The Human Eye & Optical Instruments

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Experiment 5: Microscopes

Using our advanced lab low resolution stereomicroscope and the higher resolution compound microscope, observe their optical components and note how they correspond to the schematic above. Use the calibration slides provided to quickly observe the effect of changing resolution when you change the microscope objective power and switch between the two microscopes. Record how the change in objective relates to light collection (did you have to increase or decrease the illumination intensity for a fixed intensity on the screen?), field of view (how large a region of the object gets mapped onto the entire image) and resolution. Record your resolution limit theoretically and experimentally for each microscope and objective lens setting, using the calibrated slides provided. Describe and sketch what the features look like once you approach and exceed the resolution limit, and explain why.

Comment on whether you saw any of the optical imperfections described on the next page, and if so, which configuration gave you that effect or did not.
Experiment 6: Telescopes

Using the kit provided, build a refractor telescope, noting how its design maps onto the schematic above. Also, look at the Newtonian reflector (tube only, no mount) provided. If you have time, you can disassemble and reassemble it if you wish (be VERY sure it’s the one you can take apart), similarly observing its optical design. (A four-vaned frame called a spider holds the flat secondary mirror in place. Be sure you hold this in position the entire time until you have all four screws tightened before you let go, or else it will fall on the primary mirror!) Now use the already-assembled Dobsonian Newtonian reflector and refractor telescopes to observe the world outside. You can use the windows in our lab or the Math Lounge. Comment on the smallest size object you can resolve. Comment on whether you saw any of the optical imperfections described on the next page, and if so, which configuration gave you that effect or did not. Explain how you would be able to observe each effect with an experiment. Based on the aperture size of your telescopes, what would you expect the fundamental resolution limit to be?

Even the Hubble Space telescope was famously fitted with corrective lens to fix its initial optical imperfections! (Below, images before and after the corrective lens were fitted. Ugh, who screwed up the optics on the expensive space telescope?)

  • Achromat lenses compensate for chromatic aberrations by using two lenses, one converging, one diverging, made from different index materials. As a result, the two lenses give opposite, compensating chromatic aberrations that can be designed to cancel out.

  • Using a spherical rather than a parabolic mirror can lead to spherical aberratoins, although it’s much easier to grind a spherical mirror.

  • As you’ve seen above, cylindrical lenses can be used to correct for astigmatism.

Equipment List

  1. Human eye model with its optical rail mound

  2. Pasco optical rail and light source

  3. Small desk lamp for illumination

  4. Pinhole camera and pinhole glasses

  5. Model of eye anatomy

  6. Newtonian reflector (Orion Starblast)

  7. Refractor kits from Edmund Scientific

  8. Microscopes from H106 (advanced lab)

  9. Microscope resolution target (Edmund Optics)

1 Note: actually, the angle (and hence the location) of an individual small object can be measured to higher resolution than this, because you can always locate the maximum of a diffraction spot to much higher resolution (i.e., within smaller error bars) than its width due to the resolution limit. Measurements of this sort at important in modern biology and nanotechnology.
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