Wednesday, 28 May 2014

Magic Glass (Make it invisible)!

  1. Small object made with transparent glass (Such as marble, microscope slide etc.)
  2. A glass jar or container large enough to hold the small glass object.
  3. Clear water
  4. Vegetable oil

  1. At first, place the small glass object in the large glass container.
  2. Then put some clear water inside the bowl and submerge the glass object as much as possible.
  3. After that you can record how the small object looks like underwater.
  4. Now empty the water.
  5. Then put some vegetable oil into the bowl and again submerge the object as much as possible.
  6. Then you can record how the glass object looks like again if you wish.

  1. Underwater, the small glass object might look a bit distorted.
  2. Under oil the glass object should be invisible partially or completely.


The object underwater look a bit distorted although it doesn’t change physically. The reason for this is because of the way light acts when it passes from water to air. Rays of light have 2 options when they pass from air to another transparent medium or material. One option is that, they can be reflected. This means the light rays are bounced back. Another option is being refracted. This means that, the light rays will continue to travel through the medium, but they will be a bit displaced from their initial path, will make an angel and bend towards a new path. We can see objects made with transparent materials such as glass because they reflect and refracts light rays. When a light ray pass from one medium to another medium, it changes its speed or velocity. Actually the difference in velocity among mediums determines how much refraction and reflection will occur. The smaller the difference in velocity, the less refraction and reflection. The larger the difference in velocity, the more refraction and reflection. Every medium has its own index of refraction. This is a measurement which indicates how light slows down when it passes the medium.
The index of refraction or air is usually considered to be one. This means that the light does not change speed when it travels through the air.

Although in reality light can travel at full speed only in a vacuum. But that’s completely another story so we won’t go there right now.
Now back to the discussion. When the index of refraction of a medium is greater than 1, the velocity of light in that medium will be lower than velocity of light in the air. In water, the index of refraction is 1.33, which means when light passes from air to water, it slows down.

Vegetable oil especially a specific brand called Wesson oil and glass  especially Pyrex glass have similar indices of refraction which is approximately 1.47. Light going through oil (especially Wesson oil) and glass (especially Pyrex glass) will experience minimal refraction and reflection coz the indices of reaction are similar to each other.


Light rays flows within different medium at different speed or velocity. Different medium resist the flow of light with different strengths. The characteristic that describes this property of a specific medium is called its refractive index or index of refraction.

Although oil (especially Wesson oil) and glass (especially Pyrex glass) have similar indices of refraction, the small transparent glass object will not vanish completely. It’s because the shape of an object effects the way light bends as it travels through. Another fact is, any impurities in the glass or oil will also change the index of refraction. The most glass object we will use in this experiment, submersion in the oil will produce a ghostly image of that glass object.

  1. Use vinegar instead of oil.
  2. Use glycerin instead of oil.
  3. Use different kinds of oil such as body massage oil, machine oil etc.

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