'Suspend a water drop at the lower end of a vertical pipe. Illuminate the drop using a laser pointer and observe the pattern created on a screen. Study and explain the structure of the pattern.

French Team

French Team

Material:
• Small pipe • He-Ne laser pointer (red light, 632.8 nm)

Protocol:
• Diameter of the drop: 6mm • Dark pipe to avoid many reflections inside the tube.

Observations:
• When the laser points the center of the drop  Light red circle shape  Blue circle around the main pattern • When the laser points on the edge of the drop  Irregular shape

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→ What does prevent the drop from falling down? → What does determine the shape of the drop?
Surface tension • With the air • With the edge of the tube
• Measurement of the coefficient of surface tension:  γ: coefficient of surface tension N/m  m: mass of a drop kg  r: radius of the section of the tube

Different states of the formation of a drop

 
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m.g 2 .r We found   0.034 N/m

French Team

→Why is there this brilliant pattern ?
• Geometrical optic approximation • Double refraction on spherical interfaces

Descartes law of refraction: n1 .sin i1  n2 .sin i2

i1

n1

i2

n2

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A: Amplitude of the electromagnetic wave Arefracted  2. n 1.n2 n1  n2 . A0 and Areflected  n2  n1 . A0 n1  n2
2

n n  4.n 1.n2 A2 I  I refracted  .I 0 and I reflected   2 1  .I 0 2 2  n1  n2   n1  n2  With water I reflected  0.02.I 0

i1 I0 i1

Ireflected n1

i2 Irefracted

n2 Consequence: Formation of “line shapes”

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Observation:
• An reversed image of the laser • The laser converges on one point when it is near the center.

Explanation
• The drop acts like a lens:  constant focus
For small angles: n1 .i1  n2 .i2 We have: i1  r R n  2r  . 1  1  R  n2 

r '  R.  2.i2  i1 

  2.  i2  i1  
L r'





 R  n1 .  1 2  n2  n1 

French Team