Reflection of light

Optical phenomenon

open 2 entries with the same name

Collection

zero Useful+1

zero

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

An optical phenomenon. It refers to the phenomenon that when light travels to different substances, it changes the propagation direction on the interface and returns to the original substance. Light will reflect when it meets the surface of water, glass and many other objects. When light changes its propagation direction on the interface between two substances and returns to the original substance, it is called light reflection^[1] 。

The reflected light is on the same plane as the incident light and normal; The reflected light and the incident light are separated on both sides of the normal; The reflection angle is equal to the incidence angle. It can be summarized as: "three lines are coplanar, two lines live apart, and two angles are equal". Light is reversible. In the phenomenon of light reflection, the light path is reversible.

Chinese name: Reflection of light
Foreign name: reflection of rays

Field: optics ， physics
Classification: Specular reflection And diffuse

catalog

formation

Announce

edit

When light shines on the interface between two different media, some light will return from the interface to the original media, which is called light reflection.

classification

Announce

edit

Specular: Directional light When it hits a smooth surface, the reflected light is also parallel. This reflection is called specular reflection.

Diffuse: When the parallel light shines on the uneven surface, the reflection light shines in all directions. This reflection is called diffuse reflection.

Directional reflection: the reflection between diffuse reflection and specular reflection is called directional reflection, also called non Lambertian reflection, which is reflected in all directions, and the reflection intensity in all directions is uneven.

Objects with smooth surface, easy to form light Specular reflection , forming a dazzling light, but can't see the object clearly.

Generally, objects can be identified shape The existence of harmony is due to the diffusion of light.

The reason why objects can be seen temporarily after sunset is that the dust in the air causes light diffusion. Whether specular or diffuse reflection , must obey the law of reflection.

Specular reflection and diffuse reflection both follow the reflection law of light.

distribution

Announce

edit

Point of incidence : the intersection of the incident light and the mirror;

Normal: the straight line passing through the incident point and perpendicular to the mirror surface is called normal ；

Angle of incidence : The angle between the incident light and the normal is called the incident angle;

Reflection angle : The angle between reflected light and normal is called Reflection angle 。

Reflection law

Announce

edit

Reflection of light

one Reflection angle be equal to Angle of incidence And the angle between the incident light and the plane is equal to the angle between the reflected light and the plane.

two Reflected light And Incident light occupy normal both sides.

3. Reflected light, incident light and normal are all in the same plane.^[2]

Four characteristics

Announce

edit

1. Coplanar normal is Reflected light And Incident light Of Angular bisector The line.

Reflection of light

2. Opposite side incident light and reflective surface included angle And the sum of the incident angle is 90 °

3． Equiangular The reflection angle is equal to Angle of incidence 。 The reflection angle increases with the increase of the incident angle and decreases with the decrease of the incident angle.

4. The reversible light path is reversible

As shown in Figure 1 (A) BO The reflected light shines on the interface in the opposite direction of the original reflected light (Figure B) OA Will go against the original incident light AO In the direction of.

Drawing light path according to the reflection law of light

Specific method: find out first Point of incidence , perpendicular to the interface through the incident point normal , the angular bisector of the angle between the reflected light and the incident light is the normal. If the reflected light corresponding to an incident light is determined, the size of the incident angle and the plane of the reflected light are determined by the incident light and the normal, and then the reflected light is determined according to the characteristics of the light reflection law that the reflected light is on the other side of the normal, and the reflection angle is equal to the incident angle^[3] 。

Research

Announce

edit

nature

Reflection is an important property of light. Ancient China has rich knowledge in this field, which is reflected in many practical problems.

