A physics topic in junior high school: convex lens

Knowledge point preparation: The 'object' is at the double focal length, and the 'image' is also at the double focal length.

In this way, when the 'object' is close to the double focal length from infinity, the 'image' moves from the focal point to the double focal length, of course, the 'object' and the 'image' are on both sides of the lens, the 'object' is close to the lens, and the 'image' is far away from the lens, so at the same time, the 'object' moves infinitely, and the 'image' only moves from one focal length to two the focal length, so the speed of the 'object' is greater than that of the 'image'.

However, when the 'object' reaches the second focal length and continues to move closer to the lens, the 'image' begins to move away from the double focal length. When the 'object' moves from the second focal length to the first focal length, the 'image' moves from the second focal length to infinity, (because when the 'object' is placed on the focal point, the light becomes a parallel light, and the 'image' is equivalent to being at infinity), which is the opposite of the previous one, that is, the speed of the 'image' is greater than the speed of the 'object'.

And as you said, the object is approaching the lens at a constant velocity, so the velocity of the image changes from small to large.

Far away, so it should be more than twice the focal length, what you see is an inverted, zoomed out real image (like a camera).

Then, when the focal length is doubled, it becomes an inverted, equal-sized real image.

Then, between the two focal lengths and the double focal lengths, there is an inverted, magnified real image (like a projector).

I don't image at double the focal length.

What does it mean to "between the telekinetic velocity of the image and the velocity of the movement"? I don't understand...

When the object is outside twice the focal length, the distance traveled by the image is less than the distance traveled by the object in the same time, so the velocity is small when the object is in.

When there is a focal length between one and two times, the image moving distance is greater than the object moving distance in the same time, so the speed is large.

The speed of the image is getting bigger and bigger.

Your consideration is more detailed, in fact, for junior high school students, whether it is a convex lens or a concave lens, they are just a schematic diagram, and it is okay to draw the dismantled light on the left or right. As shown in the figure below.

In junior high school, if there is no special emphasis, the convex and concave lenses encountered are all consideredThin lenses, so it is the same no matter which mirror wall you start drawing from, but if it is not a thin lens, it must be refracted twice: once on the left and once on the right.

This is just a schematic diagram, and it is best to draw a vertex in the middle.

The imaging law of convex lens is one of the compulsory test points in the high school entrance examination.

The title says that point S emits light to point n on the convex lens, and the refracted light intersects at point A on the right side of the convex lens, indicating that S is outside the focal point and point A is the real image point.

The refracted rays from the light emitted from the S point (outside the focal point) to any point on the convex lens intersect at point A.

Because. The object point S, the optical center O, and the image point A are on the same line.

The first is the convex lens, because a is biased towards the main optical axis after refraction.

Then judge the focal length, if it is a parallel light incidence, then the light will converge in the focal length after deflection, and A is obliquely incidentified, so the P point after deflection is on the left side of the focal point, choose A.

b (Experimental research on the imaging law of convex lens is: when the object distance is within one time of the focal length, the upright and magnified virtual image is obtained; Between 1x and 2x focal lengths, a handstand, magnified real image is obtained; When outside of the double focal length, a handstand and zoomed out image is obtained.

The purpose of this experiment is to study and confirm this law. In the experiment, there is the following table:

Object distance range, imaging properties, image distance range.

U>2F Handstand, Zoomed Out, Real Image, Heterolateral F2F

u=f does not image—

This is designed to prove that law**. In fact, lens imaging satisfies the lens imaging formula:

1 U (object distance) + 1 V (image distance) = 1 F (lens focal length).

The object is not imaged in focus, and the double focal length is the same.

It is smaller than the inverted two-focal pointstand, and the slide show is placed outside the focal point and inside the two.

The object is placed in focus, and a large virtual image is seen on the opposite side.

If the image can be presented on the screen, it must be a real image in handstand.

1 U f becomes a real image, u f becomes a virtual image, and the focus is the dividing point between the real image and the virtual image.

2 U 2F becomes a zoomed real image, U 2F becomes a magnified real image, and the double focal point is the dividing point between magnified real image and reduced virtual image.

3 When the object distance decreases, the image distance becomes larger, and the image becomes larger; When the object distance increases, the image distance becomes smaller, and the image becomes smaller.

4 When a real image is formed, the image and the object are on the opposite side of the convex lens, and when the image is a virtual image, the image and the object are on the same side of the convex lens.

