Microscopy-Richard

Question 1

what is a converging lens?

Answer 1

focus parallel rays of light into a single point of convergence

the distance between the lens and that point is known as the focal length

the point of convergence is known as the focal point

Question 2

What is a diverging lens?

Answer 2

cause parallel rays of light to diverge

focal point is in front of the lens which means that objects look smaller and nearer

Question 3

Refraction

Answer 3

when lenses bend light

occurs whenever a mean of light moves from one medium to another at an angle other than 90 degrees and can be seen in many everyday objects

occurs because light slows down as it enters glass from air

the amount of bending or refraction relates to the refractive index of the medium which is a measure of how much our medium slows rays of light

the greater the refractive index, the slower the light travels through the medium compared to through air

Question 4

What is the refractive index?

Answer 4

ratio of velocity of light in a vacuum to velocity of light in the medium and is always a number greater than 1

Question 5

What law is used to calculate refraction?

Answer 5

snells law

Question 6

Snell’s law

Answer 6

n1sin01=n2sin02
n1=refractive index of original medium
01=angle at which the rays enter the second medium from the first
n2=refractive index of second medium
02=angle at which the rays pass through the second medium after being refracted

Question 7

Transverse magnification

Answer 7

occurs when an object is further away from the lens that its focal point

image produced is inverted

Question 8

Angular magnification

Answer 8

when an object is closer to the lens than its focal point, a different form of magnification occurs

Question 9

Real image formation

Answer 9

when an object is further away from the lens than its focal point it forms a real image

the rays of light converge to a single point

Question 10

A virtual image

Answer 10

when an object is nearer to the lens than its focal point it forms a so-called virtual image

virtual images are not visible if we place a screen behind the lens as the rays do not come to focus

Question 11

Compound microscope

Answer 11

imaging of transparent or semi-transparent objects

mag up to 1000x

relies on light transmitted through the subject by an inbuilt light source

short working distance and low depth of field

angular and transverse magnification techniques

objective lens produces a primary real image which is then enlarged by transverse mag

image is projected into the ocular lens which acts as a magnifying glass and enlarges by angular magnification

resulting image is enlarged but is inverted

Question 12

What are 8 parts of a microscope?

Answer 12

ocular lens
objective lens
turret
stage
condender
diaphragm
light source
focussing knobs

Question 13

What is the ocular lens?

Answer 13

eye pieces

Question 14

What is the objective lens?

Answer 14

primary lens

improves resolution and removes distortions caused by chromatic and other optical aberrations

on lens it tells you mag and numerical aperture

Question 15

What is the turret?

Answer 15

where the objective lens is mounted, holds 3 or 4 lenses

Question 16

what is a stage?

Answer 16

sample is placed

Question 17

what is a condenser?

Answer 17

focusses the light passing into your sample

Question 18

what is a diaphragm?

Answer 18

control the amount of light and also the depth of field

Question 19

what is a light source?

Answer 19

may or may not be colour corrected

Question 20

what are the focusing knobs?

Answer 20

allow you to bring your samples into sharp focus

Question 21

What is the numerical aperture?

Answer 21

measure of the angular range over which the lens can gather light but also affect magnification

as numerical aperture increases, mag and resolution increases

Question 22

How do you increase numerical aperture?

Answer 22

place a drop of oil of R1 1.52 between our lens and our sample

we alter the way that light from the sample enters the lens, meaning that light takes a more direct path thereby increasing NA and hence the resolution

known as oil immersion

Question 23

what is resolution?

Answer 23

the ability of a lens to clearly distinguish fine detail or resolve minute entities lying close together in the specimen as separate

Question 24

How is resolution calculates?

