falling on the eye. So, since they are parallel and since a human eye
is a convex lens these rays converge to a point, that is the focus of this lens. And the focus of this lens is on the
retina. The Retina is a part of the human eye. Now, notice what happens if we consider a
short sighted eye, that is an eye with the defect of not being able to see far off
objects. When parallel rays of light are incident on the eye after refraction
they are converging to a point that does not lie on Retina, That is, this point is a few
distance off from the Retina. So, what is happening, the light is getting focused in
front of the retina and the image that is formed on the eye is a blurred image. Now, How can we correct this? This can be corrected with the use of
a concave lens. We use a concave lens to correct
short-sightedness. So what happens? We learnt that a concave lens is a
diverging lens. So, when these parallel rays are incident on this concave lens these rays are
diverged from their path. Now, these diverged Rays hit the lens in our eyes after this they converge at a point. Now, as you can see after the
concave lens had been introduced these rays are meeting at the retina. So,
the eye is able to obtain a clear image of the objects in front, that is the objects
that are far away. So, the concave lens adjust the rays so that the light
rays are focused on the Retina. So, taking a quick recap. What did we
learn? We learnt that when the object is placed at infinity, the image is formed at F1 for a
concave lens. The size of the images as we saw was highly diminished and the nature
of the image was virtual and erect. Further, We saw that if the object is placed
anywhere in between infinity and the optical centre. The image that was obtained was always
between the optical centre and F1. We also saw that the size of the image was
diminished and the nature of the image remained virtual and erect.

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