Question

Please briefly answer all the questions below ,and writing those more clearer and tidier (It is best to use typing)

1. This is a problem related to the transmission electron microscopy (TEM)

1. Please describe the spherical aberration and chromatic aberration. Please quantitatively compare the aberration coefficients with respect to the wavelength for the electron and optical microscopes.
2. Please describe the Abbe Theory of Microscope.
3. Please describe the Bright field and Dark field imaging and atomic resolution imaging (Hint: Using the Fourier Synthesis point of view)

Answer 1

a) Spherical aberration: Spherical aberration is related with curved surface e.g. Lens or mirror due to easier form of an image in comparison to the non-curved surface. Light ray strick to this surface reflected or refracted to the center more or less and reduces the image quality which is produced by the optical system.

Chromatic aberration: If the lens fails to focus on all color at the same point the chromatic aberration is arising which is mainly due to the dispersion and refractive index of the lens. The refractive index varies with the different wavelength of light.

b) Abbe Theory :

In 1866, Ernst Abbe and Carl Zeiss cooperated together to improve the optical performance of microscopes. Only until then microscopes and microscope objectives were being produced by trial and error; some having exceptional optical performance but others having undesirable features. Abbe and Zeiss knew that they could only get optimum and consistent performance on a complete theoretical basis.

Abbe discovered after many calculations and experiments that the diffraction image in the back focal plane of the objective is essential for image formation.

“No microscope permits components (or the features of an existing structure) to be seen separately if these are so close to each other that even the first light bundle created by diffraction can no longer enter the objective simultaneously with the non-diffracted light cone.” Ernst Abbe, 1873.

Light rays diffracted by the specimen from the objective of an optical microscope are fundamentally important in Abbe's theory. Fine details of the specimen will not be visible unless diffracted rays of light from the specimen are captured by the objective. (University, 2005)

Diffraction forms the image of the light absorbing specimen. The light shows the specimen's structure consists of a grating of different shapes of holes. A specimen will give a consistently bright image if the rays of light pass through the specimen undiffracted. Information is carried by the diffracted light over and around the structures of the specimen.

c) Bright Field Imaging: Bright-field microscopy is the simplest of all the optical microscopy illumination techniques. Sample illumination is transmitted (i.e., illuminated from below and observed from above) white light, and contrast in the sample is caused by attenuation of the transmitted light in dense areas of the sample. Bright-field microscopy is the simplest of a range of techniques used for illumination of samples in light microscopes, and its simplicity makes it a popular technique. The typical appearance of a bright-field microscopy image is a dark sample on a bright background, hence the name.

Dark Field Imaging: Dark-field microscopy (also called dark-ground microscopy) describes microscopy methods, in both light and electron microscopy, which exclude the unscattered beam from the image. As a result, the field around the specimen (i.e., where there is no specimen to scatter the beam) is generally dark.

In optical microscopy, dark-field describes an illumination technique used to enhance the contrast in unstained samples. It works by illuminating the sample with light that will not be collected by the objective lens and thus will not form part of the image. This produces the classic appearance of a dark, almost black, background with bright objects on it.

Dark-field microscopy is a very simple yet effective technique and well suited for uses involving live and unstained biological samples, such as a smear from a tissue culture or individual, water-borne, single-celled organisms. Considering the simplicity of the setup, the quality of images obtained from this technique is impressive.