The Disappearing Beaker
Publication No. 12012
Students will be amazed as various glass objects seem to vanish when immersed in a beaker of pale yellow oil.
(for each demonstration)
Canola oil, 32 oz*
Pop beads, red and green, 10*
Pyrex® beaker, 1000-mL
Pyrex beaker, unmarked, 250-mL*
Pyrex test tube with screw cap, 20 x 150 mm*
*Materials included in kit.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Pour the canola oil back into its original container. Beakers and other equipment may be washed with regular dishwashing detergent. The oil may be reused several times and then disposed of according to Flinn Suggested Disposal Method #26a.
Student Worksheet PDF
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesConstructing explanations and designing solutions
Disciplinary Core IdeasMS-PS4.A: Wave Properties
MS-PS4.B: Electromagnetic Radiation
HS-PS4.A: Wave Properties
HS-PS4.B: Electromagnetic Radiation
Crosscutting ConceptsStructure and function
Stability and change
MS-PS4-2. Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
Answers to Questions
Light travels at different speeds in various transparent media. For example, the speed of light in a vacuum is 3 x 108 m/s, in ice 2.29 x 108 m/s, in glycerin 2.04 x 108 m/s and in rock salt 1.95 x 108 m/s.
To visualize this, construct a normal line to the interface of the two media and extend the line through the second medium (see Figure 1). If the speed of light is greater in the first medium, the light ray in the second medium bends towards the normal line (see Figure 2). This bending of light is called refraction. In addition, a fraction of the light is reflected off the interface between the two. Objects that are clear can be seen because light is both refracted and reflected at the surface of the object.
The ratio of the speed of light in a vacuum over the speed of light in a specific medium is called its refractive index, n. Because the speed of light in air is so close to that in a vacuum, its refractive index, 1.000293, is usually rounded off to 1.000 for calculations. Water has a refractive index of 1.33. Pyrex glass has a refractive index of 1.474. If a Pyrex glass beaker is placed in another beaker filled with water, the beaker can be seen, but not as clearly as it is seen when the Pyrex glass beaker is in air. The clarity of a transparent object decreases as the refractive index values of two transparent media approach each other, since reflection of light off the interface surface decreases and the angle light is refracted in the second medium also decreases (see Figure 3).
When the two media have the same refractive index, as with canola oil and Pyrex glass, there is neither reflection nor refraction, (see Figure 4) and the Pyrex glass “disappears” in the canola oil.
A simple magnifying lens is a convex lens that refracts parallel light to converge at a single point called the principal focal point, F (see Figure 5). The distance from the lens to the focal point is called the focal length, f.
If an object is placed in front of the lens at a distance greater than the focal length, a real, inverted and reduced image is formed behind the lens (see Figure 6).
If the object is closer than the focal length, an erect, virtual image is created that is a magnification of the object (see Figure 7).
Canola oil has approximately the same refractive index as the magnifying lens. When the refractive index of the lens equals that of the medium in which it is immersed, no refraction of light occurs and therefore the object is not magnified (see Figure 8).
Special thanks to Lee Marek, Naperville HS, Naperville, IL, who provided Flinn Scientific with the instructions for this activity. Disappearing Glass Rods; http://www.rom.on.ca/wwatch/teachers-kit/disappearing.html (Accessed Jan 2002).