Refraction 🌈

1. What happens?

When light hits a second transparent medium at any angle other than 0°, part of the beam bounces back, and the rest bends into the new medium. We call this bending refraction. Light rays, the normal (the imaginary line drawn at 90° to the surface), and the point of incidence all lie in the same plane.:contentReference[oaicite:0]{index=0}

2. Snell’s Laws 🚦

  1. The incident ray, refracted ray, and the normal stay in one plane.:contentReference[oaicite:1]{index=1}
  2. The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r stays constant for any two given media:
    \\[ \frac{\sin i}{\sin r}=n_{21}\tag{9.10} \\] :contentReference[oaicite:2]{index=2}

What does \(n_{21}\) mean?

\(n_{21}\) is the refractive index of medium 2 with respect to medium 1. If \(n_{21}>1\), the ray bends toward the normal—the second medium is optically denser. If \(n_{21}<1\), the ray bends away.:contentReference[oaicite:3]{index=3}

Handy relationships 🛠️

  • Reverse index: \\[ n_{12}=\frac{1}{n_{21}} \\] :contentReference[oaicite:4]{index=4}
  • Chain rule: \(n_{32}=n_{31}\,n_{12}\) for three media in succession.:contentReference[oaicite:5]{index=5}

3. Optical density vs. mass density 🧐

Optical density measures how much light slows down, not how heavy a substance is. Turpentine feels lighter than water (lower mass density) yet slows light more (higher optical density).:contentReference[oaicite:6]{index=6}

4. Refraction through a parallel-sided slab 🪟

Light refracts twice—air→glass and glass→air. The emergent ray runs parallel to the original ray, but it slides sideways. We call this slide the lateral shift.:contentReference[oaicite:7]{index=7}

5. Why does a pool look shallower? 🏊‍♂️

Looking almost straight down, your eyes see the bottom raised. The apparent depth \(h_1\) relates to the real depth \(h_2\) by \\[ h_1=\frac{h_2}{n} \\] where \(n\) is the refractive index of water.:contentReference[oaicite:8]{index=8}

High-Yield Ideas for NEET 🔑

  • Memorize Snell’s law and be ready to plug angles quickly.:contentReference[oaicite:9]{index=9}
  • Know that \(n_{12}=1/n_{21}\) and use reciprocal indices to switch media.:contentReference[oaicite:10]{index=10}
  • Differentiate optical density from mass density—classic conceptual trap.:contentReference[oaicite:11]{index=11}
  • Practice problems on lateral shift in glass slabs and apparent depth in water.:contentReference[oaicite:12]{index=12}

✨ Keep experimenting and keep shining! ✨