What you need:
A glass beaker, some water, a glass test tube/small bottle, vegetable oil or glycerine and a pencil.
What you do:
1. Pour water into the beaker. Do not fill it completely.
2. Immerse the pencil into the water at a slant angle. Make a friend or a family member sit some distance away from the beaker to observe.
3. Now, empty the beaker and pour the oil/glycerine into it. Fill the test tube or bottle with the same oil and place it in the beaker.
When immersed in water, the pencil looks a little bent in the middle. In the second case, the test tube (almost) disappears.
Why does this happen?
We can see' objects because some part of light hitting an object is reflected back to our eyes. When light travels from air into another medium like water, most of it is 'refracted'.
Refraction is nothing but the bending of light as it changes mediums. It happens because the properties of the medium are different from air like, how it is harder to swim in water than walk through air. You have to adjust the angle of your body to move through water easily. It's the same with light. Light waves change their angle and speed to suit the medium they travel in. This shift in the light's path makes the pencil appear shifted' or bent.
In the case of glass and oil, the properties are almost similar, i.e. both these mediums affect light in almost the same way. So, light does not refract or reflect. It passes from oil to glass to oil at almost the same speed, smoothly. So, the eye cannot see the object anymore!
Note that there are different kinds of glasses and oils in the market Not all of them will work well together to achieve invisibility. So, keep trying and testing different test tubes made of various kinds of glass with the oil.
Picture Credit : Google