Touch Science - Experience Science Experiments

Author: Tanjou Shinji
Publisher:
Publish Date: 2006-11-01
Features: '■ A completely transparent person is invisible to the naked eye, which is common knowledge. Then, can the naked eye see transparent glass? Although glass is transparent, it can still be seen. Transparency means that light can pass through it. When light shines on glass, some of it reflects off the surface. It is because we see the light reflected from the glass that we know where the glass is. Compared to glass, what would happen if there were a transparent person? If a transparent person reflects light like glass, we could know where the transparent person is. Therefore, a completely transparent person must not reflect light. So, in reality, are there objects that are 100% transparent and do not reflect light? ◇ One each of salad oil, food-grade cling film, and acrylic ruler. Choose an appropriately sized glass cup so that you can place the small glass cup inside the large glass cup for the experiment.
(1) Place the small glass cup inside the large glass cup.
(2) Slowly pour salad oil into the large glass cup one drop at a time (Figure 1-1). At this point, almost part of the small glass cup immersed in the oil is visible. The patterns drawn on the surface of the small glass cup float in the oil, and you can barely tell that there is a glass cup there. As you continue pouring oil until the top of the small glass cup is covered, a very fascinating phenomenon occurs—the small glass cup actually becomes completely transparent!
(3) Repeat the above experiment using food-grade cling film or an acrylic ruler instead of the small glass cup. Based on the air bubbles attached to the film or the markings on the ruler, you can barely make out the position of the cling film or ruler.
[Why?] When light travels through a substance, its speed is slower than its speed in a vacuum. When the speed of light in a substance is half that in a vacuum, the refractive index of that substance is said to be 2; if it is one-third, the refractive index is 3. When light traveling in one substance hits another substance with a different refractive index, the light "realizes" it is in a different substance, and part of the light reflects while the rest refracts. At this point, the path of the refracted light, which strikes the interface at an angle, is bent. But what happens when two substances have the same refractive index? The light does not recognize the second substance as different and continues to move at the same speed in the same direction. For light, such two substances are the same, and there is no reflection at their interface. In reality, the refractive index of salad oil is almost the same as that of cling film and glass. Since there is no reflection at the interface, the small glass cup is almost invisible.
About this experiment
This is an experiment that is rarely thought of. In my teaching, when I first introduce light, I inevitably talk about transparent people and conduct this experiment. If there are no glass cups in the classroom, I use beakers instead. When the small beaker becomes invisible, the students are not surprised at all. Although this is a simple experiment that does not require large equipment, it leaves a deep impression on the students, and it is one of my favorite experiments. Please try it too. At the beginning of this section, it was mentioned that "the naked eye can see"