For humans, the largest reflection of light occurs on the moon. We know that the moon itself does not emit light, it just reflects the light of the sun. It is said to record the historical facts of Xia, Shang and Zhou dynasties《 The Book of History 》It was mentioned in. It can be seen that at that time, people had the concept of light reflection. Works in the Warring States Period Zhou Bi Li clearly pointed out that:“ sunshine The moon is born and becomes bright. " In the Western Han Dynasty, people simply said that "the moon is like a mirror", so we have a deeper understanding of the reflection of light. 《 Ink classics 》A light reflection experiment is specially recorded in: reflecting sunlight onto the human body with a mirror can make the shadow of the human body lie between the human body and the sun. This not only demonstrates the reflection of light, but also probably explains the cause of the moon spirit.

imaging

Plane mirror Imaging is the result of light reflection. Ancient China was very creative in this respect. In the earliest days, people used the still water surface as the reflection surface of light and used it as a mirror (Figure 12). This mirror was called "Jian". Western Zhou Dynasty The word "Jian" in the gold inscription looks like a person bending over to take his own picture in a plate of water. This shows that three or four thousand years ago, the method of using water surface reflection imaging was prevalent. In the Ming and Qing dynasties, some poor families also used "water mirrors". 《 The Scholars 》It's written in Hu Butcher It's not about his son in law who is too popular Fan Jin Go pee and look after your description? Although this is not elegant, it is still a legacy of the water mirror. Butcher Hu is by no means the inventor. In the middle of the Zhou Dynasty, with the progress of smelting technology, the water mirror was gradually replaced by a metal reflector, which made the word "Jian" next to the word "Jin" and became a "Jian" or "Jian", which is the so-called bronze mirror unearthed in large quantities. as for Glass Mirror, it's even later.

Ink classics

about Plane mirror The study of reflection imaging law was carried out in the late Zhou Dynasty. Mo Jing It is pointed out that there is only one image formed by a plane mirror; Like shape , color, distance, front and back are all the same as objects. It also points out that when the object moves closer to the mirror, the image also moves closer to the mirror. When the object is far away, the image is also far away Symmetry relation 。 This summary is completely correct.

influence

Announce

edit

Spherical mirror

Spring and Autumn Period and Warring States Period , also appeared sphere Reflectors are so-called spherical mirrors. The root reflection surface is concave and convex, which can be divided into Concave spherical mirror And convex spherical mirror. Objects placed in front of the mirror can be imaged in the mirror. A concave spherical mirror can make a beam of parallel light reflect and meet at a point, which is called focus. The convex spherical mirror is a divergence mirror, and the focus is a Virtual focus 。 Because there is heat energy in the sun light, if it is concentrated at one point, it will not only be bright, but also generate more heat, which can raise the temperature of the object and ignite it. (Note: Do not connect concave mirror, convex mirror and concave mirror, Convex lens Confusion. Convex lens Focus the directional light; The concave lens makes the parallel light diverge and has a virtual focus. The light path is reversible)

In Europe, legend ancient Greek At that time, the Romans launched a large group of warships to attack Syracuse. Archimedes, a physicist at that time (287-212 BC), once used a huge Concave mirror Facing the sun, gather the light on the warship and burn it, thus winning the war. Of course, this is just a legend. In ancient China, concave mirror was really a major tool for people to make fire.

Used to make fire

China knew as far back as the Zhou Dynasty to use this reflection phenomenon to make fire. At that time, the concave mirror for burning was called "flint". In Zhuangzi, it is recorded that "the sun tunnel sees the sun, and it burns into fire." In Huainanzi of the Western Han Dynasty, it was further pointed out that the concave mirror should not be placed too far away from the mirror or too close to the ignition objects such as wormwood. This actually points out that wormwood must be placed above the focus. In the Eastern Han Dynasty, people became more knowledgeable about the reflection of light. They learned that, except for the special yang tunnel, all reflective surfaces with concave spherical surfaces could focus on the sun and get fire as long as they were rubbed bright enough. At that time, someone used the bottom of a metal cup to focus on the sun and make fire, among other things. This shows that the scope of people's understanding has expanded. Shen Kuo of Song Dynasty analyzed the principle of using concave mirror to focus on the sun. He is《 Mengxi Writing 》In the middle, it said: "The sun flint has a hollow surface. When facing the sun, the light is concentrated inside. It is one or two inches away from the mirror. It is as big as a hemp tree. When something is touched, it will ignite. This is the thinnest part of the waist drum." It means that the reflecting surface of the sun flint is concave. When facing the sun, the light is reflected at one point (the focal point) Point spacing The mirror is about an inch or two, which is as big as sesame beans. If it falls on an object, it will get angry, which seems to be the thinnest part of the waist drum. Here, the reflection of light by spherical mirror is correctly described, and the focus and focal length All have been described. In particular, it can point out where the focus is, just as it is the smallest part of the waist drum, which is vivid and specific, making it easy for people to understand. According to Shen Kuo, the study of this kind of knowledge at that time was called "Geshu", which constituted a special knowledge. It must be very in-depth, but it has been lost, which is a pity.

