30 Jun, 2013
PRESENTATION: Johnson’s Baby Oil has the same refractive index as Pyrex glass (1.51-1.54). When a small Pyrex beaker is lowered into the oil, it disappears as the light does not undergo reflection or refraction because it is as though it has not changed from one medium to another. Only the scale printed on the side of the beaker, if it has one, would be visible.
- A safe pyrex ’’vanishing solution’’, Donald K. Day, Phys. Teach. 15, 438 (1977)
- Pyrex ’’vanishing solution’’, F. J. Wunderlich, D. E. Shaw, and M. J. Hones, Phys. Teach. 15, 118 (1977)
INTRODUCTION: Snell’s law was first discovered by Ibn Sahl in the XIIIth century. He used it to resolve the forms of the anaclastic lenses (lenses that focus the light with geometrical aberrations). It was discovered again in the XVI century and formulated again in the XVII by Willebrod Snel van Royen. This law relates the refractive index with an incident angle that is refracted between two different environments. The refractive index is a measure that determines the light speed reduction while it propagates through an homogeneous environment. The refractive index (n) defines as the speed quotient (c) of undulatory phenomena such as light or sound in a reference environment with respect to the phase velocity (v) in that environment.
OBJECTIVE: To demonstrate that the refractive indices of the baby oil and of the Pyrex are the same.
MATERIAL: Pyrex glass, glass container, baby oil.
SETUP: A glass is filled with oil. Then we introduce a Pyrex glass and fill it with oil again. The glass we are putting in disappears gradually.
EXPLANATION: When two substances have the same refractive index, as in the case of the baby oil and the Pyrex glass, which have a refractive index of 1.47, when we put the glass in the oil it becomes invisible. The reason is that the refractive index is the quotient between the speed of light in vacuum and the speed of light in that environment. As the speed of light in vacuum is the same and the refractive index is the same as well, the speed in the environment will be the same. That’s why we can’t see the glass.
CONCEPTS: refractive index, speed of light in vacuum and in an environment, Snell’s law.
- WIKIPEDIA 1
- WIKIPEDIA 2
- WIKIPEDIA 3
- YOUTUBE 1
- YOUTUBE 2
- UNIVERSITY OF IOWA
- R. Serway, Física, Mac Graw Hill, 2010.
- P. Tipler, Física para la Ciencia y la tecnología, Reverté, 2012.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2012-2013: Sabela Arenas, Nerea Lema, Tania López, Nerea Silva.
LINK pdf STUDENTS (in Spanish):