Answer 24

r=1.22 x wavelngth / 2 x NA

resolution is therefore affected by numerical aperture and wavelength of light

Question 25

Limits of resolution

Answer 25

to resolve an object, the object must be greater than half the wavelength of our light source in size

Question 26

Bright field compound microscopy

Answer 26

produces an image with a bright background and specimen in relief image is produced by the sample interacting with the rays of light gives rise to differences in contrast and colour when dealing with colourless or transparent samples, we can increase contrast further using stains

Question 27

dark field compounds microscopy

Answer 27

produces an image with a dark background used for specimens which do not generate enough contrast in bright-field (colourless or thin objects) dark-field image is produced by the addition of an opaque dick in the light path as light enters the sample, some is diffracted which passes into the eyepiece, but majority is transmitted unchanged light is stopped by a direct illumination block

Question 28

phase contrast compound microscopy

Answer 28

samples which lack contrast under bright field or even dark field microscopy fragment of glass or unstained cells enhances contrast and hence allows visualisation of specimens which would otherwise be virtually invisible an image is formed by light that is absorbed, scattered, reflected, refracted, diffracted thicker the object the more attenuation that occurs and thus the darker the object appears in the microscope absorption may even change the colour of our sample if all regions in the visible range are not absorbed evenly and attenuation is sufficient

Question 29

how does the phase contrast technique work?

Answer 29

the light which passes through the sample is not just attenuated, it is slowed down or diffracted with respect to light which does not pass through the sample when the light that passes through the sample and that which does not recombine, slight differences in phase between the two beams cause differences in brightness hence allowing visualisation areas of our sample which absorb light or cause significant phase differences appear dark, whereas other areas will vary in brightness according to their thickness and the way which they interact with the light

Question 30

recombined waves

Answer 30

when two light beams recombine, the respective brightness of the resulting beam depends upon the phase of the two beams if the two beams recombine out of phase, they effectively cancel each other out resulting in a darker area on specimen if the two beams recombine in phase, the two beams effectively amplify each other resulting in a brighter area on specimen the difference between the two beams will be minimal with very slight increases and decreases in brightness the greater the difference between bright and dark, the greater the contrast

Question 31

increasing contrast

Answer 31

add annular ring between light course and condenser producing a hollowed-out ring of light brighter on the outside than the inside any light which interacts with the sample is diffracted as per normal and becomes slowed by around 1/4 wavelength and then passes through the centre of a phase ring light which does not interact with the sample is un-slowed and passes through a thinner part of the phase ring the phase ring is rather clever and differentially slows down light passing through its centre more than light passing through its outside typically by 1/4 wavelength so when the two beams recombine later on, they differ by 1/2 wavelength which massively increases contrast.

Question 32

Polarising light compound microscopy

Answer 32

includes two polarising plates, the polariser and the analyser along with retardation plate allow us to determine fundamental features of evidence such as the sign of elongation and the birefringence

Question 33

stereo microscope

Answer 33

low-medium power mag long working distance and high DOF relied on light reflected from the subject so requires external or built-in reflective light source has 2 separate light paths with different viewing angles which gives 3D visualisation

Question 34

comparison microscope

Answer 34

2 microscopes connected via optical bridge

Question 35

comparison macroscope

Answer 35

used for comparison of tool marks and firearms paraphernalia

Question 36

electron microscopy

Answer 36

illuminate our sample with electrons rather than visible light allows us to produce images which become resolution limited at around 10 million times mag all images are monochrome

Question 37

transmission electron microscopy

Answer 37

ideal for high resolution imaging of thin slices of bio specimens

Question 38

scanning electron microscope

Answer 38

ideal for determining the surface structure of samples scans a beam of electrons across the sample some of which are reflected reflected electrons are picked up using a complex detector producing a high-resolution image with 3D properties

Question 39

both TEM and SEM

Answer 39

use electrons to illuminate the sample which takes away the resolution barrier which comes from using visible light allowing massive magnifications electrons cannot easily travel through air which means the EM is rather complex electrons will also not pass through a glass lens, which means we have to find an alternative means of mag and focusing

Question 40

solving the air issue with EM

Answer 40

carried out under vacuum which means that air molecules cannot interact with electron beam use of high vacuum means we can't view anything living or anything that contains moisture specimens undergo complex dehydration process

Question 41

glass lens issue

Answer 41

electrons are charged particles which means we can influence them using magnetism the lenses are electromagnets which allows us to bend the beam

Question 42

sample preparation -EM

Answer 42

staining to increase contrast stains nasty and stop electrons samples must be coated with conductive layer electrons tend to cause the sample to become negatively charged affecting image and coating allows charge to dissipate to earth

Question 43

What is EDX?§

Answer 43

determine the elements present within materials