Becke Line Method Lab
Goal: To find the index of refraction, n, of an unknown isotropic mineral using oils with known indices of refraction by using the Becke Line method.
Materials:
- Microscope with power source
- Mineral samples
- Oils with different indices of refraction
- Acetone
- Detergent
- Kim Wipes
- Paper towels
- Marker
- Scoop
- Slide
- Cover slips
Procedure:
- Set up the microscope.
- Take the microscope out of the case by holding the base and the arm. Plug in the microscope and turn the light on.
- Rotate the condensing lens out of the way.
- Rotate the Bertrand lens out of the way
- Make sure the polarizers are not crossed by moving the analyzer into the same direction as the lower polarizer (make sure light is visible).
- Make sure the microscope is on its lowest magnification.
- Set up the slide.
- Clean the slide.
- Put a couple of squirts of detergent on the slide and gently rub with fingers, then rinse.
- Be sure to only touch the slide on the edges from now on.
- Dry the slide with a paper towel.
- Spray a small amount of acetone on the slide and wipe it dry with a kim wipe. Place the kim wipe in the toxic waste bucket.
- Clean three cover slips the same way that you clean the slide.
- Choose three oils between n=1.4 and 1.64 that have n values sufficiently far apart to make it easier to narrow down your mineral's index of refraction.
- Use a marker to write on the slide the three n values as far apart from each other as possible.
- Next to the written n values, use the eye dropper in each oil bottle to put a very small amount of oil on the slide. First, rub the dropper along the inner edge of the bottle to remove excess oil, then put the drop on the slide without touching the slide. Do not touch the slide with the dropper because it could contaminate the oil when you put the dropper back in the bottle.
- Take the scoop and clean it with the acetone and kim wipe. Put it in the mineral grain sample bottle and pick up a few grains. Tap your hand that is holding the scoop over an oil drop so that a few grains fall into the oil. Repeat for each oil sample and then put the cover slips on the oil samples. Then close the mineral grain sample bottle.
- Clean the slide.
- View the sample underneath the microscope.
- Put the slide on the microscope. Look for the grain in plain light under low power. Once you find a grain, turn the analyzer on to ensure that the grain is isotropic, the grain should be black.
- Change the objective to medium power and turn the analyzer on.
- Swing the condensing lens into place and if you see a bright line bordering part of the grain, called the Becke line, then you are in the oil.
- Focus on the Becke line, you may need to close the iris to get a better view of it.
- While continuing to look at the Becke line, lower the stage by focusing
out using the coarse adjustment. Note the movement of the Becke line:
- If the Becke line moves inwards, toward the grain, then the oil's refractive index is lower than the grain's refractive index and an oil with a higher index of refraction should be tried.
- If the Becke line moves outwards, toward the oil, then the oil's refractive index is higher than the grain's refractive index and an oil with a lower index of refraction should be tried.
- If the Becke line is not visible, then the oil has the same refractive index as the oil and you can stop.
- Put everything away.
- Clean the slide and the cover slips again and place them back in their original spots. Put the microscope back under low power magnification, turn the light off, and unplug it. While holding the base and arm, place the microscope back in the case.
Results: I had mineral grain sample F9. Four oils were tried. The first one, with an index of refraction equal to 1.60 did not work because I could not find a grain. The second oil I tried had an index of refraction equal to 1.55 and the Becke line went towards the oil. So, I tried an oil with a lower index of refraction, 1.50 and the Becke line still went into the oil, but the line moved very little. So, I tried an oil with an index of refraction just a little less than the previous one at 1.49 and I did not see a Becke line.
| no | Becke line |
|---|---|
| 1.60 | Did not work |
| 1.55 | Into oil |
| 1.50 | Into oil |
| 1.49 | No Becke line |
Conclusion: Becke lines appear because of refraction at the lens-like edge of a grain. If the grain has an index of refraction lower than the oil, the grain will act like a diverging lens and the Becke line will go into the oil. If the grain has an index of refraction higher than the oil, the grain will act like a converging lens and the Becke line will move into the grain. If the grain and the oil have the same index of refraction, then the light will not be refracted and a Becke line will not appear. The F9 mineral grain sample and the oil with an index of refraction equal to 1.49 did not create a Becke line, so F9 has an index of refraction also equal to 1.49.
