The law of rectilinear propagation of light explains the formation of shadows. Formation of shadow and penumbra
Formation of shadow and penumbra The straightness of light propagation explains the formation of shadow and penumbra. If the size of the source is small or if the source is located at a distance in comparison with which the size of the source can be neglected, only a shadow is obtained. A shadow is an area of space where light does not reach. When the light source is large or if the source is close to the subject, unsharp shadows (umbra and penumbra) are created.
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Physics lesson 7th grade “Light sources. Rectilinear propagation of light. Formation of shadow and penumbra."
UMKPurysheva N.S., Vazheevskaya N.E. "Physics 7th grade"
Solvable educational tasks (in student activities):
reveal the enormous importance of light in the life of humans, animals and plants;
characterize various types light sources;
give definitions to the concepts of point and extended sources;
introduce the concept of a light ray, based on the law rectilinear propagation Sveta;
identify the conditions for obtaining shadow and penumbra, the formation of solar and lunar eclipses.
Lesson type: a lesson in discovering new knowledge.
Forms of student work : group work, individual work, independent work.
Necessary technical equipment:
pocket flashlights with one bulb and several in a row;
opaque obstacles (I had foam balls on stands made from skewers and play dough);
screens (white cardboard) .
Lesson script.
Introduction to the topic.
Teacher:On March 20, 2015, a plane with excellent students on board took off from the runway at Murmansk airport around noon.Murmansk-Murmansk. This strange flight is related to today's lesson topic. What event do you think is connected with this flight? What is the topic of the lesson?
Students:make assumptions and come to the conclusion that the event is connected with an eclipse, the topic of the lesson is with light. Formulate the topic of the lesson.
Teacher: On March 20, 2015 it was possible to observe solar eclipse. The best place observations from the territory of Russia, after being removed from the main territoryFranz Josef Land, was the cityMurmansk, where at 13:18 local time the maximum phase of the partial solareclipses. Schoolchildren-winners of the Physical Olympiadwere rewarded with the opportunity to view the eclipse from an airplane. We will try to figure out how eclipses occur today.
Light sources. Work in pairs.
Teacher:What topic did we study in lately? (the last topic studied was “Sound Waves”). What conditions are necessary for a sound wave to occur?
Students:Sound waves. For the occurrence sound waves you need a source of vibrations and an elastic medium.
Teacher:Do you need a source for light to appear? Give examples of light sources. On the tables you have cards with images of sources. Determine the types of sources and arrange the cards according to your classification.
Two students attach classification cards with magnets to the board. I write the rest in my notebook.
Law of rectilinear propagation of light. Law of independence of light propagation.
Teacher:Imagine that you are walking home from school with your friend Vasya. You turned the corner of the building, but Vasya hesitated. You shout: “Vasya!” And the friend replies: “I’m coming, I’m coming.” At the same time, can you hear your friend? Do you see him? Why is this happening?
Studentsmake assumptions.
Teacher:demonstrates an experiment showing the rectilinear and independent propagation of light (smoky glass vessel, laser pointer). You can invite two students to help.
Students:formulate the law of rectilinear propagation of light and independence of light propagation.
Light propagates rectilinearly in an optically homogeneous medium.
Teacher:E
It was noted by Euclid back in 300 BC that the ancient Egyptians used it in construction. The geometric concept of a ray arose as a result of observations of the propagation of light.
A light ray is a line along which light from a source propagates.
Beams of light rays, intersecting, do not interact with each other and propagate independently of each other.
4 . Practical task. Work in groups.
Teacher:At your disposal are two flashlights, a screen, and opaque obstacles. Using this set, determine how a shadow is formed, what determines its size, and the degree of darkening? You are given 10 minutes to answer these questions. After this time, each group presents their findings.
One of the flashlights contains one small light bulb (conditionally a point source), the second - several light bulbs arranged in a row (conditionally an extended source).
Students:With the help of the first shadow flashlight you get a clear shadow on the screen. They notice that the closer the flashlight is to the object, the larger sizes shadows. They are trying to construct an image of a shadow. They notice that with the help of a second flashlight, the shadow on the screen turns out to be fuzzy. At a certain position of the flashlight and the object, you can get two shadows. They are trying to construct an image of the shadow and penumbra and give an explanation for this result.
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students:sketch a diagram of the formation of shadow and penumbra.
Teacher:Let's draw a beam from a point source (experiment with the first flashlight) along the boundaries of the obstacle (raysS.B.AndS.C.). We got clear shadow boundaries on the screen, which proves the law of rectilinear propagation of light.
In experiments with the second flashlight (extendedsource), a partially illuminated space is formed around the shadow - penumbra. This happens when the source is extended, i.e. Consists of many points. Therefore, there are areas on the screen where light enters from some points, but not from others. This experiment also proves the linearity of light propagation.
Draw the path of rays from red and blue sources with colored pencils. Specify the shadow and penumbra areas on the screen from the opaque ball. Explain why experience proves the rectilinear propagation of light?
6. There is something to think about at home.
Teacher:demonstrates a camera obscura made from a box. Question to students: What is this?
Students:They put forward all sorts of versions that are far from the truth.
Teacher:but in fact this is the “ancestor” of the camera. With its help you can get an image and even take a picture, for example, of this window. Make a camera obscura at home and explain its operation.
7. Homework.
1.§ 49-50
make a camera obscura, explain the principle of operation (links for reading/viewing
Formation of shadow and penumbra. A shadow is that area of space into which light from a source does not fall. Penumbra is the area of space into which light falls from part of the light source. Condition for the formation of a shadow: If the size of the light source is much smaller than the distance at which we evaluate its action (the light source is a point). Condition for the formation of penumbra: If the dimensions of the light source are commensurate with the distance at which we evaluate its effect.
Slide 5 from the presentation "Refraction of Light" 8th grade. The size of the archive with the presentation is 5304 KB.Physics 8th grade
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Let us consider another experimental confirmation of the law of rectilinear propagation of light. Let's do some experiments.
Let's take an ordinary light bulb as a light source. To the right of it we will hang a ball on a thread. When conducting an experiment in a dark room, we can easily see the shadow of the ball on the screen. In addition, in the space to the right of the ball there will be a certain area into which light rays (light energy) do not penetrate. This space is called the shadow area.
Now let's use a light bulb with a white glass balloon. We will see that the ball's shadow is now surrounded by a penumbra. And in the space to the right of the ball there is both a shadow region, where the rays of light do not penetrate at all, and a penumbra region, where only some rays emitted by the lamp penetrate.
Why did penumbra arise? In the first experiment, the light source was the spiral of a lamp. It had small (they say: negligible) dimensions compared to the distance to the ball. Therefore, we can consider the spiral to be a point source of light. In the second experiment, light was emitted from a white lamp bulb. Its size compared to the distance to the ball can no longer be neglected. Therefore, we will consider the balloon an extended light source. Rays emanate from each of its points, some of which fall into the penumbra region.
So both physical phenomena– the formation of shadow and the formation of penumbra are experimental confirmation of the law of rectilinear propagation of light.
