11 Physics Experiment No. 9 Refractive Index of Prism

Experiment No. 9:  Refractive Index of Prism 

Aim:

To determine the angle of minimum deviation(δ) for a given glass prism by plotting a graph between the angle of incidence (i)

$i$

and the angle of deviation (δ)

Apparatus:

• Drawing board
• White paper sheets
• Glass prism
• Drawing pins
• Office pins (4 per reading)
• Protractor
• Ruler/Scale
• Sharp pencil
• Graph paper
• Diagram
• Theory 
• When a ray of light passes through a prism, it undergoes two refractions and deviates from its original path. The angle between the incident ray and the emergent ray is called the angle of deviation
•    by the formula: 
• Angle of incidence = Angle of emergence (e)
• At minimum deviation position: 
• The angle of deviation depends on the angle of incidence (i)
• Fix a sheet of white paper on the drawing board using drawing pins.

            Step 2: Tracing the path of light 

             and

  

  ${55}^{\circ }$

  with respect to the normal N1O using the protractor. For each angle of incidence: 

  • Fix two pins (P1 and P2) vertically on the incident ray line.
  • View the images of P1 and P2 through the other refracting surface (AC).
  • Fix two more pins (P3 and P4) such that they are in a straight line with the images of P1 and P2.
  • Remove the pins and the prism. Draw a line through P3 and P4 to represent the emergent ray.
  • Extend the incident and emergent rays to meet at a point. Measure the angle between these two rays using a protractor. This is the angle of deviation (
    

    $\delta$
    δ) for that specific angle of incidence (i
    

    $i$
    ). 
  • Step 3: Repeating measurements 
  • Repeat the procedure for all marked angles of incidence and record the angle of deviation for each. 
  • Step 4: Plotting the graph 
  • Plot a graph with the angle of incidence (i
    

    $i$
    )

   on the X-axis and the angle of deviation

  • 

    $A={60}^{\mathbf{\circ }}$
    (standard equilateral prism)


• Observations 
  • 

    $A={60}^{\mathbf{\circ }}$
    (standard equilateral prism)
  
  
  
  
  
  
  
  

  $i$

  (X-axis) and

  

  $\delta$

  (Y-axis). The graph has a shape that dips to a minimum point before rising again, forming a 'U' shape. The lowest point on the Y-axis of the curve represents the angle of minimum deviation(δ)
  

  $\delta$

  is approximately

  

  ${38}^{\mathbf{\circ }}$

  . 
  

  ${\delta }_m={38}^{\mathbf{\circ }}$

  .
• Angle of the prism, 
• A graph is plotted between 
• Thus, the angle of minimum deviation, 
  

  ${\delta }_m={38}^{\mathbf{\circ }}$

  
  

Result

The refractive index (

$\mu$

 μ ) of the prism material is  1.509.

MCQs (With Answers at the End)

1. The angle of deviation for a prism is minimum when the light ray:

A. Grazes one surface of the prism
B. Passes symmetrically through the prism
C. Is incident at 90°
D. Emerges at 0° deviation

---

2. At minimum deviation, the angle of incidence is:

A. Equal to the angle of refraction
B. Equal to the angle of emergence
C. Equal to the prism angle
D. Equal to the critical angle

---

3. Which physical quantity is plotted on the x-axis in this experiment?

A. Angle of deviation (δ)
B. Angle of incidence (i)
C. Refractive index (μ)
D. Prism angle (A)

---

4. In a graph between i and δ, the point of minimum deviation is identified as:

A. The highest point on the curve
B. The lowest point on the curve
C. The point where δ = i
D. The midpoint of the graph

---

5. The refractive index of the prism material can be determined using:

A. Snell’s law only
B. Mirror formula
C. μ = sin[(A + δₘ)/2] / sin(A/2)
D. μ = tan(A + δₘ)

---

6. The angle between the two refracting surfaces of a prism is called:

A. Angle of incidence
B. Angle of deviation
C. Prism angle (A)
D. Emergent angle

---

7. The emergent ray deviates the least from the incident ray when:

A. δ = δₘ
B. i = 0°
C. A = 90°
D. i → ∞

---

8. In the experiment, the angle of deviation is calculated by:

A. δ = e – i
B. δ = A – r
C. δ = i + e – A
D. δ = A + μ

---

9. The graph between i and δ is generally:

A. A straight line
B. A parabolic curve with a minimum
C. A horizontal line
D. A decreasing linear graph

---

10. Minimum deviation occurs when the refracted ray inside the prism:

A. Travels parallel to the base
B. Gets totally internally reflected
C. Makes equal angles with the two prism faces
D. Is perpendicular to the base

---

Answer Key

1 – B
2 – B
3 – B
4 – B
5 – C
6 – C
7 – A
8 – C
9 – B
10 – C

                                                                   

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