This time it's not the shadow disappearing, but a real object disappearing! Is it quantum mechanics? Don't worry, we will take you to find out next.
Experiment equipment
A water basin, a transparent cup, a coin (well, a coin like a dog's head is also acceptable)
Experiment procedure
First step:
Fill the cup with water, then quickly pour it over the coin in the water basin. Make sure that the water in the cup is higher than the water surface of the water basin. While moving the cup, observe the coins in the cup from an oblique perspective.
Step two:
First, keep the coin in the upside-down cup, press the cup lightly, then add water to the basin, make sure that the water in the cup is lower than the water level of the basin, and observe the cup at an oblique angle while moving the cup coins inside. Are the coins disappearing in your eyes?
Third step:
To do a comparative test, let there be no. Ha, no matter how you move it, the coins are still there!
Principle explanation
When light is incident from one medium to another, the direction of propagation will change, which is the refraction of light. The refraction of light obeys the law of refraction, that is,
Among them, n₁ and n₂ are the refractive index of the incident medium and the exit medium respectively, and θ₁ and θ₂ are the incident angle and the exit angle. When the incident angle θ₁=0, sinθ₁=0, so θ₂ is also 0, that is to say, when the light is incident perpendicular to the interface, no refraction occurs. It can be seen from the law of refraction that the angle of refraction increases with the increase of the angle of incidence. When the incident angle reaches the critical value, θ₁=arcsin(n₂/n₁), θ₂ reaches the maximum value of 90°; at this time, if the incident angle continues to increase, the refracted light will disappear, and all the light will be reflected at the interface, that is, the total reflection.
Two conditions for the phenomenon of total reflection: the incident angle is greater than the critical value; light is incident from a medium with a high refractive index to a medium with a low refractive index. In our nut disappearing experiment, the light from the nut shoots from the water into the air and then enters the human eye. The refractive index of water is higher than that of air. After we add the cup, the refraction surface changes, and the incident angle can be greater than the critical angle. Total reflection will occur, no refracted light will enter the air, and the nut will be "invisible" in our eyes.
When there is no water in the cup, it is similar to the case of water, except that the interface where total reflection occurs has changed. The light is incident into the water at θ₁, the refraction angle is θ₂ and then refracted from the water into the air. At this time, the incident angle is the complementary angle θ₃ of θ₂, and θ₃ is greater than the critical angle, so total reflection occurs, and the nut is "invisible".
