S4.10.5 For the adult
Reflection and refraction
A ray of light meets a mirror and makes an angle of 20° with its surface. At what angle was the ray arriving, in the way scientists measure it?
What this chapter teaches
Angles are measured from the normal. A ray reflects at the angle it arrives. Light bends towards the normal when it slows, in glass or water, and away from it when it speeds up. Past the critical angle, light inside glass or water is totally reflected, which is how optical fibres work.
Three questions to ask
There is no one right wording. Listen for the idea.
- A ray makes 30° with a mirror. What is its angle of incidence, and why?
- Light goes from air into water at a slant. Which way does it bend, and why?
- What two things are needed for total internal reflection, and how does a fibre use it?
After the chapter, the learner can
- Draw ray diagrams for reflection and refraction at a boundary
- Explain total internal reflection and its use in optical fibres
What they copy into their notebook
Angles are from the normal. Reflection: i = r. Slower material: bends towards the normal. Total internal reflection: denser to less dense, beyond the critical angle, sin c = 1 ÷ n.