lenses

When it comes to reflection, we have to introduce a wonderful mirror in ancient China, which is called "transparent mirror". The shape of the "transparent mirror" is exactly the same as that of the ancient ordinary copper mirror. It is also made of metal, with patterns and characters on the back. The reflective surface is polished very bright and can shine. It is reasonable to say that when a ray of light shines on the mirror and is reflected and thrown on the wall, it should be a plain round bright area. Miraculously, in this bright area, there appears a pattern on the back of the mirror, which seems to be "transparent", so it is called "transparent mirror". Shanghai Museum Treasured side Western Han Dynasty transparent mirror There are eight words on the back, "Light of the Sun, Great Brightness of the World", and even the patterns are "transparent" in the bright area, clearly visible. This is really unthinkable. Not only Chinese scientists of all ages have studied it, but also many foreign scientists in modern times have been surprised. They called it "magic mirror" and have studied it in an attempt to uncover the mystery. During a period of the nineteenth century, there were heated discussions, but none of them received satisfactory answers. In recent years, Chinese scientists have made gratifying achievements in the research of transparent mirrors by means of modern science and technology.

Ancient Mirror

A large number of bronze mirrors have been unearthed in China, most of them before the Qin Dynasty. However, the "light transmission" test was not carried out one by one, so it is not sure that there must be no light transmission mirror in it. In the Song Dynasty, Shen Kuo, the author of Six Memories of a Floating Life, hid a transparent mirror in his home. The words behind it were so ancient that he did not even know the knowledgeable and profound person himself. It can be seen that this is not an ordinary writing, but something that may have existed before the Qin Dynasty. One side of the collection of the Shanghai Museum has been identified as a relic of the Western Han Dynasty. After the Western Han Dynasty, more and more people can make it. In the Qing Dynasty, mirror workers from Jiangsu and Zhejiang could also be produced and spread to Japan. As for the written records, the novel "Ancient Mirror" in the Qing Dynasty describes an "ancient mirror". When "inheriting the sunshine, you can recite the above painting, and ink can enter the shadow without losing anything." It is obviously a transparent mirror here. Shen Kuo of the Song Dynasty recorded the light transmitting mirror in great detail. Since then, scholars of all dynasties have recorded more poems.

theory

How does a copper mirror transmit light? Shen Kuo was the first person to study and record this aspect. He is《 Mengxi Writing 》It says: "There is a kind of transparent mirror in the world... Put the mirror in the sunlight, and the patterns and twenty characters on the back of the mirror are clearly reflected on the wall of the house. It was explained that because thin areas were cooled first when casting, and the areas with patterns on the back were thicker, the cooling was slower, and the copper contracted more. Therefore, although the text was on the back, the front of the mirror also had a faint trace, so it would appear in the light. I inspected it and thought it was right. There are three such mirrors in my family. When I see the collections of other families, they are all the same. There is no difference in the patterns and inscriptions. The style is very ancient. Only this kind of mirror can transmit light. Some other mirrors, even thin, cannot transmit light. It must have been made by the ancients in another way. " In this, Shen Kuo explained the principle of "light transmission", the main point is that "although the text is in the back, there is a trace of recognition." This is very correct. Because there are patterns on the back of the mirror, the mirror surface is similarly uneven, but the fluctuation is so small that the naked eye cannot see it. When it reflects light, it can be reflected on the screen due to the long path amplification effect. This is why physicists in the Qing Dynasty Zheng Fuguang It also makes a very appropriate explanation. He pointed out that the still water surface is very flat, but when the light reflected by it is thrown on the wall, it also sees a little turbulence, because the water surface actually has undulating waves. This explanation is so specific and accurate that the British physicist Bragg made a similar explanation of this problem when he discussed the "transparent mirror" in the 1920s and 110s of this century.

