How to draw ray diagram for bi-prism experiment (step-by-step | Smart techniques)

How to draw ray diagram for bi-prism experiment (step-by-step | Smart techniques):

Author:
Prof. Kali C. S.
M.Sc., M.Ed., D.C.S.
50+ Years of Experience in Physics Teaching

1. Introduction:

     Drawing neat diagrams in physics is not just about marks—it helps you understand the experiment more clearly. Many students remember the formula λ=Xd/D  but feel nervous when the question is, “Draw and explain the Fresnel bi‑prism experiment.”

    The ray diagram looks crowded – slit bi‑prism, virtual sources, interference region, fringes, screen and eyepiece – all on one page. This post shows a simple, construction style method so that any class 11–12 student can draw the bi‑prism ray diagram neatly on A4 sheet in two to three minutes.

Let’s begin.

 2.What is a Bi-prism? (Simple Explanation):

      A Fresnel bi‑prism is a very thin glass prism whose vertex angle is almost 179⁰. So it behaves as if two small prisms are joined at the base.
  When monochromatic light from a narrow slit falls on the bi‑prism, refraction makes the slit appear as two virtual coherent sources. These two virtual sources overlap and produce interference fringes on a distant screen, just like Young’s double slit experiment, and the pattern is observed with a micrometer eyepiece.

 3. What exactly happens in the Bi-prism Experiment?

    Here’s what you need to imagine:

• A slit S emits monochromatic light of wavelength λ.
• When this light passes through the bi-prism, refraction occurs at the two inclined faces.
• Due to this refraction, the slit behaves as if it has split into two virtual coherent sources, named S₁ and S₂.
• These two virtual sources overlap and produce interference fringes on a screen or using an eyepiece on an optical bench.

   So even though you have only one slit, the bi-prism creates two coherent sources—this is the beautiful trick behind the experiment!

 4. What do we measure in the experiment?

To find the wavelength of the light, we measure:

✔ Fringe width (X)

Distance between two consecutive bright fringes.

✔ Distance between virtual sources (d)

Found using lens displacement or optical bench method.

✔ Distance between sources and screen (D)

Once these three quantities are known, wavelength is calculated using the formula:

  λ = (X × d) / D

This is the heart of the experiment.

 5. Why the ray diagram is important:

The ray diagram explains:

• How the bi-prism creates two virtual sources
• How rays emerge from these sources
• Where the fringe pattern is formed
• Why interference occurs

    A clean, properly scaled diagram makes the concept very easy to understand—this is where smart drawing techniques help.

6. Construction of the Bi-prism Ray Diagram (Step-by-   Step):

Follow these steps carefully to draw a clear, neat, and properly labeled ray diagram of the bi-prism experiment.

  6.1. Draw the Principal (Optical) Axis

   First, select 5 horizontal lines of ruled A4 page, at middle of half page. Label them as 1, 2,3,4,5 in margin region with pencil.

     Draw a long, straight horizontal line at line number 3 — this will serve as the principal (optical) axis around which the entire diagram is structured.

 6.2. Mark the Single Slit

   On the left side of the axis, draw a small dot to represent the single narrow slit S. Label it “S”. This slit is the source of monochromatic light (wavelength λ). Then mark two dots at line 1 and 5 exactly above and below of slit S. Mark them as S₁ and S₂ .

 6.3. Position and Draw the Bi-prism
    At 3 cm to the right of the slit S draw the vertical line covering slightly above of line 1 and slightly below line 5. Mark point on axis (line 3) at 0.5 cm left from vertical line. Join end points of vertical line to this point. This forms bi-prism.

 6.4. Draw Incident Rays from the Slit
   From the slit S, draw 4 straight rays directed toward the two faces of the bi-prism and touching at line 1,2,4,5 at inclined face of bi-prism. Put arrow head on these lines directing to right. These represent the light emerging from the slit heading toward the prism.

  6.5. Show Refraction through the Prism
  Join S₁ and point of intersection of ray 2 and bi-prism face by dotted line and extend this line up to line 5. From the intersection line 5 and extended line draw a vertical big line forming the screen.

  Repeat this for slit S₂ . Now draw horizontal lines from S₁ and S₂ up to screen. Label interference region as shown in the figure. Add arrow heads to all lines heading to the right.

 6.6. Label Distances
On your diagram, clearly mark and label:

• • d: the distance between S₁ and S₂ (i.e., separation of virtual sources)
  • D: the distance from the virtual sources (S₁ / S₂) to the screen

• Label bi-prism.
• Labelling measurement parameters directly links the diagram to the experimental formula λ = X d / D, reinforcing understanding.

  6.7. Final Understanding:

After completing the diagram, you have a full picture:

• How one slit becomes two sources
• How interference arises
• How the experiment leads to measurement of λ

  By using this smart, stepwise method, your diagram will look neat, logical, and exam-ready.

 8. Formula Highlight:

λ = X d / D
This formula gives the wavelength of light used in the Bi-prism experiment.

  9. Conclusion:

    Drawing the Bi-prism ray diagram becomes very simple when you follow a structured technique. Understanding the purpose of each line—slit, prism, virtual sources, rays, screen—makes the experiment clear and easy to learn.

10. Video support:

    Do you want to know  “How to draw ray diagram for bi-prism experiment (step-by-step | Smart techniques)”,using given guidelines.

Let us see from the following video for an actual smarter method of drawing a diagram.