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Prism deviation-versus-incidence plot: NEET MCQs with answers

49 questions on this topic of Experimental Skills (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. In the prism experiment, the paths of the incident and emergent rays are traced using:

  1. A travelling microscope
  2. Pins, by viewing them in a straight line through the prism
  3. A laser and photodiode
  4. A spectrometer only
Show answer and explanation

Answer: (B) Pins, by viewing them in a straight line through the prism

Pins fixed in a line along the incident ray are viewed through the prism; two more pins are placed in line with their images to mark the emergent ray.

NCERT Class 11

medium

2. A student performs the i-δ experiment using an equilateral crown glass prism. If the entire apparatus is illuminated by violet light instead of sodium yellow light, how will the minimum deviation (δ_m) and the refractive index (μ) shift?

  1. Both δ_m and μ will increase
  2. Both δ_m and μ will decrease
  3. δ_m will increase, but μ will remain unchanged
  4. δ_m will decrease, but μ will increase
Show answer and explanation

Answer: (A) Both δ_m and μ will increase

The refractive index of glass is higher for shorter wavelengths (μ_violet > μ_yellow). Because minimum deviation δ_m increases monotonically with refractive index, both μ and δ_m will increase.

hard

3. In an experimental setup, a student measures the slope dδ/di of the i-δ graph at various points. Which of the following is correct regarding the slope at the point of minimum deviation?

  1. The slope is strictly zero
  2. The slope is equal to +1
  3. The slope is undefined (vertical tangent)
  4. The slope is equal to -1
Show answer and explanation

Answer: (A) The slope is strictly zero

The angle of deviation δ reaches a minimum value at δ = δ_m. By calculus, at any local extremum (minimum) of a smooth function, the first derivative of the dependent variable with respect to the independent variable vanishes. Hence, dδ/di = 0 at the minimum deviation point, corresponding to a horizontal tangent.

medium

4. In an experiment to plot the i-δ curve, an equilateral glass prism is immersed completely in water (μ_w = 4/3). The prism is made of glass (μ_g = 3/2). Compared to the curve obtained in air, the i-δ curve obtained in water will:

  1. Shift entirely upwards with an increased minimum deviation
  2. Invert upside down, showing a maximum instead of a minimum deviation
  3. Remain completely unchanged because the prism geometry is identical
  4. Shift entirely downwards with a decreased minimum deviation
Show answer and explanation

Answer: (D) Shift entirely downwards with a decreased minimum deviation

The relative refractive index of the prism in water is μ_rel = μ_g / μ_w = (3/2) / (4/3) = 9/8 = 1.125, which is significantly less than that in air (1.5). Since the bending at both interfaces decreases, the net deviation at every incidence angle decreases, shifting the entire i-δ curve downward.

easy

5. While plotting the i-δ graph for an equilateral prism, a student measures the angle of minimum deviation to be δₘ = 38°. If the angle of incidence corresponding to minimum deviation is i, what is the value of i?

  1. 38°
  2. 49°
  3. 52°
  4. 68°
Show answer and explanation

Answer: (B) 49°

At minimum deviation for a prism of refracting angle A, we have δₘ = 2i - A. Here A = 60° and δₘ = 38°, so 2i = 60° + 38° = 98°, giving i = 49°.

hard

6. While plotting the i-δ graph for an equilateral glass prism, a student finds that when the angle of incidence is decreased below a critical threshold value i_min, no emergent ray can be observed on the other face. This occurs because:

  1. The angle of incidence inside the prism at the second face (r₂) exceeds the critical angle of the glass-air interface
  2. The light undergoes total internal reflection at the first refracting face
  3. The angle of deviation becomes imaginary because δ cannot be less than zero
  4. The prism starts absorbing the incident light completely at smaller angles
Show answer and explanation

Answer: (A) The angle of incidence inside the prism at the second face (r₂) exceeds the critical angle of the glass-air interface

As the angle of incidence i decreases, r₁ decreases according to Snell's law. Since r₁ + r₂ = A, a smaller r₁ forces r₂ to increase. When r₂ exceeds the critical angle θ_c = sin⁻¹(1/μ) for the glass-air boundary, total internal reflection occurs at the second surface, preventing the ray from emerging.

medium

7. During the i-δ experiment with an equilateral prism, the minimum deviation occurs at an angle of incidence i = 45°. What is the value of this minimum deviation δₘ?

