Junior high school optics question, a junior high school physics question about optics

You should choose B!

Analysis: To take a full-body image, it means that the image of the person to be formed 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).

Hope all the help to you and have a nice day! o(∩_o~

b focal length f becomes larger, image distance v becomes larger.

According to 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 magnification b=v u (image distance divided by object distance), so the magnification will become larger.

As the magnification increases, so does the image (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.

Outside the double focal length, the object distance becomes farther away, the image becomes smaller, and the image distance becomes smaller.

Now the bust has changed from a full-body portrait, indicating that the image has become smaller.

Therefore, the object distance should be increased, and the position of the camera and the person cannot be changed, so the focal length should be reduced, and the corresponding image distance will also be reduced.

To be aside, this person is too lazy either. This leads to such a bt topic.

Remember: the near image of the object becomes larger, and the distant image of the object becomes smaller (when the image is real).

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 area I counted was.

I don't know, right.

Picture bad point. Let's make do with it.

Let the height of the street lamp from the ground be x, and the straight-line distance of someone from the street lamp at a certain time is y, and the system of equations can be syndicated according to the known conditions:

x/y=x/(y+2)=

Solve this system of equations to get x = meters.

From this, the height of the street lamp from the ground is meters.

In the Han Dynasty of China, the principle of resonance was used to create "fish washing".This is created by the skilled craftsmen of the Han Dynasty in China, which not only has a high level of technical craftsmanship, but also has a certain understanding of the resonance phenomenon. "Fish wash" is a cast copper basin.

There is a pair of ears on the side of the copper basin, and when you rub your hands rhythmically back and forth on the ears to create a resonance, the water surface in the basin will show complex ripples.

Your question: There is a copper container called "fish washing", which contains water, and when you rub the "ears" on both sides with your hands, you can see the water droplets beating on it, and at the same time, you can also hear the sound of "zizi", and when the hands stop rubbing, you can still see this phenomenon. The root cause of this phenomenon is (a.).

Fish wash "still vibrating)

b, sound is transmitted by air vibrationsI did it in my first year of junior high school, and it was easy!

I think it's supposed to be A, because everything that makes sounds is vibrating!

1) Shadow formation.

Demonstration Experiment Hold an incandescent lamp against a wall and place a leather ball (or other convenient object) between the lamp and the wall.

Phenomenon The shadow of a ball appears on the wall.

Causes of shadow In the process of light propagation, when it encounters an opaque object, because the light travels in a straight line, the light cannot be irradiated behind the opaque object, and the dark part is the shadow.

2) Causes of solar and lunar eclipses.

Demonstration experiment Demonstration of the formation of solar and lunar eclipses with a trisphere.

Phenomenon] Phenomena similar to solar and lunar eclipses occur.

Causes of Solar Eclipse When the Moon moves between the Sun and the Earth, and the three spheres are in a straight line, the sunlight is blocked by the opaque Moon during the propagation of the straight line, and the dark shadow of the Moon falls on the Earth, forming a solar eclipse.

Causes of Lunar Eclipse When the Earth moves between the Sun and the Moon, the sun's rays are blocked by the opaque Earth, and the Earth's shadow falls on the Moon, forming a lunar eclipse.

Solution: Let the diameter of the sun be d

Then: 3 = eleven squares.

d = eleven squares.

Come on, it's just a simple graphical similarity.

Spot Diameter: Diameter of the Sun = Spot to Pore Distance: Distance from the Sun to the Earth.

The answer is to experience this little hole problem for yourself. Slowly you will be able to do it yourself.

Acquaintance with triangles, it's easy.

It is a surgical shadowless lamp.

Principle: Shadowless lamps are not actually "shadowless", it just fades the umbra so that the umbra is not obvious. Shadows are formed when light hits an object. The shadows are different everywhere on Earth.

If you look closely at the shadows under the electric light, you will also notice that the shadows are particularly dark in the middle and slightly lighter around them. The particularly dark part in the middle of the shadow is called the umbra, and the dark part around it is called the penumbra. The occurrence of these phenomena is closely related to the linear propagation of light.

If you place a cylindrical tea tube on the table and light a candle next to it, the tea tube will cast a clear shadow. If two candles are lit next to the tea tube, two shadows will be formed that overlap but do not coincide. The part where the two shadows overlap is completely black, and this is the umbra; The place where there is only one candle next to the umbra is the half-light and half-dark penumbra.

If you light three or even four candles, the umbra will gradually shrink and the penumbra will appear in many layers. The same is true for objects that can produce shadows composed of umbra and penumbra under electric light. Obviously, the larger the area of the luminous object, the smaller the umbra.

If we light a circle of candles around the above-mentioned tea tube, the umbra disappears completely, and the penumbra is too faint to be seen. According to the above principle, scientists have made a shadowless lamp for surgery. It arranges the lamps with great luminous intensity into a circle on the lamp panel to form a large area of light source.

In this way, the light can be irradiated to the operating table from different angles, which not only ensures that the surgical field of vision has enough brightness, but also does not produce obvious umbra, so it is called a shadowless lamp.

Scientists have made shadowless lamps based on this principle and used them on hospital operating tables.

A: Scientists have made (shadowless lamps) based on this principle and used them (to operate on patients).