Ray Optics

1. 1Introduce the journey: we'll follow a light ray through mirrors, glass, water, and optical instruments. Each time it meets a boundary, something happens.
2. 2Define plane reflection: angle of incidence = angle of reflection. Normal is perpendicular to the surface. Both angles measured from the normal.
3. 3Now the ray hits a curved mirror. Name the pole (P), centre of curvature (C), principal axis, and focus (F). The normal at any point on a sphere is the radius at that point.
4. 4The relationship between focal length and radius: f = R/2. Measure all distances from the pole, using the New Cartesian sign convention: left of mirror is negative, right is positive.
5. 5Work a numerical: An object is 20 cm from a concave mirror of focal length 15 cm. Use the mirror formula 1/v + 1/u = 1/f to find the image distance. Here u = −20 cm, f = −15 cm. Calculate step by step on the board.
6. 6Magnification m = −v/u tells us size and orientation. The negative sign means the image is inverted if m is negative. Use the answer from the previous problem to find m and describe the image.
7. 7Now the ray leaves the mirror and enters a new medium—glass or water. Speed of light changes: n = c/v, where c = 3 × 10⁸ m/s and n is the refractive index. Higher n means the ray slows down and bends.
8. 8Snell's law at a boundary: n₁ sin i = n₂ sin r. The angle of incidence i and refraction r are measured from the normal. Frequency stays the same; wavelength changes.
9. 9TRY IT activity: Shine a torch UP through water in a glass from below. At a certain angle, the light stops coming out and reflects back into the water. This is the critical angle. Find the angle where it happens for glass (n = 1.5): sin C = 1/n = 1/1.5. Calculate C.
10. 10Total internal reflection: when light goes from denser (higher n) to rarer (lower n) beyond the critical angle, it reflects 100%. This makes optical fibres carry signals, makes diamonds sparkle, and creates mirages on hot roads.

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