Oil and Pyrex

Public summary: 

Ordinary glass objects seem to disappear as you submerge them in a bath of oil. This experiment is spectacular and very messy!

Using the refractive index of vegetable oil to make a glass bowl 'disappear'.
Useful information
Kit List: 
  • 2 pyrex bowls (1 big, 1 small)
  • Vegetable oil (note: works less well/not at all if other types of oil are used)
  • Paper towels
  • Sodium polyacrylate spheres - optional
  • Pure water - optional
  • Coffee jar with pre-prepared spheres - optional
  • Rope to illustrate wavefronts/boundaries -optional
  • Also contains the box for Waves at Boundaries, which should in turn contain

    • A wave model
    • Plasticine for weights
    • Clamps
Packing Away: 

The oil may be recovered for reuse provided that it hasn't gained too many impurities (such as grass...)
Wash bowls in warm soapy water where possible.
Make sure outside of oil can is clean before putting in box.
Put the kit for Waves at Boundaries in its own box.

Frequency of use: 

What to do

  1. Fill the large bowl with oil.
  2. Parts of the smaller bowl which are fully surrounded by the oil become invisible - you can either pull the bowl out of the oil, or put a bowl in, the effect is the same.

Why this works

Glass is see-through, so how come we can see things made of glass at all?

It's all down to the fact that light travels through glass and air at different speeds. [1] When light moves from one material to another, it changes direction (refraction). When we look at a glass object, we don't see the glass itself, we see the bending of the light from objects behind it. [2]

Light travels through vegetable oil and pyrex glass at the same speed: they are said to have the same refractive index. When the Pyrex bowl is surrounded by oil, light passing though them doesn't change speed, and hence doesn't change direction, as it moves into the different materials. You don't have any way of seeing the smaller bowl, you only see the edge of the oil or the outer bowl.

Why does the light bend?

If the wavefronts of the light hit the interface at an angle, then one end ends up moving more slowly than the other end. This makes the light bend. This is really an example of Huygens' principle of wavelets.

Go and see the water-tank experiment for a hands-on demonstration of refraction.

(For a better explanation, take a look at the Making Pyrex Invisible experiment on the Naked Scientists).

I like to talk about this by getting the audience to consider how fast they would go in really sticky mud as opposed to on a well-made path. Then, lay some rope for a boundary and get them to hold another piece as a wavefront. Have them walk towards the boundary at an angle so one foot crosses first then say that foot is "stuck" - hopefully they will turn, carrying the wavefront with them!

If the balls have been prepared...

The same effects can be observed using sodium polyacrylate spheres in pure (de-ionised/distilled) water. Sodium polyacrylate is a super-absorbent polymer which can absorb a few 100 times its own mass in water - hence it effectively has the same refractive index as water. The spheres take a few hours to absorb the required quantity of water so this part can only be done if the spheres have been pre-prepared.

In a non-cross linked form the polymer absorbs water much more quickly but sadly isn't transparent. This form is used in baby nappies.

Waves at Boundaries (for older visitors)

At CHaOS+ events, or if you are confident at public events, this can be used to demonstrate how waves change speed indifferent mediums. To set up, clamp the wave model between 2 tables and add weights to skewers on one end. Playing around with this you should be able to see reflected waves and a noticeable change in wave speed at the boundary.

[1] The "speed of light" is defined in a vacuum; in denser materials it travels slightly more slowly.
[2] And any impurities in the glass - this is why most glass is green if you look at it side-on. Optical glass (for optical fibres, etc) is much higher purity.

Risk Assessment
Date risk assesment last checked: 
Thu, 16/01/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Sat, 18/01/2020
Risk assesment double-checked by: 
Risk Assessment: 

Putting a Pyrex bowl into vegetable oil. Since they both have the same refractive index the Pyrex bowl will disappear.:

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Vegetable oil The oil is very slippery, and could cause a nasty slip hazard if spilled. 3 3 9 Be very careful to minimise the chance of this happening, probably use a towel on top of a plastic sheet to contain any small spills if it is done inside. Mop up any spills carefully. Use slip hazard sign.
Call a first aider in the case of an injury.
2 2 4
Pyrex bowl If the Pyrex bowls broke they would be sharp and could result in cuts. 2 2 4 Check for cracks before starting demonstrating. If the Pyrex does break, dispose of it carefully. Be especially careful of any pieces that are in the oil as they will be INVISIBLE, so it is better to pour the oil out rather than fishing for them.
In the event of an accident call a first aider.
1 2 2
Sodium polyacrylate spheres Children eating sodium polyacrylate spheres and spheres bursting/choking hazard if children swallow spheres. 2 4 8 Ensure only one child is 'investigating' the spheres at a time so that they can be monitored. If they burst the sphere ensure that they don't rub their eyes and that they wipe their hands.
Call a first aider in the case of an injury, who may perform an eyewash if trained and happy to do so.
1 4 4
Skewers Ends of skewers could be sharp (could stab/cut people) 2 1 2 Don't set up in a high-traffic area (where people might walk through it), and make sure people don't lean in close. Ends of skewers have plastic on, so should be non-issue.
Call a first aider in the case of an injury.
1 1 1
Swallowing sweets/marshmallows Audience could try to eat them, which is unsanitary/a choking hazard 2 3 6 Keep an eye on children, and mention not to eat the sweets or Plasticine
In the event that something is eaten, warn parents/relevant adult.
1 3 3
Publicity photo: 
Experiment photos: