Physics · Ch 6 — Optics
Determination of Refractive Index of Material of the Prism
Determination of Refractive Index of Material of the Prism
To find the refracting angle of a prism, the prism is placed refracting-edge-first towards the collimator, and its slit, illuminated by monochromatic (e.g. sodium) light, is reflected off both faces and in turn. Turning the telescope to catch the reflection off (position ) and then off (position ), the angle turned through between and is , so is found by halving this measured angle. To find the angle of minimum deviation , the prism is instead oriented to refract the collimator's beam, and the telescope follows the deviated image as the prism table is slowly rotated; the image is seen to first approach, momentarily stop, and then recede -- the position where it turns back is the minimum-deviation position, whose reading is compared against the reading for the direct (undeviated) beam with the prism removed, giving directly as the difference. The refractive index of the prism …
What this figure shows. A prism sits on the spectrometer's prism table with its refracting edge facing the collimator, so parallel rays from the collimator strike both faces AB and AC at once and are reflected off each in turn; the telescope is swung to position T1 to catch the reflection off face AB and its reading noted, then swung to position T2 to catch the reflection off face AC. The angle between these two telescope positions, marked 2A in the figure, is exactly twice the apex angle A of the prism, so A …
What this figure shows. The prism is now turned to refract (rather than merely reflect) the collimator's beam, and the telescope tracks the resulting deviated image while the prism table is slowly rotated; as the table turns, the deviated image is shown moving in one direction, momentarily stopping and reversing, then moving back the other way -- the turning-point position is the minimum-deviation position. Comparing this turning-point telescope reading against the reading for the direct (undeviated, prism-remove …