5 The real image is formed by the convergence of actual light rays and can be displayed on the light screen, while the virtual image is the intersection point of the reverse extension line of the refracted light ray, and is not displayed on the light screen.

In camera, projector and magnifying glass, the convex lens is applied in imaging, the object is outside the focal length of one side of the lens twice the focal length of the lens, and the inverted and reduced real image is obtained on the other side through the lens, and the farther the object is from the lens, the closer the imaging is to the focal point; When the object is on one side of the lens and the distance between the focal point and the double distance, the inverted magnified real image can be obtained on the other side of the lens, and the closer the object is to the focal point, the farther away from the lens, the greater the magnification; And when the object is in the range where one side of the lens is less than the focal length of the lens, an upright magnified virtual image on the same side of the object can be observed on the other side, and the closer the object is to the focal point, the larger the magnified virtual image obtained.

As the pencil is placed horizontally and slowly exits the cylindrical glass bottle near the water, through the water bottle, the length of the pencil can be seen increasing (the distance between the left and right of the handwriting on it increases), then it becomes blurred, and finally the pencil is not visible, but the pencil does not become thicker in the process, only changes in length. Since the speed of light in water is less than the speed of light in air, this cylindrical glass bottle filled with water is equivalent to a (cylindrical mirror) that is, when the light rays on one side along the same cross-section pass through this interface, they are equivalent to being refracted by a convex lens and emitted from the other side, therefore, in the same cross-section, the same convex lens imaging law is also followed, when the pencil is in the focal length of the convex lens composed of this cylindrical bottle, the other side can see the magnified upright virtual image, but this "lens" In the direction of the cylindrical axis (up and down), it is only equivalent to observing the object on the other side through a piece of flat glass, so during the experiment, it can only be clearly observed that the image of the pencil only changes in the left and right directions, and as the distance between the pencil and the bottle increases, the real image formed by the pencil through the bottle can only be obtained on the "screen", and it is impossible to observe directly through the bottle, so the pencil will be seen "disappearing".

1. The camera uses the law that the object distance is greater than 2 times the focal length and the inverted real image is reduced. The projector uses the law that the object distance is greater than 1 times the focal length and less than 2 times the focal length to magnify the inverted real image. The magnifying glass uses the law of upright magnification of virtual images when the object distance is less than the focal length.

2. First see the tip of the pen pointing to the left, the image is getting bigger and bigger, and then see the tip of the pen pointing to the right is getting smaller and smaller.

Using the formula method 1 u+1 v = 1 f, or the drawing method, it can be proved that the shortest distance between the object and the image is 4 f. when the object is at 2 times the focal length on the main optical axis of the convex lensSo the answer is d

Because the object is an inverted and reduced real image outside the double focal length of the convex lens, and an inverted magnified real image between one and two times the focal length, a is getting bigger and bigger!

I choose AThis is because a convex lens is an inverted magnified image at one to two times the focal length, an inverted image as large as an inverted image at two times the focal length, and an inverted reduced image outside the second focal length. From this retrospective we can see a

From the real image of handstand reduction to the real image of inversion and other large images, and then to the real image of inverted enlargement, it should be A, the question is wrong.

This problem is nonsense, and the moving light screen can't see a clear image, just one reason: the position of the convex lens is wrong

If you insist on saying three, then you write:

1. The convex lens is too close, and the candle is within one double the focal length, which is a virtual image 2, the convex lens is too close, and the candle becomes a real image in a place other than the desktop, resulting in the inability to focus within the desktop 3, the convex lens is too far, and the candle is imaged very close to the convex lens, and the light screen cannot reach 4, the candle, the light and the convex lens are not strictly arranged in the same straight line (different axes)5, and the convex lens is not placed vertically along the optical axis.

6. The light screen is not placed vertically along the optical axis.

7. The candle flame is unstable.

Look at the writing, in short, it's all kinds of wrong positions.

The candle is too far away.

The candle is in focus.

In the above figure, ray a is a concave lens that takes away the previous ray.

The b ray is the concave lens, and the latter ray is removed.

After the two diagrams are re-aligned, it can be seen that the parallel light A passes through the concave lens and the light rays are reversed, just to point C. So c is the focal point f of the concave lens

20cm.The light of the point light source is in all directions, indicating that it is at the focal point of the convex lens, and the parallel light is divergent after passing through the concave lens, and its reverse extension line passes through the focus of the concave lens, and because it can be presented on the light screen, it passes through the focal point of the convex lens (parallel light), so it is calculated as 20cm