Flashcards in Lecture 8: Optics 1 Deck (65)
Describe what light may be. how it can act, properties.
Light behaves as both a particle and a wave
Can bounce (reflect) and bend (diffract or refract)
Has wave properties
– Wavelength: visible is between 400-700 nm (White light carries all visible wavelengths)
– Direction of travel
– Direction of vibration
What is diffraction and what is the product of diffraction?
•Due to its wave properties, light undergoes diffraction (deviation of the light wave direction)
•Light wavefronts diffract when they travel through narrow slits e.g. apertures
•Wavefronts spread and interact, causing constructive and destructive interference
What causes the airy disk formation?
The shape of the diffraction pattern varies according to wavelength and size of the aperture.
Thus varying the shape of the airy ring which is a property of constructive and destructive interference.
What generates an airy disk?
Diffraction through a circular aperture creates an Airy Disk
What determines the distance to the first dark ring?
The radius of the Airy disk is the distance r from the centre to the first dark ring
What is a PSF?
– intensity distribution of Airy Disk in 3D
- describes diffraction pattern of a point source of light in x, y, and z
What is an image?
An image is produced by an instrument. It is not the same as the object but is some ‘facsimile’ of the object e.g. it may be only 2D.
What does the PSF describe and the implication?
The ‘point spread function’ (PSF) describes what the instrument does to information from the object to produce an image.The image is the object convolved by the PSF.
Describe the convolution in the microscope
Image = Object X PSF
(For each point, take the object and multiply it by the entire PSF centred at that point in theimage and sum the results of the multiplication)
Describe the relationship of the PSF to the image;
The object is a large number of points arranged in space
The image is the superposition of a large number of points convolved with the PSF.
Is PSF constant?
No, it can suffer from spherical abberation
What does airy disk vary with?
Size of Airy diffraction pattern determined by resolving power of objective lens
Resolving power: Determined by NA
What is NA?
•NA = ability of lens to capture light rays
•Determines ability of lens to gather light and resolve detail
•Typically: large NA ~ small WD
What is the equation for NA?
NA = n x sin(α)
n = refractive index
alpha = one-half of the objective's opening angle (determined by focal length of objective lens)
What are some refractive indices?
Refractive index n is limiting (air 1.0, water 1.33,oil ~1.51)
What is the criterion for being able to resolve between two points?
When the intensity in the valley is reduced by ~25% compared with the two maxima, the two points are discrete (this is just sufficient for the human eye to see two separate points)
How does resolution relate to nyquist?
Nyquist sampling of an image of two points separated by the Rayleigh resolution
Nyquist sampling - sampling interval twice the highest specimen spatial frequency to preserve the spatial resolution in the resulting digital image
Describe the two plains of resolution;
Lateral (X and Y) and axial resolution
What is the equation for lateral resolution;
R(lat) = 1.22 wavelenght / (Na objective + Na cond)
if Na (ob) = Na (cond) (Epiflourescence)
then R(lat) = 0.61 wavelenght / NA
structures that lie closer than this distance cannot be resolved in the lateral plane using a light microscope
What is the equation for axial resolution?
R(axial) = 2.(wavelength).n / (Na)^2
Whats a van Leeuwenhoek microscope?
What is the property of thin lenses?
1/p + 1/q = 1.f
p = distance from object to lens
q = distance from lens to image
f = focal distance
q/p = magnification
How many lenses does a a compound microscope x10 objective have?
Microscopes/objectives contain a combination of these lenses that focus, magnify or correct aberrations
i.e heaps / tricky question
How is magnification of a microscope determined?
Objective x lens = total
What are some types of obberations?
What is spherical abberation
Spherical aberration occurs due to the increased refraction of light rays when they strike a lens near its edge, in comparison with those that strike nearer the centre.
What is chromatic abberation?
Chromatic aberration is a failure of a lens to focus all colours to the same convergence point. Lenses have a different refractive index for different wavelengths of light
What is field curvature?
a flat object normal to the optical axis cannot be brought into focus on a flat image plane.
Check the slide in that demonstrates how to read a lens and interpret it