# Resolving proteins with UV microscope

1. Nov 21, 2014

### Les talons

1. The problem statement, all variables and given/known data
Your molecular biology lab studies proteins, and you're frustrated because your microscopes can't quite resolve crystallized proteins. A sales rep touts the advantages of an expensive microscope using 200-nm ultraviolet light, saying you'll be able to resolve structures less than half the size that's resolvable with your optical microscopes.

Is the sales rep correct?
no
yes

2. Relevant equations
Rayleigh criterion
slit: Θmin = λ/a
circular aperture: Θmin = 1.22λ/D
X-ray diffraction: 2dsinΘ = mλ

3. The attempt at a solution
I looked up the wavelength ranges of visible light and found 390nm to 700nm. I also found the average size of a protein to be 531 angstroms or 5.31x10^-8 m. I also heard that you can resolve an object as small as one fourth of the wavelength from somewhere, is this true (can't find anything in the book or online anywhere on this point)? So I thought the answer is the sales rep is correct because the protein length is larger than the wavelengths of visible light. Is this correct thinking? There is not really enough given in the problem to use the equations, so I am led to believe there is some other information I missed somewhere. Thank you.

2. Nov 21, 2014

### haruspex

The rep made a specific statement, and you are asked to comment on that. The statement says nothing about proteins.
I don't know whether this helps much, though. At least it avoids any question of what wavelengths the protein crystals absorb.

3. Nov 21, 2014

### Les talons

For optical microscopes, the resolvable structures would all be in the visible spectrum of wavelengths. Comparing half of the minimum wavelength of the visible light is 390 nm /2 = 195 nm, to 200 nm of the UV light. Because 195 < 200, the sales rep. is right?

4. Nov 21, 2014

### haruspex

Based purely on that, the sales rep would be wrong, no? But most visible light has longer wavelengths, and I do not see universal agreement on the 390 figure. Remember that you need to be able to observe it somehow. For the UV, that will involve some UV detector, whereas for an optical microscope you'll be using the human eye. Not everyone has the same colour range.