Class 10 Science Notes Chapter 5 (Chapter 5) – Lab Manual (English) Book

Detailed Notes with MCQs of Experiment 5 from your Lab Manual, which deals with tracing the path of light through a glass prism. This is an important practical, and the concepts involved frequently appear in various government exams. Pay close attention to the principles, procedures, and especially the relationships between the different angles.

Experiment 5: Tracing the Path of the Rays of Light Through a Glass Prism

1. Aim:
To trace the path of a ray of light passing through a glass prism and measure the angle of deviation.

2. Theory and Concepts:

• Prism: A prism is a transparent optical element with flat, polished surfaces that refract light. Typically, it has two triangular bases and three rectangular lateral surfaces. The angle between the two lateral refracting surfaces involved is called the Angle of the Prism (A).
• Refraction: When a ray of light passes from one medium to another (e.g., air to glass and glass to air), it bends. This bending is called refraction. Light bends towards the normal when going from a rarer medium (air) to a denser medium (glass) and away from the normal when going from a denser medium (glass) to a rarer medium (air).
• Path of Light through a Prism:
  • A ray of light (PQ, the incident ray) strikes the first surface (AB) of the prism.
  • It refracts and travels inside the prism (QR, the refracted ray).
  • It strikes the second surface (AC) and refracts again, emerging out of the prism (RS, the emergent ray).
  • The angle between the incident ray and the normal at the point of incidence is the angle of incidence (i).
  • The angle between the refracted ray and the normal inside the prism is the angle of refraction (r).
  • The angle between the emergent ray and the normal at the point of emergence is the angle of emergence (e).
• Angle of Deviation (D): The angle between the direction of the incident ray (produced forward) and the direction of the emergent ray (produced backward) is called the angle of deviation. It represents the total deviation or bending produced by the prism.
  • Formula: For a prism, the relationship between these angles is: D = i + e - A
• Angle of Minimum Deviation (Dm): As the angle of incidence (i) is varied, the angle of deviation (D) also changes. It is observed that the angle of deviation first decreases, reaches a minimum value (Dm), and then increases as 'i' is further increased.
  • Condition for Minimum Deviation: At the position of minimum deviation, the angle of incidence equals the angle of emergence (i = e), and the refracted ray inside the prism travels parallel to the base of the prism (for an equilateral or isosceles prism).
• i-D Graph: A graph plotted between the angle of incidence (i) on the x-axis and the angle of deviation (D) on the y-axis is typically a U-shaped curve (or part of it). The lowest point on the curve corresponds to the angle of minimum deviation (Dm).

(Diagram Reference: Ensure you can draw and label the standard diagram showing the path of light through a prism, marking A, i, r1, r2, e, and D).

3. Apparatus Required:
Drawing board, white sheet of paper, drawing pins/adhesive tape, glass prism, pins (all pins), protractor, pencil, scale.

4. Procedure (Key Steps):
i. Fix a white sheet of paper on the drawing board.
ii. Place the prism on the paper with its triangular base downwards and draw its outline (ABC).
iii. Draw a normal (NN') to the surface AB at a point Q.
iv. Draw an incident ray (PQ) making a certain angle of incidence (e.g., 35°, 40°, etc.) with the normal.
v. Fix two pins (P1, P2) vertically on the incident ray PQ, sufficiently far apart.
vi. Look for the images of these pins through the other face (AC) of the prism.
vii. Fix two more pins (P3, P4) such that your eye, P3, P4, and the images of P1, P2 all lie in the same straight line. Ensure P3 and P4 are also far apart.
viii. Remove the prism and the pins. Mark the pin positions.
ix. Join P3 and P4 and extend the line to meet the prism face AC at R. This line RS is the emergent ray.
x. Join QR. This is the refracted ray.
xi. Draw a normal (MM') to the face AC at point R.
xii. Measure the angle of incidence (i), angle of emergence (e), and the angle of the prism (A).
xiii. Produce PQ forward and RS backward. They intersect at a point, say T. Measure the angle of deviation (D).
xiv. Verify if D = i + e - A.
xv. Repeat the experiment for different angles of incidence (e.g., 40°, 45°, 50°, 55°, 60°).
xvi. Record the observations in a table.
xvii. Plot a graph between angle of incidence (i) and angle of deviation (D).

5. Observations:

• Measure the Angle of the Prism (A) = ______ (usually 60° for an equilateral prism).
• Create a table:

• i-D Graph: Observe the shape of the graph. Identify the lowest point, which gives the angle of minimum deviation (Dm) and the corresponding angle of incidence.

