CBSE Class 12 Physics: Marking Scheme Solution
(a) Effect on the width of the beam
Step 1 (Governing Law): [0.5 Mark]
According to Snell's Law, when light travels from a rarer medium (air, $n_1 \approx 1$) to a denser medium (water, $n_2 > 1$), it bends towards the normal. The relationship is given by:
$$n_1 \sin i = n_2 \sin r$$Step 2 (Deduction): [0.5 Mark]
Since the light bends towards the normal, the angle of refraction $r$ is less than the angle of incidence $i$. Consequently, the rays converge slightly, causing the width of the beam to decrease.
(b) Apparent bending of a straw
Step 1 (Principle): [0.5 Mark]
The phenomenon is due to refraction of light. Light rays originating from the submerged part of the straw travel from a denser medium (water) to a rarer medium (air), bending away from the normal at the interface.
Step 2 (Ray Diagram Description): [0.5 Mark]
- Draw a beaker with water.
- Show a straw partially submerged.
- Draw two rays from the submerged tip of the straw hitting the water-air interface.
- Show the rays bending away from the normal as they emerge into the air.
- Extend these refracted rays backward to show they appear to originate from a point higher than the actual position, creating a virtual image.
(c) Transmission through an Optical Fibre
Step 1 (Governing Principle): [0.5 Mark]
Optical fibres work on the principle of Total Internal Reflection (TIR). For TIR to occur, two conditions must be met:
- Light must travel from a denser medium (core, $n_1$) to a rarer medium (cladding, $n_2$).
- The angle of incidence $i$ must be greater than the critical angle $i_c$, where $i_c = \sin^{-1}\left(\frac{n_2}{n_1}\right)$.
Step 2 (Diagrammatic Representation): [0.5 Mark]
[Diagram Note: Draw a cylindrical core surrounded by cladding. Show a light ray entering the core and undergoing multiple internal reflections at the core-cladding interface, maintaining the angle $i > i_c$ throughout the length of the fibre.]
💡 EL10 Concept Insight:
Think of the straw like a stick in a pool; because water bends light like a magnifying glass, your eyes get "tricked" into seeing the straw higher up than it really is. For the optical fibre, imagine a hallway with perfect mirrors on the walls—the light bounces off the sides so efficiently that it stays trapped inside the glass "pipe" until it reaches the other end!