Full reflection. Fun physical experience at home

ENTERTAINING EXPERIENCES AT HOME
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Full reflection. Physical experiments

Entertaining experiments in physics

Entertaining experiences at home / Physics experiments for children

An interesting phenomenon occurs with a beam of light that emerges from a denser medium into a less dense medium, such as water into air. A beam of light does not always succeed in doing this. It all depends on what angle he is trying to get out of the water. By angle is meant the angle that the ray makes with the perpendicular to the surface through which it wants to pass. If this angle is equal to zero, then the beam has no difficulty, it freely goes out. For example, if we put a button on the bottom of the cup and look at it exactly from above, we will see it well.

Now we will do an experiment in which the rays from an object in water will not fall into our eyes, it will seem to us that the object has simply disappeared. This can happen if you try to examine an object in the water just at such an angle when the rays are completely reflected from the surface, go into the depths and do not reach our eyes.

This phenomenon is called total internal reflection or total reflection.

Let's get to the experience. Make a ball with a diameter of 10-12 millimeters from plasticine and stick a match into it. From thick paper or cardboard, cut a circle with a diameter of 65 millimeters. Take a deep plate and pull on it two threads parallel to the diameter at a distance of three centimeters from each other. Fasten the ends of the threads to the edges of the plate with plasticine or adhesive tape. Then, piercing a circle in the very center with an awl, insert a match with a ball into the hole. Make the distance between the ball and the circle about two millimeters. Place the circle ball-side down on the stretched threads in the center of the plate. When viewed from the side, the ball should be visible. Now pour water into the plate up to the mug. The ball has disappeared. The light rays with his image no longer reached our eyes. They, reflected from the inner surface of the water, went deep into the plate. There was a complete reflection.

The phenomenon of total reflection is perfectly obtained if the ball is smoked with soot. It is necessary to find a metal ball with an eye or a hole, hang it on a piece of wire and cover it with soot (it is best to set fire to a piece of cotton wool moistened with turpentine, machine or vegetable oil). Next, pour into a thin glass of water and, when the ball has cooled, lower it into the water. A shiny ball with a "black bone" will be visible. This is because the soot particles retain air, which creates a gaseous envelope around the balloon.

Total reflection can be observed in another interesting experiment. Only for its success you need to take care of good lighting.

Pour water into a glass and dip a glass pipette into it. If viewed from above, slightly tilted in the water so that its glass part is clearly visible, it will reflect the light rays so strongly that it will become like a mirror, as if made of silver. But as soon as you press the rubber band with your fingers and draw water into the pipette, the illusion will immediately disappear, and we will see only a glass pipette - without a mirror outfit. It was mirrored by the surface of the water in contact with the glass, behind which there was air. From this boundary between water and air (glass is not taken into account in this case), light rays were completely reflected and created the impression of mirroring. When the pipette was filled with water, the air in it disappeared, the total internal reflection of the rays ceased, because they simply began to pass into the water that filled the pipette.

total reflection

Pay attention to the air bubbles that sometimes appear in the water on the inside of the glass. The brilliance of these bubbles is also the result of total internal reflection of light from the boundary of water and air in the bubble.

Author: Rabiza F.V.

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