6. Result/Inference:

• The path of the light ray through the prism is traced.
• The emergent ray (RS) bends towards the base (BC) of the prism relative to the incident ray (PQ).
• The angle of deviation (D) initially decreases with an increase in the angle of incidence (i), reaches a minimum value (Dm), and then increases with a further increase in the angle of incidence.
• From the graph, the angle of minimum deviation, Dm = ______.
• At minimum deviation, i ≈ e.

7. Precautions:

• Use a soft drawing board and sharp pins.
• Fix the pins vertically and straight.
• Maintain a distance of at least 5-6 cm between the pins (P1, P2 and P3, P4) to trace the ray direction accurately.
• Use a sharp pencil to draw thin lines for the prism outline and rays.
• Measure angles carefully using a good protractor, aligning the baseline correctly.
• Ensure the prism is not disturbed during the experiment.
• View the pin images with one eye closed, keeping the eye at the same level as the pins to avoid parallax error.
• The angle of incidence should preferably be between 30° and 60°.

Multiple Choice Questions (MCQs)

1. When a ray of light passes through a glass prism, it bends:
   a) Towards the vertex
   b) Towards the base
   c) Parallel to the base
   d) It does not bend

2. The angle between the two refracting surfaces of a prism is called the:
   a) Angle of deviation
   b) Angle of emergence
   c) Angle of the prism
   d) Angle of incidence

3. The relationship between the angle of incidence (i), angle of emergence (e), angle of the prism (A), and angle of deviation (D) is:
   a) D = i - e + A
   b) D = i + e + A
   c) D = A - i - e
   d) D = i + e - A

4. In the condition of minimum deviation through a prism:
   a) i > e
   b) i < e
   c) i = e
   d) i = 0

5. While tracing the path of a ray of light through a prism, pins P1, P2, P3, and P4 are fixed. The distance between pins P1 and P2 (and P3 and P4) should be:
   a) As small as possible (1-2 cm)
   b) Moderate (around 3 cm)
   c) Sufficiently large (5 cm or more)
   d) Does not matter

6. The graph between the angle of incidence (i) and the angle of deviation (D) for a prism is typically a:
   a) Straight line passing through the origin
   b) Straight line parallel to the i-axis
   c) Parabola or U-shaped curve
   d) Hyperbola

7. What is the angle of deviation (D) if the incident ray strikes the prism normally (i=0°)? (Assume A=60°, and refractive index allows emergence)
   Note: This is a conceptual question. Normal incidence means i=0, so r1=0. Then r2=A. Using Snell's law at the second face sin(e)/sin(r2) = n_air/n_glass = 1/n. So sin(e) = nsin(r2) = nsin(A). D = i + e - A = 0 + e - A = e - A.
   This might be too complex for a direct MCQ without more info, let's rephrase.
   Alternative Q7: If the incident ray retraces its path after reflecting from the second surface (AC) silvered, what is the angle of emergence 'e'?
   a) 90°
   b) 0°
   c) Equal to 'i'
   d) Equal to 'A'
   (Answer: b. If it retraces, it must hit AC normally, so e=0)

8. Which precaution is essential for accurately tracing the path of light through a prism?
   a) Using a blunt pencil for drawing lines
   b) Placing the pins very close to each other
   c) Ensuring pins are fixed vertically
   d) Using a prism with scratched surfaces

9. The angle of deviation depends on:
   a) Angle of incidence
   b) Angle of the prism
   c) Material of the prism (refractive index)
   d) All of the above

10. If a ray of light passes through an equilateral glass prism (A=60°) such that the angle of incidence is equal to the angle of emergence and each is equal to 3/4 of the angle of the prism, what is the angle of deviation?
    a) 60°
    b) 45°
    c) 30°
    d) 90°

Answer Key for MCQs:

1. b) Towards the base
2. c) Angle of the prism
3. d) D = i + e - A
4. c) i = e
5. c) Sufficiently large (5 cm or more)
6. c) Parabola or U-shaped curve
7. b) 0° (For the alternative Q7)
8. c) Ensuring pins are fixed vertically
9. d) All of the above
10. c) 30° (Calculation: i = e = (3/4)A = (3/4)*60° = 45°. D = i + e - A = 45° + 45° - 60° = 90° - 60° = 30°)

Study these notes thoroughly. Understand the definitions, the formula D = i + e - A, the concept of minimum deviation, and the precautions. Being able to draw the ray diagram correctly is also crucial. Good luck with your preparation!