2. Microscopy

Question 1

What are focal points and focal lengths of convex lenses?

Answer 1

Convex lenses cause parallel light rays from a distant source to converge at a specific point termed the focal point. The focal length is the distance from the centre of the lens to the focal point.

Question 2

What does the shape of a lens affect?

Answer 2

Smaller rounder lenses tend to have higher magnification and shorter focal lengths

Working distance is the distance from the specimen to the objective lens. High mag have shorter working distances and narrower depth of field (range of focus)

Question 3

What does the resulting image look like if the object is located more than 2 focal lengths away?

Answer 3

It will be real, smaller, and inverted

Question 4

What does the resulting image look like if the object is located 2 focal lengths away?

Answer 4

The image will be real, the same size, and inverted

Question 5

What does the resulting image look like if the object is located between 1 and 2 focal lengths away?

Answer 5

The image will be real, magnified, and inverted

Question 6

What does the resulting image look like if the object is located exactly 1 focal length away?

Answer 6

The light rays emerge parallel and image can no longer be focused on a screen

Question 7

What does the resulting image look like if the object is located within 1 focal length away?

Answer 7

The image will be virtual, magnified, and erect (not inverted)
Can only be seen by looking through the lens, the image forms on the same side of the lens as the object itself

Question 8

What is total magnification?

Answer 8

When you multiply the magnifications of the two lenses (objective and ocular)

E.g. 40x X 10x = 400x

Question 9

What is a convex lens?

Answer 9

Causes light rays to converge
Magnification is related to the curvature of the lens
Looks like an eye shape

Question 10

What are two lens aberrations?

Answer 10

Chromatic aberration - shorter wavelengths come to focus closer to the lens than longer wavelengths, causing colour distortion and a fringe of colours around the periphery of the field.

Spherical aberration - light passing through lens does not bend as much as those passing through the periphery, causing a blurred image on the outer edges.

Question 11

How do you correct spherical aberrations?

Answer 11

Use a combination of convex and concave lenses

Termed “plan-“

Question 12

What is the amount of refraction dependent on?

Answer 12

Angle of incidence
Refractive index (density)

Question 13

What is the angle of incidence?

Answer 13

The angle at which approaching light strikes the surface

Light entering more dense mediums bends towards normal , air to glass
Light entering less dense mediums bends away from normal , glass to air

Question 14

What is the refractive index?

Answer 14

Is an expression of the density of a medium and its effect on light rays

Grown glass = 1.52
Immersion oil = 1.52

The denser a medium the higher the refractive index

Question 15

What is immersion oil? What does it do?

Answer 15

Immersion oil can help control the refraction of light preventing the loss of light rays, maintaining image sharpness and brightness

Question 16

What is resolution?

Answer 16

Is expressed as the minimum distance two objects must be apart in order to be seen as distinct

Question 17

What is resolution dependent on?

Answer 17

Wavelength of light used
Numerical aperture of the lens (ability of a lens to gather light)

Resolution = wavelength / 2 NA (numerical aperture)

Question 18

What is numerical aperture? What does it do?

Answer 18

Ability of a lens to gather light
Higher the numerical aperture, the better resolution
Oil lens NA = 1.25
The wider a cone of light gathered by the lens, the higher the NA
The NA of the condenser should match the NA of the objective lens to obtain the best resolution

Question 19

What is Useful vs. Empty magnification?

Answer 19

Useful magnification occurs when 1000 X NA is greater than the total magnification, results in loss of resolution

Empty magnification occurs when 1000 X NA is less than the total magnification, results in too much magnification

Question 20

What is the procedure for setting Kohler?

Answer 20

1. Close radiant field diaphragm
2. Open condenser aperture
3. Raise condenser to highest position
4. Lower condenser until diaphragm image is in sharp focus (small circle crisp edges)
5. Centre image using condenser centering screws
6. Open field diaphragm until field is filled with light (stop when edge is just out of field of view)
7. Remove ocular and close condenser aperture until 3/4 of circle is light
8. Adjust brightness

Question 21

What region of the light spectrum is visible?

Answer 21

380nm to 780nm

Question 22

What can light waves be described as?

Answer 22

Wavelength
Amplitude
Frequency
Velocity

Question 23

What are the key components of a compound microscope?

Answer 23

Light source
Radiant field diaphragm 
Condenser assembly
Aperture diaphragm 
Stage assembly
Nosepiece and objectives
Body tube
Oculars

Question 24

What does parfocal mean?

Answer 24

The object stays in focus even when the objective is switched, requires only minimal fine focusing