Experiment 6 : To determine refractive index of a glass slab using travelling microscope.
Aim :
To determine refractive index of a glass slab using travelling microscope.
Apparatus :
Travelling microscope, a glass slab, lycopodium powder, spirit level.
Theory :
When an object mark X is placed at the bottom of the glass slab of refractive index n, it appears to be raised, when viewed obliquely. The actual depth of the mark X is called real depth and the raised depth is called apparent depth.
$\therefore$ Refractive index of the glass slab
$n= \frac{Real \ depth}{Apparent \ depth}$
Procedure
- Calculate the vernier constant of the travelling microscope.
- Level the travelling microscope using a spirit level and the base screws. Set its axis to the vertical scale.
- Move the eye piece of the microscope so that a sharp image of the cross-wires is obtained.
- Put an ink mark X on the plateform of the travelling microscope (M) and focus the microscope on it. Note the reading $R_1$.
- Raise the microscope and put the glass slab on the mark.
- Focus the microscope on the image of the mark and note the reading $R_2$.
- Now sprinkle lycopodium powder on the upper face of the glass slab and focus again to record $R_3$.
- Repeat three times.
Observations :
Refractive index of glass
Mean $n= \frac{n_{1}+n_{2}+n_{3}}{3}$
$n= \frac{1.55+1.58+1.46}{3} = \frac{4.59}{3}$
$n=1.53$
Refractive index of glass slab $n = 1.53$
Actual refractive index of glass slab = 1.5
Precautions
- The cross-mark should be made on the inner side of the bottom of the beaker.
- Microscope should be parallel to the vertical scale.
- The cross-wires must be focussed before making any use of the travelling microscope.
- The rack and pinion arrangement not be touched while taking observations for $R_2$ and $R_3$.
- Only a fine layer of lycopodium powder should be sprinkled.
- There should be no parallax between the cross-wires and the image of the mark.