Weather Changes – Investigation 2 CAP
Weather Changes: Investigation 2 – CAP
In this CAP we would like for you to consider the differences in the transparency of objects. Some objects are transparent and transmit light through them. Such objects include glass, plastic wrap, and water. Light passes through such objects and they produce little, if any, shadow. Other objects do not transmit light but rather reflect or absorb light. Such objects produce shadows.
Actually, a non-transparent object both reflects and absorbs light. We do not see the colored wavelengths of light that an object absorbs but rather see only the wavelengths of light that the object reflects. Thus, an apple absorbs most wavelengths of light but reflects those wavelengths in the red range. We see only the reflected light, thus the apple appears red to us. A green leaf absorbs most wavelengths of light but reflects green wavelengths. Thus, our eyes detect the reflected green light and we perceive the leaf to be green.
In this CAP we will NOT discuss light absorption but rather focus on the difference between reflected and transmitted light and the formation of shadows.
This slide introduces both the concept of reflection and transmittance of light. Essentially, reflected light is that light that allows us to see an object. Transmitted light is light that passes right through the object. The position of the two eye icons tries to explain that we see the light that is transmitted through a transparent object (lower eye) whereas we see the reflected light that bounces off the surface of a non-transparent object.
Some objects are not totally transparent. Colored water and sunglass lenses are examples of objects that transmit some light and reflect some light.
This slide begins the discussion of shadows. You may have noticed that it appears darker during the day when there are many clouds in the sky as compared to a bright, cloudless day. This is mainly because the clouds are not completely transparent and therefore block some of the Sun’s rays from reaching the surface of the Earth. Therefore, clouds cast shadows on the Earth’s surface.
If one looks out across a large open field on a partly cloudy day, one can actually see the shadow of individual clouds on the field. The shadows move across the field as the clouds move past the Sun. The video below shows this effect on hilly terrain.
This slide simply extends the discussion from above by showing a break in the cloud cover through which Sun rays can pass unimpeded and reach the Earth, creating patches of brighter spots on the ground. Which of the Sun’s rays in this picture will most easily reach the Earth?
This slide introduces the vocabulary word shadow. You will be very familiar with shadows from your previous experiences. For the rest of this CAP, we wish to use previous experience with shadows and teach you that shadows are caused by objects that do not transmit light; that is non-transparent objects like the people that produced the shadows shown in this slide.
Try to identify the location of the shadow that is cast by this sphere. Based on the location of the shadow and the highlights produced on the sphere’s surface, can you tell the approximate direction of the light source? In this case, the light source is from to left side of the slide.
Notice that the deepest part of the shadow is black because the solid sphere blocks all of the light from reaching the surface behind the ball. If more than one light source is present (as is likely the case in the classroom) complete shadows will not be seen, but rather areas slightly darker than those surrounding the shadow.
This slide is identical to the previous one except that the direction that the light comes from is different. In this case, the light source comes approximately from the lower right. In both of these slides, one can see that artists have learned to use shadows and highlights to add realism and depth to their drawings.
This slide asks you to do an experiment. You are to find objects in the classroom that cast shadows. Almost any solid object will fall into this category and cast a shadow. You are also asked to find objects that do not cast a shadow. This is more difficult. Glass windows do not cast a shadow. A drinking glass will also allow most of the light to pass through it and cast only a weak shadow. The air in the room does not cast a shadow, but this concept is quite abstract for students at this age. They cannot see the air. But the very fact that they cannot see it means that light is not reflected from the air. Instead, the light is transmitted through the air and therefore it does not cast a shadow.
This slide shows the shadow produced by the window frame. Notice that the glass panes of the window do not cast a shadow but instead transmit the sunlight to the floor.
This final slide will be fun to discuss. It suggests the presence of an “invisible” person wearing the pair of sneakers. If there really is an invisible person in the picture we are not able to see them because the light is transmitted through them. The slide then asks, “What is wrong with this picture?” The answer is that if the person was truly invisible and allowed all light to pass right through them, then there would be no reason for them to cast a shadow! Their invisible body would not reflect or absorb light. They would be more like a window!