Determination of Refractive Index of Glass Prism
What you will learn
Determine the angle of minimum deviation (Dm) for a given glass triangular prism by plotting the i-D curve and calculate the refractive index (μ) of the prism material.
Materials needed
- Equilateral Triangular Glass Prism (60°)What is it?Equilateral Triangular Glass Prism (60°)₹50
High-purity optical crown glass equilateral prism with polished refracting faces and ground base.
Specifications- Refracting Angle (A)
- 60° ± 0.1°
- Face Dimensions
- 38 mm × 38 mm
- Glass Type
- Dense Crown (Schott BK7 equivalent)
- Optical Flatness
- λ/4 wave
- Drawing Board with Thumb PinsWhat is it?Drawing Board with Thumb Pins₹95
Seasoned soft pine wood drawing board with non-warping laminated construction.
Specifications- Size
- 450 mm × 300 mm × 15 mm
- Material
- Soft pine wood core
- Surface
- Smooth sanded natural grain
- Edge
- True-edge ebony strip
- White Drawing Sheet (A3/A4)What is it?White Drawing Sheet (A3/A4)₹140
Rigid stainless steel entomological laboratory pins with sharp points.
Specifications- Length
- 38 mm
- Diameter
- 0.65 mm
- Head
- Solid brass round head
- Corrosion Resistance
- Nickel plated finish
- Optical Fixing PinsWhat is it?Optical Fixing Pins₹185
Transparent acrylic 360-degree precision protractor and bevelled millimeter ruler.
Specifications- Protractor Diameter
- 150 mm
- Angular Accuracy
- 0.5 degree divisions
- Scale Length
- 30 cm / 1 mm divisions
- Material
- Optical grade acrylic
- Protractor & 30cm Metric Scale setWhat is it?Protractor & 30cm Metric Scale set₹230
Standard laboratory apparatus used in Determination of Refractive Index of Glass Prism.
Specifications- Category
- Physics Apparatus
- Grade
- CBSE / NEP 2020 Laboratory Standard
- Sharp Pencil & EraserWhat is it?Sharp Pencil & Eraser₹275
Standard laboratory apparatus used in Determination of Refractive Index of Glass Prism.
Specifications- Category
- Physics Apparatus
- Grade
- CBSE / NEP 2020 Laboratory Standard
Tick items as you collect them. Tap + to see the product, specifications and buy options.
Stay safe
- Optical pins must be fixed strictly vertically into the board.
- Distance between pins on the same ray must not be less than 5 cm to ensure angular accuracy.
- Align pin pricks viewing their feet/bases, not the bent tops.
- Do not disturb the prism position while sighting pins.
Steps
- 1Fix the white paper firmly on the wooden drawing board with thumb pins.Observation
Drawing board is perfectly flat with paper tautly pinned down without any wrinkles.
SafetyPress board pins straight downward using a thumb pin press to protect fingertips.
- 2Place the prism on its triangular base and draw its boundary ABC with a sharp pencil.Observation
Pencil outline shows crisp 60° triangular vertices without multiple overlapping traces.
SafetyHandle optical prism only by its frosted ground base, never touching polished refracting faces.
- 3Draw a normal on face AB and erect an incident ray at an angle i = 35°.Observation
Normal drawn at 90° using circular protractor; incident ray accurately marked at chosen angle.
SafetyUse a sharpened 2H pencil for high precision line width < 0.2 mm.
- 4Fix two optical pins P1 and P2 vertically along the incident ray separated by at least 5 cm.Observation
Fix pins P1 and P2 vertically separated by at least 5 cm for minimal parallax angular error.
SafetyCheck pin straightness before fixing into board.
- 5Look through face AC of the prism and fix pins P3 and P4 such that their bases align with images of P1 and P2.Observation
Looking through opposite face: feet of images P1', P2' align strictly with pins P3 and P4.
SafetyKeep eye level with the horizontal plane of the prism table to avoid vertical tilt error.
- 6Remove the prism, join the pin pricks, produce incident and emergent rays to measure the deviation angle (D).Observation
Incident ray and emergent ray extended to intersect at deviation angle D.
SafetyStore prism immediately into its velvet-lined box after completing tracings.
- 7Repeat for incident angles 40°, 45°, 50°, 55°, and 60°, then plot the angle of incidence vs deviation (i-D) graph.
Tap + on a step to see what you will observe and any safety tips.
Record your observations
| Angle of Incidence (i) | Angle of Emergence (e) | Angle of Deviation (D = i + e - A) | i + e Sum |
|---|---|---|---|
| 35° | 65.5° | 40.5° | 100.5° |
| 40° | 59.8° | 39.8° | 99.8° |
| 45° | 54.5° | 39.5° (Dm) | 99.5° |
| 50° | 50.0° | 40.0° | 100.0° |
| 55° | 46.2° | 41.2° | 101.2° |
| 60° | 43.0° | 43.0° | 103.0° |
Tap a highlighted cell to write your own observation.
Result and conclusion
The i-D graph forms a smooth parabola with a distinct minimum. The refractive index of crown glass is determined to be approximately 1.51 ± 0.02 using the prism formula.
| Observation | What it means |
|---|---|
| Minimum Deviation Point | At i = 45°, angle of deviation reaches its unique minimum value Dm = 39.5°. |
| Symmetry Condition | At minimum deviation Dm, the angle of incidence equals angle of emergence (i ≈ e = 45°-50°). |
| Internal Ray Parallelism | At Dm, the refracted ray travels exactly parallel to the base of the equilateral prism. |
| Refractive Index of Glass | Formula μ = sin(49.75°)/sin(30°) yields μ = 1.524, confirming standard crown glass. |
Sample readings. Replace with your own results.