Experimental Procedure:
1. Rotate the turntable until the refracting edge of the prism is approximately pointing
towards the collimator. Light from the collimator will be reflected from both sides of
the prism.
2. Rotate the telescope until the first of the reflected slit images is centered on the
crosswires. Record the angle of the telescope.
3. Rotate the telescope until the second of the reflected slit images is centered on the
crosswires. Record the angle of the telescope.
4. The angle between the two telescope positions is twice the prism (apex) angle ϕ.
The error of apex angle ∆ϕ is a sum of two elements: one being the smallest-markedscale-division
(referring to precision of angle measurement) and the second is given
by the half of the angular width of the slit.
Figure 7: Measurement of Apex Angle ϕ.
Measurement of the Minimum Angle of Deviation εmin
For this part of the experiment the light of the wavelength at which n is to be
measured is required. The task is to find the refractive index n of the glass for
different wavelengths being the spectral components of neon lamp.
The prism and the spectrometer can be used to measure the refractive index at a
given wavelength using the formula:
⎟
⎠
⎞ ⎜
⎝
⎛ ϕ
⎟
⎠
⎞ ⎜
⎝
⎛ + ϕ
=
2
sin
2
ε sin
n
min
, (7)
where ϕ is the prism angle and εmin is angle of minimum deviation for that
wavelength.
Experimental Procedure:
1. Rotate the turntable and telescope until light will pass approximately symmetrically
through the prism (see Figure 5)
2. Rotate the telescope until the succession of views 1-5 shown in Figure 8 is
observed as the turntable is rotated consistently in the same direction. Record the
angle of the telescope. Repeat this procedure for different wavelengths.
3. Remove a prism and measure the angle for the direction of incident ray. Record
the angle of the telescope.
4. The angle between the two telescope positions from step 2 and 3 is the minimum
deviation angle εmin.
The error of the measured angles of minimum deviation ∆εmin is a sum of three
elements: two of them are identical as in the case of ∆ϕ and third is given by the half
of the angular dead interval. The death interval is an angle of the turntable rotation
(constantly in the same direction) from the moment when the slit image is stopped in
the field of view until the instant when it starts to move back.
Figure 8: Successive views as the turntable is rotated through the angle of
minimum deviation.
CALCULATIONS AND DATA ANALYSES:
Basing on Equation (6) calculate the apex angle ϕ of the prism and give the error
∆ϕ of obtain value.
Calculate the angle of minimum deviation εmin for different observed lines of neon
spectrum and give the errors ∆εmin of calculated values.
From Equation (7) calculate the refractive
indices nλ for following lines of neon
spectrum. Calculate the errors ∆nλ, which
depend on the errors of measurements of ϕ
and εmin angles, basing on the total
differential method. In your calculations
remember to use the values of ∆ϕ and ∆εmin
which should be expressed in radians.
Plot the curve of dispersion i.e. the
dependence of the calculated refractive
index as a function of the wavelength
(Figure 9). Do your experimental results
show the same trend of the theoretical
curve of normal dispersion?