šŸ”· Refraction through Glass Slab

Verify Snell's Law and Calculate Refractive Index of Glass

āœ… FREE Experiment ā€¢ šŸ“Š Real-Time Ray Diagrams ā€¢ šŸŽ“ NEB Class 11 Practical

Interactive Simulation

Adjust angle and watch how light bends through the glass slab!

Experiment Controls

Range: 10Ā° - 80Ā°

Common glasses: 1.50 - 1.70

Affects lateral displacement

Real-Time Results

Angle of Incidence (i)
30
degrees
Angle of Refraction (r)
19.47
degrees
Angle of Emergence (e)
30
degrees
Lateral Displacement
2.54
cm

Observation Table

S.No. Angle of Incidence (i)
degrees		 Angle of Refraction (r)
degrees		 sin i sin r Refractive Index (n)
n = sin i / sin r
No observations yet. Run the experiment and click "Add to Table"

šŸ“Š Mean Calculation

Mean Refractive Index (n) = 0.00

Standard value for glass = 1.50 - 1.52

Percentage error = 0.00%

šŸ“š Theory & Concepts

What is Refraction?

Refraction is the bending of light when it passes from one transparent medium to another. This happens because light travels at different speeds in different media. When light enters a denser medium (like glass from air), it slows down and bends towards the normal.

Snell's Law of Refraction

Snell's Law states that the ratio of the sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media. This constant is called the refractive index.

nā‚ sin i = nā‚‚ sin r

For air to glass:
ā€¢ nā‚ = 1.00 (refractive index of air)
ā€¢ nā‚‚ = n (refractive index of glass)
ā€¢ Therefore: n = sin i / sin r

Refraction through a Glass Slab

When light passes through a parallel-sided glass slab, it undergoes refraction twice:

Lateral Displacement

Lateral displacement is the perpendicular distance between the incident ray and the emergent ray. It depends on:

d = t Ɨ sin(i - r) / cos r

Where:
ā€¢ d = lateral displacement
ā€¢ t = thickness of glass slab
ā€¢ i = angle of incidence
ā€¢ r = angle of refraction

Refractive Index

The refractive index (n) of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium. Common values:

Key Observations

šŸ”¬ Procedure

  1. Set the initial angle of incidence (e.g., 30Ā°) using the slider
  2. Choose the refractive index of glass (typically 1.50 for common glass)
  3. Select the thickness of the glass slab (thin, medium, or thick)
  4. Click "Run Experiment" to see the light ray path through the glass slab
  5. Observe the incident ray, refracted ray, and emergent ray on the diagram
  6. Note the angle of refraction (r) and angle of emergence (e)
  7. Verify that angle of emergence equals angle of incidence (e = i)
  8. Observe the lateral displacement of the emergent ray
  9. Click "Add to Table" to record this observation
  10. Repeat steps 1-9 for different angles (20Ā°, 30Ā°, 40Ā°, 50Ā°, 60Ā°)
  11. Calculate sin i and sin r for each observation
  12. Find the refractive index n = sin i / sin r for each case
  13. Click "Calculate Mean n" to find the average refractive index
  14. Compare with standard value and calculate percentage error

šŸ’¬ Viva Questions & Answers

Q1: What is refraction of light?
Refraction is the phenomenon of bending of light when it passes from one transparent medium to another. This bending occurs because light travels at different speeds in different media.
Q2: State Snell's Law of refraction.
Snell's Law states that the ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media. Mathematically: nā‚ sin i = nā‚‚ sin r, or for air to glass: n = sin i / sin r.
Q3: What is refractive index?
Refractive index of a medium is the ratio of speed of light in vacuum to the speed of light in that medium. It is denoted by 'n' and is a dimensionless quantity. It indicates how much light bends when entering the medium.
Q4: Why does light bend when it enters glass from air?
Light bends because it changes speed when moving between media. When light enters glass from air, it slows down because glass is optically denser. This change in speed causes the light to bend towards the normal according to Snell's Law.
Q5: What happens to the emergent ray in a glass slab?
The emergent ray is parallel to the incident ray. This is because the two surfaces of the glass slab are parallel, so the refraction at the second surface exactly cancels the refraction at the first surface. However, there is a lateral displacement.
Q6: What is lateral displacement?
Lateral displacement is the perpendicular distance between the incident ray (extended) and the emergent ray. It occurs because light takes a different path inside the glass slab. It depends on the thickness of the slab, angle of incidence, and refractive index.
Q7: Does the angle of incidence equal the angle of emergence?
Yes, for a parallel-sided glass slab, the angle of incidence (i) always equals the angle of emergence (e). This is because the two refracting surfaces are parallel to each other. However, the emergent ray is laterally displaced from the incident ray.
Q8: What factors affect lateral displacement?
Lateral displacement depends on: (1) Thickness of the glass slab - greater thickness means greater displacement, (2) Angle of incidence - larger angle means greater displacement, (3) Refractive index - higher refractive index causes more bending and greater displacement.
Q9: Why do we use sin i and sin r instead of just i and r?
Snell's Law is based on the wave nature of light. The relationship between angles is not linear but involves sines because of how wavefronts bend at the interface. The ratio sin i / sin r remains constant for a given pair of media, not the ratio i / r.
Q10: What is the critical angle and total internal reflection?
Critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90Ā°. When the angle of incidence exceeds the critical angle, light does not refract but reflects back into the denser medium. This is called total internal reflection. For glass-air interface, critical angle is about 42Ā°.
Q11: What are the laws of refraction?
The two laws of refraction are: (1) The incident ray, refracted ray, and the normal at the point of incidence all lie in the same plane. (2) The ratio of sine of angle of incidence to sine of angle of refraction is constant for a given pair of media (Snell's Law).
Q12: How does refractive index relate to optical density?
Optical density is a measure of how much a medium can slow down light. A medium with higher refractive index is optically denser. Glass (n ā‰ˆ 1.5) is optically denser than air (n ā‰ˆ 1.0), so light travels slower in glass and bends towards the normal when entering.
Q13: What are sources of error in this experiment?
Main sources of error: (1) Parallax error in measuring angles, (2) Imperfect parallelism of glass slab surfaces, (3) Non-uniform refractive index of glass, (4) Thickness variation in glass slab, (5) Human error in drawing normal and measuring angles, (6) Using thick incident light beam instead of a thin ray.
Q14: What are practical applications of refraction?
Applications include: (1) Lenses in eyeglasses, cameras, and microscopes, (2) Optical fibers for communication, (3) Prisms for splitting light into colors, (4) Correcting vision defects, (5) Magnifying glasses, (6) Telescopes for astronomy, (7) Mirage formation in deserts, (8) Twinkling of stars.
Q15: Why does a glass slab not disperse white light into colors?
A parallel-sided glass slab does not produce visible dispersion because the deviation at the first surface is exactly cancelled by the deviation at the second surface. Although different colors refract by different amounts inside, they all emerge parallel to their original direction. Only lateral displacement differs slightly for different colors.