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Home Physics Engineering Physics To determine the dispersive power of the material of a given prism by the spectrometer
Engineering Physics Lab Experiments

To determine the dispersive power of the material of a given prism by the spectrometer



Aim

To determine the dispersive power of the material of a given prism by the spectrometer

Apparatus Required

Spectrometer, Prism and Mercury Vapour Lamp

Principle

The Dispersive power of the material of the given prism is expressed as

ω = (μ 2 - μ 1)/(μ-1)

Where μ 1 and μ 2 are the refractive indices of two colors

μ = (μ 1 + μ 2)/2

Usually the colors chosen are blue and red so that

ω = (μ b - μ r)/(μ - 1)

Where μ = (μ b + μ r)/2

Procedure

  1. The usual adjustments of the spectrometer are made .The refractive angle of the Prism is found.
  2. Then the prism is mounted on the prism table and the position of prism is adjusted to observe the spectrum of the mercury vapor.
  3. Observing the blue line in the spectrum through the telescope, the prism is adjusted for minimum deviation position.
Telescope

4. Working with the tangent screw of the telescope, the position of the prism is adjusted so that the blue line is just one point of refracting its path after coming to the point of intersection of the cross wires.

5. The readings of the telescope for the minimum deviation of red line are noted.

6. The telescope is brought in line with the collimator and removing the prism, the direct readings on both verniers are noted.

7. The respective differences give the minimum deviations for blue and red colors.

Their refractive indices are found by

μ b = [Sin (A+D b) / 2] / [Sin (A/2)] & μ r = [Sin (A + Dr) / 2] / Sin (A / 2)

The Dispersive power of the material of the prism, for blue and red colors is found by the relation

ω = (μ b - μ r)/(μ - 1)

Observations

The observations of the above experiment are as follows

V 1= MSRo+ (LC') VC

Spectral Line LHS V1 RHS V2 LHS V1' RHS V2' Dm(LHS)[V1 - V'1] Dm(RHS)[V2 - V'2] AVG Dm μ = [Sin(A + Dm) / 2] / Sin (A / 2)
Blue
Green
Yellow
Red

Dispersive power ω = (μ b - μ r)/(μ - 1)

Precautions

  1. The prism should be adjusted for each colour separately.
  2. Readings are noted without any parallax error.

Result

The Dispersive power of the material of the prism = ...............