Learning objectives:
1, which can distinguish between light source and non-light source.
2. Know that light travels in a straight line in a uniform medium.
3. The formation of shadow and pinhole imaging can be studied by using the linear propagation of light and light.
Know that light travels fastest in a vacuum. Remember the speed of light in vacuum and calculate it according to it.
Preview outline:
1. You are the standard to measure everything, and you are the speed champion in the universe. It was you who lit up the eyes of all the elves, who sent away the night and ushered in the dawn. It is you who light up the road for the vehicles driving at night, and you let scientists see the distant and mysterious space. With you, the earth will breed all things and create brilliant modern civilization.
What does "you" mean in the poem above? What is the truth in poetry?
2. It is called light source. For example,, and are all light sources. The light from the light source enters our eyes and we see this luminous body.
3. Light travels along the same homogeneous medium.
In physics, a straight line with an arrow represents the sum of light, and this straight line with an arrow is called light, and many of them are called light beams when combined.
5. The phenomenon of shadow formation, solar eclipse and lunar eclipse can be explained by the propagation of light along a straight line.
6. The propagation speed of light in vacuum is about the fastest in vacuum, the fastest in water and the fastest in glass.
Learning process:
First of all, the magic of light
Observe and think
Observe the following figure (as shown in figure 4- 1- 1, 4- 1-2, 4- 1-3, 4- 1-4).
Figure 4- 1- 1 Light makes us see objects Figure 4- 1-2 Photosynthesis of flowers and trees
Fig. 4- 1-3 solar-powered vehicles run by light energy fig. 4- 1-4 optical fiber
Combine figures 4- 1, 4-2, 4-3 and 4-4 on page 50 of the textbook with the above figure (as shown in figure 4-1-kloc-0/,4- 1-2, 4- 1-3,
Second, the propagation path of light.
1, light source
Read the "gas station" column on page 5 1 in the textbook to understand what a light source is.
Discuss and think.
The starry sky at night is beautiful and magical. Are these twinkling stars all light sources? Write your opinions and reasons:
(Hint: Our common stars are stars, planets and satellites. )
Among the stars, the star is the light source, and other stars emit light by reflecting light.
2. Light travels in a straight line
(1) experimental exploration
Use the following equipment to read and refer to "Experimental Inquiry" on page 5 1 of the textbook. Can you design several experiments to explore the propagation path of light in "air", "water" and "glass"? Write your experiment plan and conclusion.
The following equipment can be used for reference: laser pen, mosquito-repellent incense, matches, big beaker filled with clear water, milk, dropper, glass brick, etc.
Experiment 1: Explore the propagation path of light in the air.
Equipment you choose: mosquito-repellent incense, laser pen, matches.
Your experimental process: light mosquito-repellent incense, illuminate the smoke emitted by mosquito-repellent incense with a laser pen, and find a straight line.
Your conclusion: Light travels in a straight line in air with uniform density.
Experiment 2: Explore the propagation path of light in water.
The equipment you choose: a big beaker, clear water in the laser pen and a milk dropper.
Your experiment process: fill half a cup of water, add some milk with a dropper to make the solution transparent and white, and shine it on the cup with a laser pen, and it is found to be a straight line.
Your conclusion: Light travels in a straight line (uniform density) in water.
Experiment 3: Explore the propagation path of light in glass.
Your choice of equipment: laser pen glass brick
Your experimental process:
In the darkroom, the laser pointer shines on the glass brick and finds that the light in the glass brick is a straight line.
Your conclusion is that light travels in a straight line in a (uniform density) glass brick.
Sublimation of thinking In the above experiment, what method did you use to represent the light path? Please sum up your experience in time: light can't see the way of propagation in transparent media and needs to be transformed into translucent media.
(2) Light and light beam
Read the "gas station" column on page 52 of the textbook to understand what light is and what light beam is.
Think about whether light really exists?
No, it's just drawn for the convenience of research.
The lamp of gas station is a physical model, and the model method is an important scientific method in physics. In life, we often build models, as shown in Figure 4- 1-5. We can draw the basic characteristics of things with lines, representing the sun, smiling face, anger, running and lifting weights respectively. This method is simple and convenient.
Figure 5- 1-5
(3) the formation of shadows
Read the first paragraph on page 52 of the textbook and know the reason for the shadow formation.
practical activity
Can you play hand shadow games? Please refer to the textbook Figure 4-9 and Figure 4- 1-6 below to try bravely.
Let's compare which classmate performed the most vivid hand shadow.
Figure 4- 1-6
Exercise The following is a student's analysis of the causes of shadows by drawing a light path diagram (as shown in Figure 4- 1-7).
Figure 4- 1-7 Causes of Shadow Generation
Please write down the cause of the shadow in words:
Light travels in a straight line in the same uniform medium and cannot pass through the dark area formed by opaque objects.
(4) solar and lunar eclipses
Read the second paragraph on page 52 and Figure 4-10,4-11to understand the causes of solar and lunar eclipses.
practice
Referring to the textbook Figure 4- 1 1, can you draw a schematic diagram of the cause of the eclipse?
Third, the speed of light propagation.
Read paragraph 1 on page 53 of the textbook, and then answer the following questions:
1. What is the propagation speed of light in a vacuum? 3× 10 to the 8th power m/s
2. What is the relationship between the propagation speed of light in vacuum, air, water and glass?
Vacuum > air > water > glass
3. Observe Figure 4- 1-8. what do you think?
Light travels very fast.
