Tools of the Laboratory

What is the role of lenses in microscopy?

Lenses focus either light or electrons to create a magnified image of a specimen.

Why is a specimen smaller than 200 nm not visible with a light microscope?

Anything smaller than 200 nm cannot interact with visible light.

What happens to the light rays when they hit the specimen?

They are reflected, refracted, or absorbed by the specimen.

What is the role of the ocular lens?

To recreate the image in the viewer's eye

What is meant by light rays being divergent?

It is spreading out

In a typical brightfield microscope (seen in the animation), at which point does magnification begin?

the objective lens

What is the total magnification of a specimen viewed with a 10x ocular lens and a 45x objective lens?


A student is looking at a bacterial specimen using the oil immersion lens, but has forgotten to put immersion oil on the slide. The specimen will appear

somewhat fuzzy and have poor resolution.

What does resolution mean?

the ability of a microscope to distinguish fine details and differentiate between two very close objects

Which of the following statements comparing electron microscopy and light microscopy is FALSE?

Both the electron microscope and the light microscope use the same wavelengths for illumination.

If acid-fast bacteria are stained with the Gram stain, they will stain gram-negative.


The purpose of a mordant in the Gram stain is to

prevent the crystal violet from leaving the cells

Which of the following places the steps in the correct sequence?1-Staining2-Making a smear3-Fixing


You are performing a Gram stain on gram-positive bacteria and you stop after the addition of the first dye. What is the appearance of the bacteria at this point?


Which step in the Gram stain is the critical step in differentiating gram-positive cells from gram-negative cells?


Bacterial smears are fixed before staining to

affix the cells to the slide

In a completed Gram stain, gram-negative bacteria are colorless.


The Gram stain, acid-fast stain and endospore stain have the following in common:

are differential stains

In the Gram-staining procedure, the primary stain is

crystal violet

Gelatin is used as a solidifying agent in microbiological media since few bacteria can degrade it.


An isolated colony on a streak plate contains millions (or even billions) of identical cells all arising from one initial cell.