Lenses are very common objects in our daily lives. That’s because many people wear glasses, which are made up of a pair of this optical device . In addition to the glasses, the magnifying glass, the telescope, the photo cameras and cell phone cameras also have lenses.
But how does light behave as it passes through them and what is the difference between converging and diverging lenses ? These are some of the questions that we will answer with today’s text. Follow!
What are diverging and converging lenses?
The divergent and convergent lenses are manufactured objects with a solid piece of glass with curved surfaces, which follow a circular curve. This way, when the light rays reach the lens, they can converge (converging lens) or diverge (divergent lens).
How do converging and diverging lenses work?
The converging lens , as mentioned, has the characteristic of converging the light rays at a specific point. So, look at the image below. In it we have a converging lens and rays of light that are approaching it. When they pass it they start to converge to a point, which is known as a focus.
The divergent lens, on the other hand, has the characteristic of diverging light rays. Look at the image below. There is a divergent lens on it. The rays of light reach the lens, but as they pass through, they spread.
Lens elements (focus, anti-main, optical center)
Every lens, whether convergent or divergent, has three main elements: focus, anti-main and optical center .
See the image below:
The vertical line is a converging lens and the horizontal line is drawn right in the middle of the lens. In this way, the optical point (O) is always marked at the meeting of the two lines, that is, in the center of the lens. The focus (F) is where the lens rays converge.
The anti-main point (A) is designed with the focus in mind. Thus, the distance from A to the lens is equal to twice the distance from the focus to the lens (focal length) .
The focus and anti-main are points that, to be drawn, depend on the lens. That’s because each lens has its own focus .
Below you can see how the image of an object is formed on a converging lens and a diverging lens:
There are some rules to draw each ray you are seeing in the image above:
- pink ray: the ray departs from the object, in parallel with the horizontal axis and, when reaching the lens, it makes an angle with which it reaches the focus F ‘.
- green ray: the ray leaves the object, in a straight line through the focus and reaches the lens – after the lens, it follows in parallel with the horizontal line.
- lilac ray: the ray departs from the object towards the optical center and continues in the same direction and direction.
- salmon ray: the ray leaves the object, passing through the anti-main point and reaching the lens – after that, it follows a straight line passing through A ‘.
These are four standard rays that can be used to form any image on converging or diverging lenses .
After drawing the four rays, it is possible to see where the image of the object will be formed. The points F ‘and A’ are called image focus and anti-main image, respectively, and are always opposite the object, when it is a converging lens. When the lens is divergent these two points are on the side where the object is.
Types of converging and diverging lenses
The lenses do not have a single shape. Therefore, we will see each of these formats below.
When the refractive index of the lens is greater than the refractive index of the medium in which it is immersed, converging lenses have thin edges. See what we have in this case!
In biconvex lenses the edges are thin and their two sides are convex.
In plane-convex lenses , one side is convex and the other is flat.
In concave-convex lenses , one side is concave and the other is convex.
Considering that the refractive index of the lens is higher than that of the middle , the diverging lenses will have thick edges. See what we have!
In biconcave lenses , the two sides are concave.
In plane-concave lenses , one side of the lens is flat and the other is concave.
In convex-concave lenses , one side of the lens is convex and the other is concave. The difference in the shape of this lens for the concave-convex, which is convergent, is the width of the edges.
Is the magnifying glass lens convergent or divergent?
The magnifying glass is an instrument composed of a lens, and is used to enlarge objects, letters in a book or anything else that is close to the lens. This is because your focus is just inches from the lens . So, to enlarge the objects you need to place them in focus.
But how do you know if the magnifying glass lens is convergent or divergent ? There is an experiment that can answer that question and one that is easy to do, with a magnifying glass, a sheet of bond paper, a black pen and a sunny day.
With the black pen, draw a 1 cm ball on the sheet and paint it black. Now, go where there is sun and place the leaf on a dry surface – it could be on the ground. Take the magnifying glass and position it above the leaf and move it closer or further apart until you can see a well-lit spot on the leaf.
Now, take that point to the black ball on the sheet and wait a few seconds. A little smoke will start to come out of the leaf until it really starts to burn. At that time, have a mug of water to put out the leaf fire.
Some questions may arise from this experiment , such as: “why did the leaf catch on fire?”, “Why did an illuminated point appear on the leaf?”, “Why take the illuminated point to the black point?”.
When the magnifying glass is positioned at a certain height on the leaf, the sun’s rays pass through it and converge at the illuminated point. Therefore, it is necessary that the magnifying glass is at an ideal distance from the leaf so that it is possible to see its focus, which is where all the rays pass.
Thus, if it is too far from the leaf, the rays will converge in the air and, if it is too close to the leaf, the rays will not be able to converge, as they will be reflected by the leaf itself.
The black dot is drawn and you are asked to bring the illuminated dot to it, as the black color absorbs all colors. Sunlight, being formed by all colors, will be absorbed by the black dot and, thus, the leaf will catch fire faster.
So, we have that the magnifying glass is a converging lens , that’s why the illuminated point appears.
Is the eye lens system convergent or divergent?
The human eye is a very complex system, composed of several transparent media. It is considered an optical system because the lens is a natural lens.
The eye captures light in the following way: the light that enters our eyes is converged by the lens to a focus, which is within the eye. The rays then reach the retina, forming an inverted image. However, we don’t see it inverted because our brain inverts the image, leaving it in the correct position.
So, our eye has a converging lens .
Myopia and hyperopia
The myopia and hyperopia are vision problems caused by the formation of the image before or after the retina. In the case of myopia, the image is formed before the retina. In this way, objects that are further away are blurred.
In the case of hyperopia, the problem happens for close objects. This is because the image is formed after the retina, so it is blurred.
To correct both problems, it is necessary to wear glasses, contact lenses or have refractive surgery.