skill

What method did the ancients use to make the front of the bronze mirror have patterns similar to the back of the mirror? According to Shen Kuo's records, people before the Song Dynasty believed that it was because there were protruding patterns on the back of the mirror, so the thickness of each part was different, the cooling time was different when casting, and the shrinkage degree was different, so the mirror surface was "vaguely marked". Shen Kuo agreed with this explanation, and many foreign scientists in this century also agreed. In 1975, someone used experimental methods to prove that it was correct. But an archaeologist in the Yuan Dynasty was named Wu Qiuyan , propose another explanation. He said that another copper material was used to insert a pattern exactly the same as the back on the mirror surface, and then the mirror surface could be "vaguely marked" after polishing. In this way, the ability of each part of the mirror to reflect light is different, so the pattern can be seen in the reflected bright area. This explanation is also reasonable, and I Qiuyan personally saw someone break a transparent lens to check it in order to verify his statement, which proved to be true. Later, scientists in the Ming Dynasty Fang Yizhi , support this explanation and supplement it. There is no doubt about it. In addition to these two methods, there is another method, which is to scrape and grind the mirror surface with a couch grinding stick after casting the bronze mirror. The thin part is pressed and grinded, and slightly bulges toward one place, pressure After removal, these thin parts still protrude slightly. If the mirror is polished with mercury paste, the thin parts will expand slightly and become more bulged, so the mirror surface will be "indistinct". This method was spread to Japan, and they are still using it to make transparent mirrors. The trial production in Europe according to law has also been successful. It can be seen from the above that there were various methods of making transparent mirrors in ancient China. No matter which method is used, the process requirements for making a transparent mirror are very high. In ancient times, it was amazing to be able to do so. It's a pity that such a unique skill, "the final secret is not announced", has lost the manufacturing skill of transparent mirror. What a pity!

Different light reflection

8. When light is reflected, some of it will be absorbed by the object.

Each light is a mixture of red, green and blue light in different proportions. All objects we can see except black must be reversed Chromatic light 。 A white object reflects all the light, which looks white. A gray object also reflects light of all colors, but only partially. As long as other objects are of any color, they will reflect light that can be mixed in a certain proportion and is natural color.

attach : Red light plus green light is Yellow light Green light plus blue light is cyan light, red light plus blue light is purple light, and red, green and blue light together is white light.

Through white light prism Different color Monochromatic light The deflection of red light is the smallest, so a disk is at the top when imaging on the light screen, and the blue light is the largest.

The color of the transparent body is determined by the color of the light transmitted. (When the light passes through the transparent body, the color of the transparent body can pass through the light of any color)

The color of an opaque object is determined by the reflected light color. (When light encounters an opaque body, the color of the opaque body can reflect the light of the same color)

experiment

Announce

edit

Purpose and requirements

Summarize the rules of reflection when light strikes the object surface:

1. The reflected light is in the same plane as the incident light and normal, and the reflected light and incident light are separated on both sides of the normal.

2. The reflection angle is equal to the incidence angle.

Instruments and equipment

Fig. 2.3-1

The structure of the light reflection demonstrator is shown in Figure 2.3-1, M It is a flat mirror, inlaid on a board, with a white light screen E Vertically fixed on board, white light screen F It can wrap around the ON axis turn ， E 、 F The screen is painted with O It is an arc at the center of a circle, marked with a scale.

