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Photography > Microscopy-Richard > Flashcards

Microscopy-Richard Flashcards

1
Q

what is a converging lens?

A

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

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2
Q

What is a diverging lens?

A

cause parallel rays of light to diverge

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

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3
Q

Refraction

A

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

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4
Q

What is the refractive index?

A

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

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5
Q

What law is used to calculate refraction?

A

snells law

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6
Q

Snell’s law

A

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

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7
Q

Transverse magnification

A

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

image produced is inverted

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8
Q

Angular magnification

A

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

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9
Q

Real image formation

A

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

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10
Q

A virtual image

A

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

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11
Q

Compound microscope

A

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

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12
Q

What are 8 parts of a microscope?

A 
ocular lens
objective lens
turret
stage
condender
diaphragm
light source
focussing knobs
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13
Q

What is the ocular lens?

A

eye pieces

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14
Q

What is the objective lens?

A

primary lens

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

on lens it tells you mag and numerical aperture

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15
Q

What is the turret?

A

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

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16
Q

what is a stage?

A

sample is placed

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17
Q

what is a condenser?

A

focusses the light passing into your sample

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18
Q

what is a diaphragm?

A

control the amount of light and also the depth of field

19
Q

what is a light source?

A

may or may not be colour corrected

20
Q

what are the focusing knobs?

A

allow you to bring your samples into sharp focus

21
Q

What is the numerical aperture?

A

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

as numerical aperture increases, mag and resolution increases

22
Q

How do you increase numerical aperture?

A

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

23
Q

what is resolution?

A

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

24
Q

How is resolution calculates?

A

r=1.22 x wavelngth / 2 x NA

resolution is therefore affected by numerical aperture and wavelength of light

25
Q

Limits of resolution

A

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

26
Q

Bright field compound microscopy

A

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

27
Q

dark field compounds microscopy

A

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

28
Q

phase contrast compound microscopy

A

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

29
Q

how does the phase contrast technique work?

A

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

30
Q

recombined waves

A

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

31
Q

increasing contrast

A

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.

32
Q

Polarising light compound microscopy

A

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

33
Q

stereo microscope

A

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

34
Q

comparison microscope

A

2 microscopes connected via optical bridge

35
Q

comparison macroscope

A

used for comparison of tool marks and firearms paraphernalia

36
Q

electron microscopy

A

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

37
Q

transmission electron microscopy

A

ideal for high resolution imaging of thin slices of bio specimens

38
Q

scanning electron microscope

A

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

39
Q

both TEM and SEM

A

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

40
Q

solving the air issue with EM

A

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

41
Q

glass lens issue

A

electrons are charged particles which means we can influence them using magnetism

the lenses are electromagnets which allows us to bend the beam

42
Q

sample preparation -EM

A

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

43
Q

What is EDX?§

A

determine the elements present within materials

Photography (19 decks)

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