Question

1. Name at least 3 ways you can control the amount of light that reaches the ocular lens.

2. Define resolution. What is the limit of resolution for the light microscope?

3. What effect does the wavelength of light used have on resolution?

4. If you view a bacterial smear under the oil immersion lens, but forget to add oil, will the image still be just about as clear as if you had added the oil.

5. Define parfocal.

6. What is the magnification of the oil immersion objective lens in our lab’s microscopes?

7. What is the total magnification when using the oil immersion objective lens?

8. Is it permissible for you to use the coarse focus knob with the oil immersion and high dry lenses? EXPLAIN why or why not.

9. When observing the prepared bacterial smears at 1000X magnification, should your iris diaphragm be all the way open or stopped down for minimal light? EXPLAIN why or why not.

10. Why should you start first with the scanning lens when viewing a slide?

11. When observing a slide, should the specimen be on the top surface or bottom surface of the slide? EXPLAIN your reason.

12. As working distance increases, does magnification decrease or increase?

13. When observing the wet mount of the pond water:

a) What was the optimum magnification for seeing the protists?

b) How should you adjust the light for optimal viewing?

14. What are the three general shapes you will see among the bacteria? Please illustrate and label.

15. You are viewing a sample under the 4x (scanning) objective lens. What is the total magnification of the sample as you see it?

16. You are viewing a sample under oil immersion objective, and your microscope gets bumped, causing the stage to move, and the sample to go out of focus. What should you do?

a)use the fine focus knob by turning it up to 10 full revolutions to bring your sample back into focus under the oil immersion objective

b)use the coarse focus knob to try to bring your sample back into focus under the oil immersion objective

c)move the scanning (4x) lens into place, and bring your sample back into focus with this lens before moving sequentially back to the oil immersion lens

17. Define the term colony in your own words, as it is used to describe bacteria.

18. Say you have placed one single bacterium onto your agar plate. It divides in two every hour.

• How many bacterial cells would there be at the end of 6 hours?
• How many colonies would there be at the end of 6 hours?

Answer 1

1. The amount of light reaching the ocular lens of a light microscope is dependent on several factors. First, the light source determines the illumination power of the microscope. The condenser focuses the light into a beam to pass through the sample. Finally, the thickness or size of the sample will determine if light can pass through it in order to be viewed through the eyepiece.

2.  The resolution of an optical microscope is defined as the shortest distance between two points on a specimen that can still be distinguished by the observer or camera system as separate entities.

In a compound microscope, the wavelength of the light waves that illuminate the specimen limits the resolution. The wavelength of visible light ranges from about 400 to 700 nanometers. The best compound microscopes cannot resolve parts of a specimen that are closer together than about 200 nanometers.

3. Microscope resolution is also impacted by the wavelength of light being used to illuminate the specimen. Longer wavelengths of light offer less resolution than short wavelength illumination. Near-ultraviolet light has the shortest usable wavelength and offers the greatest resolution.

4. In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens.