An optics problem in the second year of junior high school, and a junior high school physics problem

To draw an optical path diagram, first determine the length of the object, the distance between the object and the lens, the axis, and the focal length. Remember: the over-focus light passes through the lens to become parallel, and the parallel light passes through the lens to cross the focus, in two steps:

1. Make one end of the object a point, and make a parallel and axis line to intersect with the lens. Make a straight line from this intersection point to the focal point of the lens (on the other side of the object and the lens).

2. The same end of the object intersects the lens with a straight line (the object is on the same side as the lens). The straight lines passing through this intersection point are parallel to the axis.

The intersection point of the line drawn in the first and second steps is the position of the image at the end of the object, and the other end of the object also does the same step, that is, the optical path diagram can be obtained!! You'll also notice that the line that passes through the center of the lens doesn't turn!!

You always note the following (assuming the path of light is from left to right):

1.The left side of the lens is the object space, and the right side of the lens is the image space;

2.The object focus of the positive lens is on the left, and the image focus is on the right side of the lens;

3.The object focus of the negative lens is to the right, and the elephant focus is to the left of the lens;

The above is the basic definition of a lens, and the following is a drawing.

1。The light rays parallel to the optical axis pass through the lens through the "elephant square focus";

2。The light passing through the "object focus" passes through the lens and is parallel to the optical axis;

3。The light passing through the center of the lens (commonly known as a node) does not change.

The above 3 are the drawing guidelines for the optical path diagram, and the size and position of the image can be drawn by following 1 or 2 points of the object, regardless of whether it is a positive or negative lens. Distinguish between positive and negative lenses, like the correct position of the focal point.

The basic drawing method is to draw a horizontal line starting from the apex of the object height, and connect it to the "elephant square focus" after passing through the lens

Also take this point as the starting point, draw a straight line through the "object focus", after passing through the lens is a horizontal line, the intersection point of the two rays is the image of the point of the object height, and the vertical line of the optical axis is a few positions at the height of the image.

The optical path outside the focus of the positive lens is easy to draw, and there are skills in drawing in the focus of the positive lens and the negative lens, but follow 1 and 2.

If the object distance is greater than one time the focal length, just draw a line from one end of the object through the optical center, and the light direction of the optical center will not change, and then you can know whether to zoom in or out according to the image distance.

Damn, I'm most familiar with this, but how do I draw, depressed!

To take a full-body image means that the image of the person to be made should be reduced. So either stay away from Xiaolan or turn the focus down. However, the position cannot be changed, so the focal length can only be reduced. When the focal length is smaller, the distance is also reduced.

The reason for the smaller focal length: The theory is not easy to demonstrate, let's take an example, if the object distance is originally between 2x the focal length and 3x the focal length. We reduced the focal length, but the object distance didn't change, so it was between 3x and 4x focal lengths.

At this time, it is equivalent to the object distance becomes larger, so the actual image becomes smaller. (just an example).

Reasons for the decrease in image distance: As mentioned above, decreasing the focal length is equivalent to increasing the object distance, and according to the law of convex lens imaging, if the object distance becomes larger, the image distance must be reduced).

Technically, it is based on Gauss's formula: (1 u) + (1 v) = (1 f).

When f becomes larger, the right side of the equation becomes smaller, while u is basically the same, so (1 v) is smaller, that is, the image distance v becomes larger.

Then according to the formula of magnification.

The magnification b=v u (image distance divided by object distance), so the magnification will be larger.

The magnification increases, and the image of course becomes larger (the definition of horizontal magnification is.

The ratio of the size of the image to the size of the object).

The focal length is determined by the lens. The focal length f changes because the camera's objective lens is a lens group of multiple lenses (as you can see from the diagram), and by changing the distance of several lenses, the focal length f of the entire objective lens is changed

Correct answer: B

Because I took a full-body photo later, the image is actually smaller than the original; According to the change law of the real image when the object moves: the object is near, the image is far, and the image becomes larger; If the object is far away, like near, and like smaller, and the object distance does not change, the focal length becomes smaller, which is equivalent to the object becoming farther away.

Because the image becomes smaller, the image distance becomes smaller. Magnification = image distance object distance, so choose: b

The picture is finished for you, and I hope to give points.

The picture upstairs is wrong, mine is the correct solution.

The shade of blue is the range that can be seen.

1. The glass has a certain thickness, and the two sides become two images.

The height of the candle is not on a horizontal line.

2. Connect OC (apostrophe) to get the intersection point of OC (apostrophe) and AB, and then connect C to that intersection point.

1.Flat mirrors can only multiply virtual images.

2. Extend AB to find the symmetry point D of C with respect to AB, connect OD to AB and connect E to OE and CE.

Diffuse reflection is a way to see inclearly and incompletely, because the reflected light in only one direction is received by the human eye.

Non-luminous objects are those that cannot emit light on their own, and some can reflect light, such as the moon, which does not emit light itself, but only reflects sunlight. Diffuse reflection is due to the irregularity of the reflector, and the flatter the surface, the stronger the directionality of the light, such as a plane mirror.