2.2 Magnification and calibration

Question 1

magnification definition

Answer 1

how many times larger the image is than the actual size of the object being viewed.

Question 2

resolution definition

Answer 2

the ability to see individual objects as separate entities.

Question 3

Resolution

Answer 3

Resolution if limited by the diffraction of light as it passes through samples (and lenses).

Diffraction is the tendency of light waves to spread as they pass tp physical structures such as those present in specimens.

The structures are very close to each other.

The light reflected from individual structures can overlap due to diffraction.

This means the structures are no longer seen as separate entities.

In optical microscopy, structures that are closer than half the wavelength of light cannot be seen separately.

Question 4

Increasing resolution

Answer 4

Resolution can be increased by using beams of electrons which have a wavelength thousands of times shorter than light.

Electron beams are still diffreacted but the shorter wavelength means that individual beams can be much slower before they overlap.

This means objects which are much smaller and closer together can be seen separately.

Question 5

magnification formula

Answer 5

Magnification = image size / actual size

Question 6

units of conversion

Answer 6

cm
mm
micrometre
nanometre

Question 7

Using a graticule to calibrate a light microscope

Answer 7

Eyepiece graticule:
Glass disk marked with a fine scale of 1 - 100.
No units
Remains unchanged whichever objective lens in place.
The relative size of the divisions increases with each increase in magnification.

Stage micrometer:
Accurate scale in micrometres.
100 division = 1mm so 1 division = 10 micrometre

Question 8

Suggest why you should put all measurements into the same units before carrying out calculations

Answer 8

simplifies calculation (1) /
reduces errors (1)

Question 9

Contrast vs resolution

Answer 9

Contrast is difference in colour/shade between two objects (1)

Resolution is the smallest distance between two objects that can still be seen as separate (1)

Question 10

Explain how diffraction limits resolution

Answer 10

Diffraction happens when light passes through structures (1) /

light waves spread out (1) / (light waves) overlap (1) /

individual objects do not appear separate (1) /
causes blurring (1)

Question 11

Explain why eyepirce graticules have no units

Answer 11

(eyepiece graticule is) arbitrary scale / calibrated for each lens (1) /

using stage micrometer (1)

Question 12

end of 2.2