Travelling microscope
Focus successively on a bare mark, its image through glass and a marker on the upper surface; read the same vertical carriage scale each time.
Physics / Glass slab refractive index
FREE TO EXPLOREEXPERIMENT 10 · PHYSICS LABORATORY
Refractive index of a glass slab using a travelling microscope
Determine the refractive index of a plane-parallel glass slab from microscope measurements of real and apparent depth, then compare individual ratios with a straight-line estimate. Explore a virtual apparatus, collect readings and analyse a graph.
CHANGE · OBSERVE · MEASURE
Your virtual workbench.
APPARENT DEPTH A—
MEASUREMENT STATUS
Ready for a virtual observation.
Adjust the apparatus, then take a reading.
Readings are generated by an ideal model. The diagram is schematic and is not to scale.
Simulation assumptions and limits
The lesson assumes a uniform plane-parallel slab, paraxial viewing, fixed illumination and air index one. Apparent-depth values are deterministic simulated differences rounded to 0.01 mm; independent microscope focusing, vernier reading, backlash and repeated-trial scatter are not modelled. A plotted fit does not include uncertainty in real depth or covariance between shared focus readings. The reference glass-to-air index is n = 1.500. Real thickness t ranges from 3 to 15 mm. The generator uses a = t/n, rounded to 0.01 mm. Under the virtual height datum R1 = 0 and R2 = t, the apparent focus height would be R3 = t − a. These are illustrative heights, not three independently measured scale readings. The fitted x and y are real and apparent depth in metres; n is the reciprocal of the fitted gradient.
Know your apparatus.
Plane-parallel glass slabs
Provide different real thicknesses of the same transparent material, with model refractive index 1.500 relative to air.
Fine base mark and surface marker
Identify the lower object and the upper surface without confusing their focus positions.
Levelled support and scale
Keep the viewing axis close to the surface normal and express all vertical differences in consistent units.
FOLLOW THE METHOD
A reason behind every step.
Determine the refractive index of a plane-parallel glass slab from microscope measurements of real and apparent depth, then compare individual ratios with a straight-line estimate. Suggested independent settings: 3, 5.5, 8, 11.5, 15 mm.
Define the scale and focus references
Level the microscope and identify an upward-increasing vertical scale. Focus the crosshairs first, then focus the bare base mark and label its carriage height R1.
Why this step matters
A clear reference and sharp crosshairs prevent sign mistakes and focus ambiguity.
Choose a thickness
Place a clean slab over the mark without moving the base. Select real thickness t in the simulator; the simplified virtual datum is R1 = 0 and R2 = t.
Why this step matters
The real and apparent depths must refer to the same base and upper surface.
Locate image and surface
In the physical method focus the apparent mark through glass to obtain R3, then a fine marker on the upper surface to obtain R2. Approach final focus from the same carriage direction. The virtual reading returns their apparent-depth difference directly.
Why this step matters
Consistent motion reduces backlash; the displayed virtual depth is not a simulation of three independent vernier readings.
Build a thickness series
Measure and record five well-spaced thicknesses of the same model material. For example use 3, 6, 9, 12 and 15 mm. Repeat focus judgements in a real apparatus.
Why this step matters
A broad thickness range constrains the slope; repeats reveal focus scatter absent from this ideal model.
Check a row and fit
Calculate n = t/a for one row. Convert both depths to metres for the stored fit, then fit a = st + b without forcing the intercept to zero.
Why this step matters
A hand calculation checks units and the difference between apparent rise and apparent depth.
Report what the model supports
Calculate n = 1/s, inspect residuals and intercept, and state the resolution, fixed-material assumptions and omitted uncertainties. Label all virtual readings simulated.
Why this step matters
The numerical estimate needs a clear account of its origin and limits.
YOUR NOTEBOOK · 0 OBSERVATIONS
Turn measurements into a result.
Points are your observations. The fit uses SI x and y values before display scaling.
LINEAR FIT · y = mx + b
n = t/a; a = mt + b; n = 1/m
| # | Real slab depth t (mm) | Apparent depth a (mm) | Real depth t (mm) | Apparent depth a (mm) | Remove |
|---|
No readings yet. Take a virtual measurement and record it.
Calculate it yourself.
CHECK YOUR UNDERSTANDING