1,
In my lecture, I will give all of you a description of thin lenses, especially converging
lenses and their application.
2,
This is the outline of my lecture. Please review it and you will know the main content of
my lecture.
3,
What are thin lenses?
A typical thin lens consists of a piece of glass or plastic, ground so that each of its two
refraction surface is a segment of either a sphere or a plane.
What are these lenses used to do?
Lenses are commonly used to form images by refraction in optical instruments, such as
cameras, telescopes and microscopes.
This figure shows some representative shapes of lenses. Notice that we have placed these
lenses in two groups. Those in upper group are thicker as the center that as the rim, and
those in lower group are thinner at the center that at the rim. The lenses in the first group
are examples of converging lenses, and those in the second group are diverging lenses.
Now let me introduce these lenses to you.
Then let us focus on the converging lenses.
4, Focal length
It is convenient to define a point called the focal point for a lens. For example, when a
group of rays parallel to the axis passes through the converging lenses, they will be
converged and passes through the focal point. The distance from the focal point to the
lens is called the focal length. The focal length is usually symbolized by the letter f. The
focal length is the image distance that corresponds to an infinite object distance. Recall
that we are considering the lens to be very thin. As a result, it makes no difference
whether we take the focal length to be the distance from the focal point to the surface of
the lens or the distance from the focal point to the center of the lens, because the
difference between the two lengths is negligible. I must emphasize that a thin lens has
two focal points corresponding to parallel rays traveling from the left and from the right.
But the focal lengths are the same.
5, Geometric Construction
Now let us look at the geometric construction for developing the thin-lens equation. We
get it from experiments. Now let us observe the figure carefully. We will find two
important properties of the thin converging lens. The first is…., and the second is ….
6, thin lens equation
From these two properties, we can develop the thin-lens equation. It is listed here. Let me
explain the symbols to you. ……. Now we define the magnification. It is …..
7
We can get three relations between objects and images from the thin-lens equation.
Maybe there are difficult for you to understand, especially it is the first time you contact
them. So I will give you some vivid examples to explain them..
8 Cameras
The first example is the camera. It is one of the thin-lens’ most important application.
The single-lens photographic camera is a simple optical instrument. Its essential features
are shown here.
9 How cameras work?
It consists of a light-tight box, a converging lens that produces a real inverted image, and
a film behind the lens to receive the image. What is real image? A real image is one in
which light intersects, or passes through, an image point. Focusing is accomplished by
varying the distance between lens and film. Proper focusing will lead to sharp images. To
achieve it, the lens-to-film distance will depend on the object distance as well as on the
focal length of the lens.
The shutter, which is not shown in the picture, located behind the lens, is a mechanical
device that is opened for selected time intervals. With this arrangement, moving object
can be photographed with the use of short exposure times, and dark scenes (low light
levels) with the use of long exposure times. Without this control, it would be impossible
to take stop-action photographs. For example, a speeding race car would move far
enough while the shutter was open to produce a blurred image.
It is obvious that the object distance is bigger than the image distance. According our
equation, the magnification is smaller that 1, which accords with the reality that the
image is smaller that the object.
10 another application of converging lens is projector. These are three kinds of projectors
but their principles are the same. On the contrary to the camera, the film, located inside of
the projector, is the object and it is closer to the lens than the screen, which receive the
real image. The distance between the film and the lens is between one focal length and
two focal lengths. Our equation shows that the magnification is bigger than one and the
reality is that the image is bigger than the film. They agree on each other.
11 The last example is the simple magnifier.
The simple magnifier is one of the most basic of all optical instruments because it
consists only of a single converging lens. As the name implies, this device is use to
increase the apparent size of an object. The object is close to the lens and the object is
less than the focal length. At this location, the lens forms a virtual, upright and enlarged
image. I must explain what a virtual image is. A virtual image is one in which the light
dose not pass through the image point but appears to diverge from that point. So we
cannot receive the images. Of course the image is bigger than the object which accords
on the equation.
12 in fact, converging lens has more applications. All examples I gave before consist of
only one converging lens. We can use a series of lens, such as telescope, microscope and
so on. But no matter how complicated these setups are, the basic principle is the equation.