JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MATERIALS CHARACTERIZATION Previous Question Paper (August/September - 2021 - R18)

A metallurgical microscope operates on the principle of reflected light (incident light) rather than transmitted light, as metal specimens are opaque. Light from a source passes through a collector lens and a field diaphragm, then reflects off a glass plate (vertical illuminator) located above the objective lens. This light is directed downward through the objective lens onto the specimen surface. The reflected light from the specimen travels back up through the objective lens, passes through the glass plate, and is magnified by the eyepiece for observation.

Key components include: Light source, collector lens, vertical illuminator (glass plate), objective lens, specimen stage, and eyepiece.

Phase contrast microscopy is an optical imaging technique that converts phase shifts in light passing through a transparent specimen into brightness changes in the image. It is based on the principle that light waves passing through different parts of a specimen experience different phase shifts due to variations in refractive index and thickness.

The system uses a specialized annular diaphragm in the condenser and a phase plate in the objective lens. The annular diaphragm produces a hollow cone of light. When this light hits the specimen, some light passes through undeviated (background light), while light passing through the specimen is diffracted and phase-shifted. The phase plate in the objective lens selectively retards the phase of the undeviated light by a quarter-wavelength and reduces its intensity. When the undeviated and diffracted light rays recombine at the image plane, they undergo constructive or destructive interference, creating contrast that makes internal structures visible without the need for staining.