To figure out which lens gives which image, you first need to remember what a lens is. The main physical phenomenon that is used to create a lens is the refraction of light passing through a medium. It is this phenomenon that made it possible to create such a device that can control the direction of light fluxes. The principles of such management are explained to children back in school, in the course of eighth grade physics.
Definition of the word lens and the material used to make it
Lenses are used so that a person can see an enlarged or reduced image of a certain object. For example, using a telescope or microscope. Therefore, this device is transparent. This is done in order to see objects as we really are, only changed in size. It will not be color, distorted, if not required. That is, the lens is a transparent body. Next, we move on to its components. The lens consists of two surfaces. They can be curved, often spherical, or one of them will be curved and the second flat. It is these planes that determine which lens gives which image. The material for the manufacture of lenses in a wide household are glass or plastic. Further we will talk specifically about glass lenses for a general understanding.
Separation into convex and concave lenses
This separation depends on the shape of the lens. If the lens has a middle wider than the edges, it is called convex. If on the contrary, the middle is thinner than the edges, such a device is called concave. What is more important? What matters is the environment in which the transparent body is located. After all, what kind of lens which image gives depends on the refraction in two media - in the lens itself and in the matter surrounding it. Next, we will consider only airspace, since the refractive index of the lens from glass or plastic is higher than the established indicator of the environment.
Collecting lens
Take a convex lens and pass a stream of light through it (parallel rays). After passing through the surface plane, the flow is collected at one point, therefore the lens is called collecting.
To understand what image the collecting lens gives, and any other, you need to remember its main parameters.
Important parameters for understanding the properties of a given glass body
If the lens is bounded by two spherical surfaces, then its spheres, of course, have a certain radius. These radii are called the radii of curvature that extend from the centers of the spheres. The straight line that connects both centers is called the optical axis. A thin lens has a point through which the beam passes without any deviations from its previous direction. It is called the optical center of the lens. Through this center, perpendicular to the optical axis, a perpendicular plane can be drawn. It is called the main plane of the lens. There is also a point called the main focus - the place where the rays will collect after passing through the glass body. When parsing the question of what image the collecting lens gives, it is important to remember that its focus is on the back side of the rays. For a scattering lens, the focus is imaginary.
What image of an object does a collecting lens give?
This directly depends on how far the object is placed relative to the lens. There will be no valid image if you place the subject between the focus of the lens and the lens itself.
The image is imaginary, direct, and significantly enlarged. An elementary example of such an image is a magnifier.
If you place objects behind the focus, then two options are possible, but in both cases the image will be inverted and actual in the first place. The difference is only in size. If you place objects between focus and double focus, the image is enlarged. If placed behind a double focus, it will become reduced.
In some cases, it may happen that no image is obtained at all. As can be seen from the figure above, if you place the subject just in place of the focus of the lens, the lines whose intersection gives the top point of the subject go in parallel. Accordingly, the intersection is out of the question, because the image can only turn out somewhere in infinity. Also interesting is the case when the subject is placed in place of double focus. In this case, the image is turned upside down, valid, but identical in size to the original item.
In the figures, this lens is schematically depicted as a segment with arrows at the ends directed outward.
Scattering lens
Logically, a concave lens is scattering. Its difference is that it gives an imaginary image. The rays of light after its passage are scattered in different directions, because there is no real image. The answer to the question of which image the diffuser lens gives is always one. In any case, the image will not be inverted, that is, direct, it will be imaginary and reduced.
In the figures, this lens is schematically depicted as a segment with arrows at the ends that look inward.
What is the principle of imaging
There are several construction steps for a collective lens . The object, the image of which will be built, has a vertex. Two lines must be drawn from it: one through the optical center of the lens, the other parallel to the optical axis to the lens, and then through the focus. The intersection of these lines will give the top of the image. All that is needed next is to connect the optical axis and the resulting point, parallel to the original object. In the case when the subject is in front of the focus of the lens, the image will be imaginary and located on the same side as the subject.
We remember what image the scattering lens gives, therefore we construct the image for a concave lens, according to the same principle, with only one difference. The focus of the lens used to build is on the same side as the subject whose image you want to build.
conclusions
Let us summarize the above materials in order to understand which lens gives which image. It is clear that the lens can increase and decrease, but the questions are different.
Question number one: which lenses give a valid image? The answer is only collective. It is a concave collective lens that can give a valid image.
Question number two: which lens gives an imaginary image? The answer is scattering, and in some cases, when the subject is between the focus and the lens, it is collective.