Parallel light source (J2501 type Optical disk Light source), low-voltage power supply (J1201 type).

[Experimental method]

1. Adjust the position of the light source to make a beam of light along the plane E Any line inside AO Projected on a plane mirror O Point. Around ON Shaft rotation plane F , looking for O The light reflected by a point is only available when the plane F Flat surface E In the same plane F Reflected light seen above, plane F In other locations, F There is no reflected light on the. It is concluded that the reflected light is in the same plane as the incident light and the normal, and the reflected light and the incident light are separated on both sides of the normal.

2. Plane F and E When in the same plane, the reflection of light is observed E 、 F Read the values of the entrance and exit angle and the reflection angle on the screen, and get that the reflection angle is equal to the incidence angle.

3. Change the angle of incident light in plane E and repeat step 2. Conclude Reflection law 。

4. Make the plane first F and E At any included angle. Rotate the position of the light source at E Reflected light is seen in the plane. Rotate plane E to find the plane where the incident ray and normal lie, and deepen the understanding of the reflection law.

matters needing attention

1. Lack of junior high school students space Imagination, understanding the spatial relationship between the line and the surface of the reflection law is a difficulty. During the experiment, attention should be paid to helping students establish the concept of space and intuitively establish the reflection image of light.

2. The incident light is a light band with a certain width. In order to show the route of light propagation on the light screen, the axis of the incident light band should form a small angle with the light screen. Similarly, the reflected light also has a certain width of light band, so when the plane E And F When there is a small angle, the light screen F can still have short bright lines. During the experiment, we should try to reduce the width of the light band and avoid the position of this small angle to avoid confusion.

Experimental requirements

Fig. 2.3-2

1. In addition to the J2507 light source, the parallel light source can also use laser Flashlight (Good polymerization is required Photoefficiency Fruit) or sunlight. There are two requirements for light source in this experiment:

(1) Wide beam light sources need to be equipped with slits, through which a thin beam of parallel light is taken as the incident light;

(2) It can change the angle of light cast on the mirror. For example, taking sunlight as light source, you can use a pair of plane mirrors Q one and Q two To project a beam of sunlight onto a flat mirror M Up, slightly move and rotate the flat mirror Q two , you can make projection to O The light angle of the point changes (Fig. 2.3-2).

Fig. 2.3-3

2. Light reflection experiment is also available Miller Instrument (Fig. 2.3-3) or circular disc (included in J2501 optical disc). Because the mirror in these instruments can be around the incident point O It is convenient to verify that the reflection angle is equal to the incidence angle, but it is not easy to explain the spatial relationship 。

Tip: This small experiment can be supplemented by“ Photophenomenon ”Part of the physical experiment teaching, so as to cultivate and improve students' Experimental ability And literacy.

Let the light ray shoot against the direction of the reflected light ray to the mirror. It can be seen that the reflected light ray shoots against the direction of the original incident light ray.

This shows that, In reflection, the light path is reversible.

principle

Announce

edit

Light is also a kind of wave. The reflection of light can be caused by the famous phenomenological theory Huygens principle Explain^[4] 。

