Prism Explained for Re-NEET 2026: Angle of Deviation, Minimum Deviation and Refractive Index

Ray Optics is one of the highest-scoring topics in NEET Physics, and within it, the prism is a concept that appears almost every year. The good news is that prism questions follow a very predictable pattern. If you understand the geometry, know three or four key formulas, and can solve a standard numerical, you can confidently pick up marks from this topic in Re-NEET 2026.

This prism revision for Re-NEET 2026 covers everything from basic terminology to the minimum deviation condition and refractive index calculation, with a fully solved numerical at the end.

Basic Setup of a Prism: Terminology You Must Know

Before any formula makes sense, you need to be clear on what each angle in the prism diagram represents. This is the foundation of all prism ray optics revision.

A ray of light enters one refracting surface of the prism and exits through the other. Here is what each angle is called:

• Angle of Incidence (i): The angle at which the incoming ray hits the first refracting surface of the prism.
• Angle of Refraction at the first surface (r1): The angle of refraction as the ray enters the prism.
• Angle of Incidence at the second surface (r2): The angle at which the ray inside the prism hits the second refracting surface.
• Angle of Emergence (e): The angle at which the ray exits the prism from the second surface.
• Angle of Deviation (delta): The angle between the original direction of the incident ray and the final direction of the emergent ray. This tells you how much the prism has bent the light.
• Angle of Prism (A): The apex angle of the triangular prism.

Lock these six terms in your memory. Nearly every prism important question for NEET begins by giving you two or three of these values and asking for the rest.

Key Formulas for Prism

These two relationships connect all the angles and must be memorised for any prism short notes revision:

Formula 1: Angle of Deviation

delta = i + e – (r1 + r2)

Formula 2: Prism Angle Relationship

A = r1 + r2

These two formulas are the core of the entire prism chapter. Most numericals reduce to substituting values into these two expressions.

Minimum Deviation Condition: The Most Tested Case

The minimum deviation condition is the single most important special case in prism optics revision, and it shows up almost every year in NEET.

When a prism is set at the position of minimum deviation, three specific conditions hold simultaneously:

1. i = e (angle of incidence equals angle of emergence)
2. r1 = r2 = A/2 (the ray travels symmetrically through the prism)
3. Delta minimum = 2i – A

These three conditions define the minimum deviation position. If a question tells you the ray undergoes minimum deviation, you can immediately apply all three.

The refractive index of the prism in the minimum deviation condition is given by:

mu = sin[(A + D_min) / 2] / sin(A / 2)

This is derived directly from Snell’s Law and is the standard formula for calculating the refractive index of a prism from experimental data. It appears in both theory and numerical questions.

Solved Numerical: Prism with Minimum Deviation

This is the type of standard numerical that appears in NEET and is solved step by step in this prism revision video for Re-NEET 2026.

Problem: The angle of incidence for a ray of light at a refracting surface of a prism is 45 degrees. The angle of the prism is 60 degrees. If the ray suffers minimum deviation through the prism, find the angle of minimum deviation and the refractive index of the prism.

Step 1: Find the Angle of Minimum Deviation

Using the minimum deviation formula:

D_min = 2i – A

D_min = 2 x 45 – 60 = 90 – 60 = 30 degrees

Step 2: Find the Refractive Index

mu = sin[(A + D_min) / 2] / sin(A / 2)

mu = sin[(60 + 30) / 2] / sin(60 / 2)

mu = sin(45) / sin(30)

mu = (1/root 2) / (1/2)

mu = root 2 (approximately 1.414)

So the angle of minimum deviation is 30 degrees and the refractive index is root 2.

This is a direct NEET-pattern numerical. Practice it until you can solve it in under two minutes.

Dispersion of Light Through a Prism

When white light passes through a prism, it splits into its constituent colours. This is called dispersion of light. The order of colours in the spectrum, from least deviated to most deviated, is:

VIBGYOR from bottom to top (Violet, Indigo, Blue, Green, Yellow, Orange, Red)

Violet light deviates the most because it has the shortest wavelength. Red light deviates the least because it has the longest wavelength.

For NEET, the key facts about dispersion are:

• Violet has the highest refractive index in a given medium
• Red has the lowest refractive index
• The angle of deviation depends on the wavelength of light
• Dispersion occurs because the refractive index of a medium varies with wavelength

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