Lec 21 Flashcards
What is similar in diffraction and scattering
They are both physical phenomena related to the interaction of waves with objects
What is the difference in diffraction and scattering
Diffraction is when The waves bend or spread due to passing through an aperature or around and obstacles
Scattering is the redirection of waves in different direction because of interaction with particles
What is the requirement for the waves that are being diffracted
The wave has to fit the size of the aperature/opening
If don’t fit, you only get scattering and no diffraction
Diffraction is
A form of scattering
Why do we use x rays for x ray crystalline
The wavelength of x rays (0.5-2 A) match the sizes that are being repeated over and over again (in the carbon carbon bond)
Which is Around 1.5-2 A
This allows for diffraction to occur
What are the pros and cons of using visible light instead of x rays
Pros: not damaging, easy to focus, eyes are a great detector of visible light
Cons: limited by the long wavelength (400-700), so limited in the resolution you can see
What are the pros and cons of using x rays
Pros: small wavelength, good penetration, easy to generate
Cons: hard to focus, damage the sample
What are the pros and cons of using electrons instead of x rays
Pros: Very small wavlngth (pm), can be focused at a specific point due to use of magnets
Cons: need very strong electron beam because they don’t fully penetrate the sample, damage sample
What are the pros and cons of using neutrons instead of x rays
Pros: small wavelength (pm), low damage kf sample
Cons: how do you get it? Only source is the sun, also it’s hard to focus/manipulate
What is unique to x ray crystallography
It is an iterative process
We can correct errors that we see and refine it
What happens to the phases of the wave as you go from the object to the diffraction pattern on x ray crystallography
The phases are lost which is why we do the method of getting a similar object with known phases
What is a Fourier transform
Ex. A song can be split into a bunch of different notes (frequency’s of waves)
Taking a complex wave (image) and decinvolute it into its own sin wave frequencies
What is the Bragg angle
The highest resolution images are scattered further away from the centre at different angles (Bragg angles)
The larger the Bragg angle , the higher the resolution of the image
So the further diffraction pattern from the centre has higher resolution
Fournier space is ________ to real space
Reciprocal
If a spot is further away in Fourier space, what is its distance in real space
What is its resolution
In real space it is closer since inversely proportional
Highest resolution if further away in fourier space