  1. 15°
  2. 45°
  3. 60°
  4. 30°
Show answer and explanation

Answer: (D) 30°

For an equilateral prism, A = 60°. At minimum deviation, i = e. Using δₘ = 2i - A, we get δₘ = 2(45°) - 60° = 90° - 60° = 30°.

easy

8. While plotting the i-δ graph for an equilateral prism, a student records the angle of emergence e = 55° when the angle of incidence is i = 35°. The corresponding angle of deviation δ is:

  1. 20°
  2. 50°
  3. 40°
  4. 30°
Show answer and explanation

Answer: (D) 30°

For a prism, i + e = A + δ. Here, A = 60° (equilateral prism), i = 35°, and e = 55°. Thus, δ = i + e - A = 35° + 55° - 60° = 30°.

easy

9. During the i-δ experiment with a prism of refracting angle A = 60°, a student measures that at minimum deviation, the angle of refraction inside the first face is r₁. If the student increases the angle of incidence i from the minimum deviation value, what happens to the internal refraction angles r₁ and r₂?

  1. r₁ increases and r₂ decreases
  2. r₁ decreases and r₂ increases
  3. Both r₁ and r₂ increase
  4. Both r₁ and r₂ decrease
Show answer and explanation

Answer: (A) r₁ increases and r₂ decreases

According to Snell's law at the first surface, sin i = μ sin r₁. Increasing i increases r₁. Since r₁ + r₂ = A (a constant), an increase in r₁ necessarily leads to a decrease in r₂.

medium

10. In an i-δ experiment, if an equilateral prism produces a minimum deviation δ_m = 30°, what is the speed of light inside the prism? (Take speed of light in vacuum c = 3 × 10⁸ m/s)

  1. 1.50 × 10⁸ m/s
  2. 2.12 × 10⁸ m/s
  3. 2.00 × 10⁸ m/s
  4. 2.45 × 10⁸ m/s
Show answer and explanation

Answer: (B) 2.12 × 10⁸ m/s

μ = sin[(60° + 30°)/2] / sin(60°/2) = sin(45°)/sin(30°) = (1/√2)/(1/2) = √2 ≈ 1.414. The speed of light in the prism is v = c/μ = 3 × 10⁸ / √2 ≈ 2.12 × 10⁸ m/s.

easy

11. While performing the experiment to trace the path of rays through a glass prism, what is the recommended minimum distance between the pins placed on the incident ray to ensure an accurate measurement of angles?

  1. 1 cm
  2. 2 cm
  3. 5 cm
  4. 15 cm
Show answer and explanation

Answer: (C) 5 cm

To minimize angular error and clearly establish the direction of the ray, the distance between pins should typically be at least 5 cm (or preferably 8-10 cm). Keeping pins too close (like 1-2 cm) leads to large parallax and alignment errors.

hard

12. In a prism experiment, the value of minimum deviation is δₘ = 37° for a prism of angle A = 60°. If the least count of the protractor used to measure angles is 1°, what is the maximum percentage error in the measurement of (A + δₘ)?

  1. 1.03%
  2. 2.06%
  3. 1.50%
  4. 0.51%
Show answer and explanation

Answer: (B) 2.06%

Let S = A + δₘ = 60° + 37° = 97°. The maximum absolute error in measuring two independent angles is ΔS = ΔA + Δδₘ = 1° + 1° = 2°. Therefore, the maximum percentage error is (ΔS / S) × 100 = (2 / 97) × 100 ≈ 2.06%.

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