Light (Copy)

 1. Core Concepts

 Laws of Reflection

• Angle of incidence = Angle of reflection
• Reflected ray lies in the same plane as incident ray and normal

 Refraction

• Bending of light as it passes from one medium to another due to change in speed
• Towards the normal when entering a denser medium
• Away from the normal when entering a less dense medium

 Refractive Index (n):

• n = sin(i) / sin(r)
  Where:
  • i = angle of incidence
  • r = angle of refraction
  • n = refractive index (no unit)

 Critical Angle & Total Internal Reflection:

• n = 1 / sin(c)
  • c = critical angle
• If i > c → Total internal reflection occurs

 2. Objectives of ATP Light Experiments

• To verify the laws of reflection
• To investigate refraction through glass blocks
• To measure angles accurately using a protractor
• To calculate refractive index using experimental data
• To determine the focal length of a convex lens

 3. Apparatus Required

 4. Experiment 1: Reflection from a Plane Mirror

Method:

1. Draw a straight line and place mirror vertically along it.
2. Mark normal line (perpendicular) at point of incidence.
3. Shine ray at an angle of incidence (i) (e.g. 30°).
4. Mark reflected ray and measure angle of reflection (r).
5. Repeat for multiple angles (e.g. 20°, 40°, 50°).
6. Verify: i = r

✔️ Use sharp pencil and draw thin lines
✔️ Label all angles clearly

 5. Experiment 2: Refraction through a Glass Block

Method:

1. Place rectangular glass block on paper and trace outline.
2. Draw a normal line at one edge.
3. Shine ray at angle i (e.g. 20°) to the normal.
4. Trace emergent ray and measure angle of refraction (r) inside the block.
5. Extend incident and emergent rays to show lateral displacement
6. Repeat with several angles of incidence (e.g. 30°, 40°)

Calculation:

• Use formula: n = sin(i) / sin(r)
• Use calculator in degree mode

 6. Experiment 3: Finding Refractive Index Using Semicircular Block

Objective:

• To measure critical angle (c) and calculate refractive index

Method:

1. Place semicircular block on paper (flat side facing ray).
2. Direct ray towards the center of flat face.
3. Increase angle of incidence until emergent ray disappears (Total Internal Reflection).
4. Measure angle = critical angle (c)
5. Calculate: n = 1 / sin(c)

 7. Experiment 4: Finding Focal Length of Convex Lens

Method:

1. Place lit object (e.g. candle) at known distance from convex lens.
2. Place screen on the other side and move until a sharp image is formed.
3. Measure object distance (u) and image distance (v).
4. Use lens formula:
   1/f = 1/u + 1/v
   → All distances in cm or m
5. Repeat with different values of u (e.g. 15 cm, 20 cm, 25 cm)
6. Calculate focal length (f) for each and take average

 8. Data Tables (Examples)

For Refraction:

 9. Graphical Analysis

✔️ Axes must be labelled:
→ sin(i), sin(r) – no units
→ All distances in cm/m
✔️ Draw best-fit line, not dot-to-dot

 10. Variables

 11. Common Errors and Corrections

 12. Improvement Suggestions

• Use dark room to improve ray visibility
• Use colored ray box for clear tracing
• Repeat measurements and average sin(i)/sin(r)
• Use laser for single wavelength (monochromatic light)
• Ensure glass block or lens sits flat on paper

 13. ATP-Style Questions

14. Exam Tips

• Always label:
  → Incident ray, normal, reflected ray, refracted ray, angle i, angle r
• Show working for sin(i) and sin(r) to get full marks
• Quote:
  → n = sin(i)/sin(r)
  → n = 1 / sin(c)
  → 1/f = 1/u + 1/v
• Use correct units:
  → Degrees for angles
  → cm/m for distances
  → No units for sine values
• Write:
  

  “Repeat readings for different i and average calculated n for reliability.”
  “Use sharp pencil and ruler for precise ray tracing.”