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The Magic of Optics: Now you see it, now you don't

Nathaniel Lasry, John Abbott College, Montreal Canada
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


This interactive lecture demonstration is used as an introduction to the concept of refraction. A piece of Pyrex tube seems to disappear when submerged into a beaker half-full of mineral oil because Pyrex and mineral oil have a highly similar refraction index. The same Pyrex tube is clearly visible in a beaker of water. Usually, this demonstration is used as an example of refraction (or lack thereof) after the topic has been introduced. However, a different, magical approach is suggested here to better engage students.
Students are presented with a "magic" liquid that binds broken pieces of glass back together. The instructor takes a Pyrex tube and after carefully wrapping it well in brown paper, proceeds to smash the test tube with a hammer. The remaining fragments are poured into a beaker filled with the "magic" liquid (which unbeknownst to students has a hidden intact Pyrex tube submerged). After a little stirring, the instructor pulls out the fully "repaired" Pyrex tube with a pair of tongs.
Students are guided through an inquiry cycle (is this liquid to good to be true?) through which the concept of refraction is constructed and the magic trick debunked.

Learning Goals

Students should be able to use their critical thinking skills to question the existence of a miracle fluid that repairs broken glass with a stir.
Through guided inquiry students will construct the concept of refraction as the deflection of light (and accompanied partial reflection) as it passes from one medium to another.

Context for Use

Educational level: grade 8 to college-level intro to optics course
Setting: Classroom
Time required: 15-30min
Special equipment: 500 mL Beaker (2) ; Pyrex test tubes (2); mineral Oil (500mL)
Pre-requisite knowledge: None! Preferably, the students have not been lectured or assigned reading on the topic. Optimal situations involve students having to construct the concept of refraction (and/or partial reflection) in trying to debunk the magic trick.

Description and Teaching Materials


Before entering class:
  • Fill a 500mL beaker with 200mL of mineral oil.
  • Cut the rounded end of two Pyrex test tubes
  • Submerge delicately one of the resulting Pyrex cylinders in the beaker so that no air bubbles are trapped in the cylinder
In class:
  • Show the beaker with dissimulated Pyrex cylinder
  • Add a little more "magic fluid" to the beaker (the mineral oil bottle should have its label removed for a better effect)
  • Wrap the second Pyrex tube (that has had its end removed) into thick paper
  • Crush the Pyrex with a hammer
  • Pour the fragments into the beaker
  • Stir, wave your hand (or say magic words, incantation, or mantra chant)
  • Remove intact Pyrex cylinder with tongs
  • Bow for applause

Teaching Notes and Tips

This interactive class demo is designed to engage students, cognitively and affectively. It is a truism to claim that students learn best when engaged; disengaged individuals do not participate in much and therefore cannot achieve much. Less obvious are the characteristics of environments that are optimally engaging to a large number of students.

One approach is to seek activities that are known to actively engage people cognitively and affectively. The format chosen here is magic. What makes a trick magical is the unexpected nature of the resulting observation (Huh? What just happened?). A violation of one's expectation. In psychological terms, a cognitive conflict. Interestingly, cognitive conflict is at the root of learning through conceptual change. The purpose of magic demos are therefore to create environments that make use of magic to engender cognitive conflict and actively engage students.

Traditionally, a demo is presented after a concept is explained. The demo proceeds to show that the given concept is verified in the particular setting demonstrated. Instructors are essentially saying: Situation X fits into concept Y, look indeed it does. But this leads to no "incongruities", or cognitive conflicts and is therefore less engaging.

The purpose of the strategy here is applicable in most interesting demos. What makes a demo interesting is that to some extent it shows something unexpected. The unexpected feature can be enhanced (reinforcing a misconception prior to demo) so as to make the resulting demonstration optimally unexpected, interesting and therefore engaging.


This activity is geared to engage and interest students in optics, specifically refraction and partial reflection. All assessment forms are formative.

In a think-pair-share mode students are asked:
1) Do you believe that this fluid finds all the broken pieces and seals them together with one stir?
2) How do you think the tube was taken out whole?
3) What kind of physics do you think is at work here

Once these questions are addressed by small groups, students are asked to send a representative to explain their group responses. At least one group will usually put forward the hypothesis that a whole Pyrex tube was already in there. This leads to the next set of questions:

4) If there is absolutely no light in a room, can you see light colored objects better than darker ones? (identifies whether students conceive of seeing as a reception of reflected light or an outgoing process emerging from one's eyes. This can lead to follow-up demos of dark and light objects in a dark room; photo-lab if the school has one)

5) If the Pyrex tube was already there, where are all the small bit of Pyrex? (Make beaker available to students)

6) Why can you see the same Pyrex tube in the second beaker full of water but not in the magic fluid?

References and Resources