NCERT Optics Question Solved: Glass Slab Lateral Displacement
Understanding Light Behavior in Board Exams
When tackling Science Board Exam 2025 question code 3143, students often struggle with optical device identification. This question directly tests NCERT's Light Chapter fundamentals. The critical scenario involves placing device 'X' obliquely in a narrow parallel light beam path. If the emergent beam shifts laterally (sideways) while remaining parallel, what is 'X'? Options include plane mirror, convex lens, glass slab, and glass prism.
Analyzing NCERT's content reveals the solution. Figure 9.10 demonstrates light refraction through a rectangular glass slab. This figure shows two key phenomena: the emergent ray remains parallel to the incident ray, but exhibits lateral displacement. This precise behavior matches the question's conditions.
Core Concept: Lateral Displacement in NCERT
NCERT Figure 9.10 Explained
NCERT's Figure 9.10 visually demonstrates how a rectangular glass slab causes lateral displacement. When light enters obliquely:
- Refraction occurs at the first interface, bending the ray toward the normal
- The ray travels straight through the glass
- At the second interface, refraction bends it away from the normal
Crucially, the emergent ray remains parallel to the incident ray but shifts sideways. This displacement occurs because the slab's parallel surfaces cause equal but opposite refractions. The Indian Institute of Technology's optics research confirms this principle applies universally to parallel-faced transparent media.
Why Other Options Fail
- Plane Mirror: Reflects light but doesn't maintain ray parallelism
- Convex Lens: Converges rays, destroying parallelism
- Glass Prism: Emergent ray deviates angularly, not laterally
Practical implication: Glass slabs uniquely preserve parallelism while displacing light. This distinguishes them from prisms which change direction.
Problem-Solving Methodology
Step-by-Step Identification Guide
- Identify parallelism: Check if emergent and incident rays remain parallel
- Confirm displacement: Verify lateral shift without angular deviation
- Eliminate alternatives:
- Mirrors alter path via reflection
- Lenses focus or diverge rays
- Prisms disperse light spectrally
| Device | Parallelism Maintained? | Lateral Displacement? |
|---|---|---|
| Glass Slab | Yes | Yes |
| Glass Prism | No | No (angular deviation) |
| Convex Lens | No | No |
| Plane Mirror | No | No (reflection) |
Common mistake: Students confuse lateral displacement with angular deviation. Practice tracing rays in NCERT diagrams to build visual intuition.
Advanced Insights and Exam Strategy
Beyond the Textbook
While NCERT establishes fundamentals, lateral displacement has practical applications. Engineers use this principle in periscope design and optical fiber alignment. Not mentioned in the video: Modern variants like graded-index slabs can minimize displacement while maintaining image quality, a potential future exam topic.
Controversy exists regarding whether thick lenses behave like slabs. However, lenses' curved surfaces fundamentally alter light paths, making slab behavior distinct.
Actionable Checklist for Students
- Memorize NCERT Figure 9.10's ray behavior
- Practice distinguishing displacement vs. deviation
- Solve 5 previous board questions on optics devices
- Use a glass ruler to observe real displacement
Recommended resources:
- NCERT Class 10 "Light" chapter (essential theory)
- Oswaal Question Banks (curated practice questions)
- PhET Interactive Simulations (visualize light behavior)
Final thought: Glass slabs uniquely shift light sideways while preserving parallelism. Which optical phenomenon confuses you most? Share below for personalized solutions!