Reflection of light

As shown in the right figure, a column of parallel light waves shoots at the obstacle (or plane mirror), a ， b ， c It's the three lines of this light wave Waveline (light), because the wave line has an intersection angle with the obstacle a ， b ， c It takes different time to reach the obstacle, wave line a Arriving at obstacles A Point A Wavemaking line b and c Vertical line of AC 。 Then when the wave line a To A Point time, wave line b Just arrived B Point, wave line c Just arrived C Point. Waveline a Transmitted to obstacles A Point, a new wave source (i.e. wavelet source) will be formed A ）, and circumference Type to spread new wave lines in all directions. hypothesis c from C Point to obstacle C ’The time required for the point is t ， a，b，c If the velocities of the three wave lines are identical, the wave source A Around t Propagation distance in time (i.e. circle A Radius) and wave line b stay t Time passed B Point transmitted to obstacles B ’The sum of the distance that a point travels around in the form of a wavelet source（ BB ’+Circle B ’) are equal, both are CC ’Because the three waves have the same velocity and time, the propagation distance is equal. At this time C ’（ P Point) Make a circle A And circle B ’Of tangent , tangent point is M ， N ， P (Because C ’It will also form a sub wave source, which is about to launch waves around, but it has not yet formed Wavefront , so C ’Points can be treated as circles C ’）。 according to Huygens principle The blue line in the figure is the three sub wave sources A ， B ’， C ’Three formed wavefront Of Envelope surface (new wave front formed after reflection), the new wave line formed is always perpendicular to the envelope surface, then connect AM ， B ’ N ， C ’（ P ）。 AM ， B ’ N ， C ’（ P ）The length of is equal to the respective radius.

Certificate: ray AM ， B ’N， C ’(P) Three wave lines a，b，c Reflected wave line of

Solution: Use junior high school congruent triangle to prove that three wave lines a，b，c They are parallel to each other, so ∠ 1=∠ BB ’ A =∠ CC ’ A 。 Because in right angle △ ACP And right angle △ PMA Medium, AM = CP ， AC ’= AC ’, so Rt △ ACP ≌Rt△ PMA (HL), so ∠ 1=∠ CC ’ A =∠ 2, so the incident angle is equal to the reflection angle to prove the wave line AM It's really a wave line a Of reflected wave Line, similarly provable wave line BN ， C ’（ P ）It is also a reflection wave line.

The above process explains the reflection principle of light as a wave. total reflection The phenomenon that light is reflected back into the original medium when it is reflected from the dense medium (i.e. the medium in which light travels at a lower speed) to the interface of the sparse medium (i.e. the medium in which light travels at a higher speed).

When the light hits the interface of two media, it will only reflect without refraction. When the light shines from the dense medium to the sparse medium, the refraction angle will be greater than the incidence angle. When the incidence angle increases to a certain value, the refraction angle will reach 90 °. At this time, there will be no refraction light in the sparse medium. As long as the incidence angle is greater than the above value, there will be no refraction, This is total reflection. Therefore, the conditions for generating total reflection are: ① light must be emitted from the dense medium to the sparse medium; ② the incidence angle must be greater than the critical angle

The critical angle is the corresponding incident angle when the refraction angle is 90 degrees (total reflection will occur only when the light enters the light sparse medium from the light dense medium and the incident angle is greater than the critical angle)

essence

Announce

edit

Light wave is a kind of high-frequency electromagnetic radiation^[5] 。 The interaction between light waves and atoms and molecules leads to the change of propagation direction, phase and polarization, as well as the redistribution of energy and energy flow.

The atomic limit is in the order of ∨, 1mm two Surface arrangement approx. 10 seven ×10 seven Atoms. Under the action of light wave alternating electric field, atoms on the interface make forced oscillation. Each atom is an electric dipole oscillator, forming a plane antenna array. Due to interference, radiation in a certain direction is formed. At normal incidence, all vibration elements are excited in the same amplitude and phase, and the beam is perpendicular to the vibration surface and has two lines: one is called reflection wave, and the other is called refraction wave. Oblique incidence is equivalent to different excitation phases, similar to phased array antenna.

Typical examples

Announce

edit

Example 1

Why close one eye when shooting and aiming?

Resolution: We know that there is a collimator for aiming at the front end of the barrel. When we close one eye and observe that the collimator blocks the target, it means that the collimator, the target and the eye are in the same straight line, that is, they are aiming at the target. In fact, this is the reason why the applied light travels along a straight line in the same uniform medium.

Example 2

When the light is vertically incident on the plane mirror, the included angle between the reflected light and the incident light is ____. If the direction of the incident light is unchanged and the plane mirror rotates at angle a, the included angle between the reflected light and the incident light is ______.