4. When it thunders, why do we always see the lightning first and then hear the thunder?
The speed of light is greater than the speed of sound
Mini-lab
Refer to the "mini-lab" column on page 53 of the textbook and the "gas station" below to complete the production of "coronation instrument" and "pinhole imager"
gas station
1, Corona: The sundial, also known as the sundial, is an ancient instrument for measuring time, which consists of a dial and a dial. The dial is a graduated disk, and the dial pointer is a thin rod installed in the center of the dial and perpendicular to the disk surface. The sundial invented by China in ancient times is unique. Its disk is parallel to the equatorial plane and installed obliquely. In the sun, the shadow of the needle moves on the disk surface with the movement of the sun, indicating different positions and different moments.
2. Pinhole imaging: the production and principle of pinhole camera (as shown in Figure 4- 1-9).
Materials: empty cans, translucent plastic film (or paper), hard paper, etc.
Manufacturing method: make a small hole in the center of the cylinder bottom (be careful not to make the hole too big). Cut off the top of the can and cover it with translucent plastic film (or paper) as a light shield. Then make a paper tube with a thick paper roll just to cover the cans, which makes a pinhole camera.
Usage: aim the small hole at a luminous object, such as candle flame, with a distance of 10~20cm. Observe the image on the screen.
Working principle: As the light propagates along a straight line, the light emitted from the upper part of the candle flame irradiates the lower part of the light screen through the small hole, and the light emitted from the lower part of the candle flame irradiates the upper part of the light screen through the small hole. So the reflection of candle flame will appear on the screen, which we call pinhole imaging. Mo Zhai, a great thinker and scientist in ancient China, was the first scientist in the world to explore the law of straight-line propagation of light and conduct pinhole imaging experiments.
Self-test: (there is an answer later)
1. It is difficult for us to directly see the propagation path of light. What methods are used to study and describe the propagation path of light? What does it inspire you? model method
Zhang Qiang pierced a small hole in the cardboard and looked out through it. As his eyes approached the small hole, he saw the scenery outside (B)
A. get smaller. Become bigger
C. Invariant D. First big and then small
3. Please find out through social investigation or reference materials: Apart from the examples mentioned in the textbook, what other examples have been applied to the linear propagation of light in life? Laser calibration shooting 2 points 1 line
In the school sports meeting, if you are asked to be a timekeeper at the end of the 100-meter race, how can you accurately record the departure time? State your reasons. When I saw the smoke from the starting gun, I began to remember. The speed of light is greater than the speed of sound.
5. Observe Figure 4- 1- 10. What conclusion can you draw?
Light travels fast.
There is a big tree near the playground. On a sunny summer day, Wang Xia found that the sun shone through the cracks in the leaves and made many spots on the ground. Although some spots are big and some are small, they are almost round. Why is this? The sun is round, and the pattern on the ground is based on the shape of the sun imaged by pinhole.
Wang Xia went to consult the teacher. The teacher told her that these light spots are similar to the shape of the sun and are the "images" of the sun; You can use a lighted candle as a light source, use a pointed pencil to make a small hole (about 3mm in diameter) in the center of a piece of paper, try to stand on the table (as shown in Figure 3. 1-5), and put a piece of white paper behind the hole. Try to see if you can see the image of the candle on the white paper at the back. Draw curtains during the experiment to dim the indoor light. )
(1) Please do the pinhole imaging experiment at the same time. Explore how to make the image of a candle bigger or smaller. Write down your findings.
(2) Is the candle made of holes positive or negative? Can you explain it?
Reference answer
1. For example, when studying the propagation path of light in water, the method of "dropping a little milk in clear water" is used to represent the light path; In order to express the propagation of light, people introduce "light" to describe the propagation path and direction of light vividly. People can try to study what they can't see directly and express it intuitively. This is the power of scientific method.
2. analysis: choose b, which can be illustrated by drawing. As shown in Figure 3-L-8, two straight lines are drawn from the eyes to both sides of the hole. According to the law of straight-line propagation of light, within the included angle formed by these two lines, the light emitted by the object enters the eyes through the hole, causing vision. Outside the included angle formed by these two lines, the light emitted by objects can't enter the eyes after passing through the holes, so the eyes can't see these objects. The closer the eyes are to the mouth of the cave, the greater the angle formed by these two lines, and the greater the range that the eyes can see.
3. For example, if a classmate in front of you blocks all the people in front, the team will be lined up; When shooting and aiming, when the eyes see the gap between the target, the sight and the ruler overlap, they can be guaranteed to be in the same straight line; The carpenter can check whether the wooden board is straight at a glance; The sundial used by the ancients to measure time, the shadow cast by the sun to measure time and so on.
4. Start timing when you see the smoke from the starting gun, instead of starting timing when you hear the gunshot, because the speed of light in air is much faster than the speed of sound.
5. Light travels at an extremely fast speed.
6.( 1) The closer the candle is to the hole, the farther the screen is from the hole, and the larger the image will be; Conversely, the smaller the image. (2) As shown in Figure 3. 1-6, since light travels in a straight line in the air, the light emitted from the upper part of the object is projected below the light screen through the small hole; The light emitted from the lower part of the object passes through the small hole and is projected above the light screen, so the image is inverted.
Learning points:
Summary and reflection:
My harvest:
My confusion: