Question

state which objective lens would be better to use and why.

A) Measuring the distance a Tetrahymena swim in search of food in a Petri dish. Tetrahymena can grow to hundreds of µm in length.

10x with N.A. 0.25 or 40x with N.A. 0.65

B) Measuring the dimensions of a chloroplast in Elodea cells.

40x with N.A. 0.65 or 50x with N.A. 0.60

Answer 1

A. The distance of a Tetrahymena should be measured by 50x with N.A-0.60

B. Measuring the dimensions of a chloroplast in Elodea cells should be by 40x with N.A.-0.65

Because, cells are very small and are seen under compound microscope. With a compound microscope, the magnification is the product of both lenses. So if the microscope has a 10x eyepiece and a 40x objective, the total magnification is 10 x40 =400x.  

Magnification is the ratio of the size of the image to the size of the object.

We use micrometres for specimen size under the microscope. The conversion is 1mm =1000 micrometres.

For ex: if the image size is 2 cm and the actual size is 0.5 mm, then the magnification = 200/5= 40x