Difficult Conceptual Numericals
5 min read

Ray Optics And Optical Instruments

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1
Rotating plane mirrors:
Questions involving rotation of mirrors can be difficult at times. The very thing to remember is, if the plane mirror rotates by an angle , then the reflected ray rotates by an angle 2 in the same direction.
2
Combination of prism and mirror:
The total deviation of light in any combination can simply be found by the algebraic sum of the deviations produced by each optical element. For combination of prism and mirror, deviation produced by prism is added to the deviation produced by the mirror.
3
Finding refractive index of prism when angle of minimum deviation is given:
Sometimes, the angle of minimum deviation is not given. In fact, we will have to apply simple geometry to find the relation between the angle of prism A and angle of minimum deviation. There can be many layer to the question. Instead of asking the refracting index directly, the examiner might ask the factor by which light has slowed down in that medium, which is obviously, the R.I. of the medium.
4
An optical system immersed in a medium of given refractive index.
Usually, we come across questions where the optical systems are in placed in air. So using the absolute refractive indices for the lenses or prisms involved in place of relative R.I. doesn't make any difference. However we should be careful with equations where the system is immersed in an optical medium other than air.
5
Compound microscope which has another combination of lenses:
In such questions, we use the very basic concepts step by step. We first reduce the combination to a single equivalent lens. And then use the results obtained for compound microscope for the final two lenses: the objective and the eyepiece.