Light and shadow
What do we teach here?

Let's clarify the basic concepts of light education, such as light source and light beam ​​​​​​​. Let's see what light sources exist and how to apply the light emitted by them to drawings.

But what happens when there are several light sources and their lights merge?

Light travels in a straight line. If none of the light rays of the light source reach an area, a full shadow is created there; if only some of the rays of the light source reach it, a penumbra is created there.


Learning path

Learning focus

We describe light sources and the light emitted by light sources using light rays. Describing the appearance of shadows in the drawings.



Metoodilised juhised

In the study of light, a light beam is the basic physical model used to explain phenomena. So it is very important that the student understands what it is that he is drawing as a light beam.

Another important issue is the light emitted by the light source. This is very often the root cause of misunderstandings - students do not understand how the light source emits light, which is expressed in the fact that they cannot reasonably transfer the light rays to the drawing.

Finally, it is particularly difficult to understand the light emitted by a large light source. So you could start with simpler topics and move on to more complex ones. The basis of everything is a point light source - this is generally understood. If there are several point light sources, their lights merge; this is the next stage. Finally, there is a transition to a light source, which consists of point light sources placed close to each other, the lights of which again merge.

Putting the drawings on paper by hand could give confidence that the student can do it for real. The videos show how it can be done. One good option seems to be to carry out the discussion on the board using the same magnet system as the students.



Warm-up

Would you draw the Sun?

Everyone understands that the picture shows the Sun. But does this image also convey, for example, what shadows are created in the light of the Sun? Or that we are warmer in the summer than in the winter?
Small light source: light and shadow

Väikese valgusallika valguse katseid saab teha kas üksiku LED-lambiga või taskulambiga, kui kasutada selle otsas olevaid LED-lampe ja keerata see küljele, nii et üksikute LED-lampide tekitatud valgusvihud kattuvad. Kui kasutada üksikut LED-lampi, siis järgnev tööleht näitab, kuidas seda töökorda seada. 

Let's set up a single LED lamp

Connect the LED lamp to the battery, attach the resulting light source to a paper clip and create a light trail on the base. Answer the question.

Alustame punktvalgusallika valguse uurimisega. Siin võiksid õpilased veenduda, et kiired tõepoolest koonduvad valgusallika ehk LED-lambi juures.

Light emitted by a small light source

Examine the light emitted by a small light source using slitted paper. Answer the question.

Oleme nüüd uurinud väikese valgusallika valgust ning kandnud selle joonisele. Edasi kasutame seda teadmist ja oskust selleks, et konstrueerida väikese valgusallika valguses tekkivaid varjusid.

A shadow created by the light of a small light source

Do an experiment where a shadow appears behind an obstacle under the light of a point light source. Make a drawing. Answer the question.
Large light source: light and shadow

Igapäevases kasutuses olevad valgusallikad ei ole peaaegu kunagi väikesed, nagu üksikud LED-lambid. Liigume nende teiste, "suurte" valgusallikate valguse kirjeldamise poole läbi selle, et uurime mitmest LED-lambist koosneva valgusallika kiiratud valgust.

A shadow created by the light of several point light sources

Place a resistor in the light of several LEDs. Study the resulting shadow. Answer the question.

Kui mahutame valgusallikasse rohkem ja rohkem väikseid valgusallikaid, saame lõpuks nn suure valgusallika. Proovime nüüd konstrueerida ja ennustada sellise valgusallika valgusesse asetatud takistuse taha tekkivat varju.

A shadow created by the light of a large light source

On paper, construct the shadow of a resistor placed in the light of a large LED lamp. Do an experiment and make sure that the drawn shadow actually appears.

Järgnevad kaks töölehte on usutavasti kasulikud, kui tahame õpilastes kujundada oskust valgusõpetuses õpitud teadmistega reaalse maailma nähtuseid selgitada. Sest reaalselt oleme me ümbritsetud suurte valgusallikatega, st valgusallikatega, mille mõõtmeid ei saa arvestamata jätta. Aga kuidas selliste valgusallikate kiiratud valgust kirjeldada?

Study of light from a large light source

Explore the light of a larger LED flashlight with a slit system. Explain the result of the experiment. Answer the question.

Selle tunni viimase harjutusega anname füüsikalise selgituse kõige tavapärasemale Päikese ja valguskiirte kujutamisele. Mõistame, et kui joonistame Päikesest igas suunas väljuvaid valguskiiri, siis kujutame Päikest kui kõigis suundades valgust kiirgavat punktvalgusallikat. Ja see ei ole väga vale ettekujutlus - ainus „patt" on see, et Päike ei asugi nii kaugel.

A large light source is not large from a distance

Do the following thought exercise so that you can better understand what you have drawn the next time you draw the Sun.
Knowledge control

Esimene neist töölehtedest on lihtsam - uuritakse ainult väikeste valgusallikate valgust ja varjusid. Teine on keerulisem, seal tekivad ka poolvarjud.

Knowledge test: light and shadow I

Check your knowledge and understanding by answering the questions.

Knowledge test: light and shadow II

Check your knowledge and understanding by answering the questions.
Summary

Light propagation and light beam

The light spreads in a straight line in the room. We represent the direction of propagation of light rays in the drawings as straight lines, which we call light rays .

Point light source

A point light source is a light source in which light is emitted from only one point.

Combining light from multiple light sources

The lights emitted from several light sources merge and do not "interfere" with each other. If, for example, two identical lamps illuminate the paper from the same distance, we see the paper twice as bright.

A real light source as the sum of point light sources

We treat a light source of any shape and size as a sum of point light sources. The light produced by a light source is the sum of the lights of the point light sources that make it up. When depicting such light sources in the figure, we mark only the rays that are important for the performance of one or another specific task or purpose.

Shadow

Light travels in a straight line. If none of the light rays of the light source reach an area, a full shadow is created there; if only some of the rays of the light source reach it, a penumbra is created there.

"Important rays" in the construction of shadows

Since light spreads in a straight line, when constructing shadows, you can find the rays that separate the shadow area from the illuminated area - light cannot get to one side of such rays, because no straight line can be drawn there from the light source. Such rays thus define the shadow region and can be called "significant rays".

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