Resolution: When the ray is perpendicular to the incident ray, the reflected ray will be reflected against the incident ray. At this time, the incident ray, the normal ray and the reflected ray coincide. The reflection angle is 0, and the incident angle is 0. Therefore, the included angle between the reflected ray and the incident ray is 0. When the plane mirror rotates, there is an included angle between the normal line and the incident light. This included angle is the angle that the plane mirror rotates. Then the incident angle is a , the reflection angle is also a Therefore, the included angle between reflected light and incident light is 2a.

Example 3

To make the included angle between the reflected light and the incident light at a right angle, the included angle between the incident light and the plane mirror should be _____.

Resolution: According to the reflection law, the reflection angle is equal to the incident angle. If the included angle between the reflected light and the incident light is a right angle, then the reflection angle is equal to 45 °, the incident angle is also equal to 45 °, and the included angle between the incident light and the mirror should be 45 °.

Example 4

If the angle between the incident light and the plane mirror is, the reflection angle should be ______.

Resolution: First of all, we should make clear the angle of incidence. The angle between the incident light and the plane mirror is, so the incident angle is, because the incident angle is the angle between the incident light and the normal, the normal is perpendicular to the mirror surface, and the reflection angle is equal to the incident angle, so the reflection angle is also equal to.

Example 5

As shown in Figure 2, when the light shines on the plane mirror, the reflected light shines in the direction. Please draw the position of the plane mirror.

Resolution: Connect the reflected light with the incident light, and the point is the incident point. Because the reflection angle is equal to the incident angle, the normal is the angular bisector, and the normal is perpendicular to the plane mirror, so the position of the plane mirror can be drawn.

The specific method is: intersect and connect at the point, then draw the bisector of the angle, that is, the normal line (pay attention to draw dotted lines), and then make it perpendicular to the normal line to represent the plane mirror. As shown in Figure 3.

Example 6

out-of-service Linear propagation of light The phenomenon to be explained is ()

A、 solar eclipse and lunar eclipse B. Shadow formation

C、 pinhole imaging D. In case of thunder and lightning, see the lightning first and then hear the thunder

Resolution: When the moon moves between the sun and the earth and is on the same straight line, because the light travels along a straight line, the moon blocks the light from the sun to the earth, a solar eclipse occurs. When the earth moves between the sun and the moon, and is on the same straight line, the earth blocks the light from the sun to the moon, and a lunar eclipse occurs. The formation of shadow is the shadow area left by the opaque object when light travels along a straight line. The keyhole imaging is also due to the light propagating along a straight line. The light of the candle passes through the keyhole and forms an inverted candle flame image on the screen. It can be seen that options A, B and C can be explained by the linear propagation of light. Occurred thunder When you first see the lightning and then hear the thunder, the reason is that the speed of light propagation in the air is much faster than the speed of sound propagation, so option D cannot be explained by the linear propagation of light.

Example 7

At night, spread a white cloth on the table, place a small flat mirror on the white cloth with the mirror facing up, and let the light of the flashlight shine directly against the flat mirror. From the side, the white cloth is illuminated, but the flat mirror is relatively dark. Why?

Resolution: Because the mirror surface is very smooth, the light perpendicular to the mirror surface is reflected back vertically, and there is very little light in other directions. From the side, there is basically no light in the eye, so the plane mirror looks dark. The surface of the white cloth is rough. The light incident on the white cloth has diffuse reflection instead of specular reflection. The reflected light can reach all directions, so the white cloth is bright from the side.

Diffuse reflection

The uneven surface will reflect light in all directions, which is called diffuse reflection

For example, when the sun shines on a mirror, you can see dazzling light in the direction of reflected light. If the sun shines on the white paper, no matter in which direction, it will not be dazzling.

It turns out that the mirror surface is very smooth, while the white paper that looks very flat is actually uneven in the details. This is the use of diffuse reflection.

Novice on the road

Growth task Getting Started with Editing Edit Rule Edit by myself