2 thoughts on “give scientific reason. (any two)<br />(1) a reflecting glass prism is better reflector than highly polished mirror.”
Answer:
This was true before the advent of enhanced aluminum coatings and all dielectric coated mirrors which can reflect 99.9% of the light.
A right angle prism will reflect 100% of the light through total internal reflection, but unless it has a really good antireflection coating on each of the right angle prism faces it will loose about 4.25% of the light at each air glass interface. There is also a small amount of light absorbed by the glass, about 0.2% absolute.
A prism requires making 3 surfaces optically flat within 1/4 of a wavelength of light. A plane mirror only requires one flat surface with a flatness requirement that is an 1/8 of a wavelength of light.
In corrosive environments, the unmetallized prism will last longer than an optical flat with an aluminum coating on the first surface.
It is superior under a few conditions, but of course it could be inferior if any of those conditions were violated.
Total internal reflection depends on the rays striking the inside surface of the prism to be above the critical angle. The following conditions will give a superior image to a plane mirror:
The flatness and smoothness of the prism surface is as good as or better than the surface of the plane mirror
The outside surface of the prism is clean, free of all molecular layers, fingerprints, and there is an air gap of at least 10 wavelengths thickness
The wavelengths comprising the image are within the transmission range of the glass
You can tolerate an image at the high incidence angles required by the TIR prism
You can compensate for the optical depth added by the path through glass
The glass is ultra-high homogeneity to prevent wavefront errors
In most of my work, one or more conditions is not met and a plane mirror image will be superior. We can make plane mirrors with high reflectivity coatings that are protected from oxidation with an oxide layer overcoat. In addition, prisms are usually very problematic with high power laser beams as they tend to get extremely hot.
That said, the penta-mirror image in my Canon T3i is inferior to a pentaprism image because Canon used cheap aluminum coatings on the mirrors rather than higher quality silver or MLD coatings. It is only inferior in terms of brightness, but that is an important difference.
Answer:
This was true before the advent of enhanced aluminum coatings and all dielectric coated mirrors which can reflect 99.9% of the light.
A right angle prism will reflect 100% of the light through total internal reflection, but unless it has a really good antireflection coating on each of the right angle prism faces it will loose about 4.25% of the light at each air glass interface. There is also a small amount of light absorbed by the glass, about 0.2% absolute.
A prism requires making 3 surfaces optically flat within 1/4 of a wavelength of light. A plane mirror only requires one flat surface with a flatness requirement that is an 1/8 of a wavelength of light.
In corrosive environments, the unmetallized prism will last longer than an optical flat with an aluminum coating on the first surface.
Answer:
It is superior under a few conditions, but of course it could be inferior if any of those conditions were violated.
Total internal reflection depends on the rays striking the inside surface of the prism to be above the critical angle. The following conditions will give a superior image to a plane mirror:
The flatness and smoothness of the prism surface is as good as or better than the surface of the plane mirror
The outside surface of the prism is clean, free of all molecular layers, fingerprints, and there is an air gap of at least 10 wavelengths thickness
The wavelengths comprising the image are within the transmission range of the glass
You can tolerate an image at the high incidence angles required by the TIR prism
You can compensate for the optical depth added by the path through glass
The glass is ultra-high homogeneity to prevent wavefront errors
In most of my work, one or more conditions is not met and a plane mirror image will be superior. We can make plane mirrors with high reflectivity coatings that are protected from oxidation with an oxide layer overcoat. In addition, prisms are usually very problematic with high power laser beams as they tend to get extremely hot.
That said, the penta-mirror image in my Canon T3i is inferior to a pentaprism image because Canon used cheap aluminum coatings on the mirrors rather than higher quality silver or MLD coatings. It is only inferior in terms of brightness, but that is an important difference.