Question

1) What objective lens is best when you are first starting out on a slide?

2) What does 'Par focal' mean? Why is it useful?

3) The equation for the Numerical Aperture is NA=nsinθ. What does 'n' stand for? What does 'θ' stand for? If we want to get a better resolution, should we increase or decrease these values? Feel free to refer back to the lecture slides.

Answer 1

1) When focusing on a slide, always start with either the 4X or 10X objective. Once you have the object in focus, then switch to the next higher power objective.

2) Parfocal means that the microscope is binocular and Parfocal means that when one objective lens is in focus, then the other objectives will also be in focus. For example, if the focal point of a microscope is changed from a low power objective (10x) to a higher power (40x or 100x), the object stays in focus. Parfocal is a desirable feature for a microscope, as the constant re-adjusting of the focus while viewing a sample can be overwhelming. A Parfocal lens allows for more accurate focusing at the maximum focal length, and then quick zooming back to a shorter focal length.

3) Here 'n' is the refractive index of the medium between the objective lens and the object.

'θ' is half the angular aperture( or acceptance angle of image-forming rays) of the objective lens(Jenkins and white 1957).

The bigger a cone of light that can be brought into the lens, the higher it's numerical aperture is. Therefore the higher the numerical aperture of lens, the better the resolution of a specimen will be which can be obtained with that lens.