Here is a thin converging lens, thinner at the edges, and thicker at the center. We say that the surface is convex. You can get the image of an object using a lens. It will form on the other side of the lens.
We observe that it is overturned and that it is captured by a screen. When it does, it is said to be a real image. Not all optical systems offer a real image.
The magnifying glass, even if it is a converging lens, in its normal use, offers us an image which is on the same side as the object and which is not on a screen. It's a virtual image.
The same goes for the microscope, the astronomical telescope.
The same goes for corrective lenses.
But, in the case of the projector, the image of course forms onto a screen. It’s real.
With the camera, too, the image is formed on a sensor. It’s real.
For the smartphone, which has a camera, this is also the case.
And for our eye, which works a bit like a camera, it's the crystalline lens, here, that is the converging lens and the image is formed on the retina.
I isolated two rays, here, coming from B, a particular point of the object. Two rays among the infinity of rays since B emits in all directions.
These two rays, at the exit of the lens, converge towards the same point B prime, which is the image of B. If the image is formed in front, or behind the screen, we cannot see it.
If the image is on the same side as the object, we can see it, but the eye will be placed on the right, here. The image is virtual. This is the case with the magnifying glass I was talking about, for example.
I come back to the real image case.
Here, I let this appear in order to explain, but what you see is what is being scattered by the screen, which is a scattering object. The information from B, here, does not arrive in the same place. And so, we see something blurry, since at that moment, the image of a point is not a point.
To get the sharpness, I can move the object around, that is called focusing, move the lens, possibly the screen, or even, as is the case in our eye, make the curvature of the lens more or less convex.
This is what the ciliary muscles in our eye do, we will see that in another video. They manage to modify this curvature.
A little vocabulary. Here is the symbol of the lens.
This particular point, which is the optical center, is the origin of the principal axis, which is an axis oriented in the direction of propagation of light, so in my example, to the right, and graduated according to a chosen scale. I also wanted to talk to you about the principal focus,or focal point, here, which is an important point because its position is characteristic of this lens. It is important. Its name, focus, means, hearth, in latin. If we form the image of the Sun on a leaf, a tree leaf, a dry leaf or even a sheet of paper, we can start a combustion because all the rays here, and therefore all the energy they carry, will be concentrated at the same point.
This allows me to speak here of the focal length between O and F prime.
It is a characteristic of the lens.
It is here + 125 mm but this lens is only an example. It is an algebraic value. The focal length is O F prime.
How to draw the image of an object on a sheet?
So I chose small squares here, and remember, there are three particular rays you must know.
Any incident ray travelling parallel to the principal axis will refract through the lens and travel through the focal point, on the opposite side of the lens.
A single ray is not enough to get a point. I need a second ray. The one passing through the center keep his direction. So I can get B prime from this and already represent the image A prime B prime.
To reassure myself, I can represent the ray which passes through the focal point F.
It comes out parallel to the principal axis and I check that it also passes through B prime. So that I can build my image here. I remind you that the image and the screen must coincide to obtain sharpness.
🎧 Thanks to Gilles FOURNAT for his contribution 😲🎸
https://www.youtube.com/channel/UCPnKRvO0Wfq5QXKlm5ZsSQA
Subscribe for more licks ;-)
FIND HIM:
► INSTAGRAM : https://www.gillesfournat.com/instagram/
► FACEBOOK : https://www.gillesfournat.com/facebook/
► YOUTUBE : https://www.gillesfournat.com/youtube/
0 Comments