Physics
Optics, lenses & reflection — quick study summary
Light behaves as both wave and particle. Reflection: angle of incidence = angle of reflection (both measured from the normal). Refraction: light bends when entering a different medium — Snell's law n₁ sinθ₁ = n₂ sinθ₂. Converging (convex) lenses focus parallel rays to a focal point and form real images of distant objects. Diverging (concave) lenses spread rays and always form virtual images. Thin-lens equation: 1/f = 1/d_o + 1/d_i.
Key points
- Reflection: θᵢ = θᵣ, measured from the normal
- Refraction (Snell): n₁ sinθ₁ = n₂ sinθ₂
- Total internal reflection if θ > critical angle; basis of fibre optics
- Convex (converging) lens forms real images of distant objects
- Thin-lens eq: 1/f = 1/d_o + 1/d_i; positive d_i means real image
Practice quiz
Click each question to reveal the answer.
1. Light passes from air (n=1.00) into glass (n=1.50) at 30°. What's the refraction angle in glass?
- 10°
- ≈19.5°
- 30°
- 45°
Answer: ≈19.5°
Snell: 1×sin30 = 1.5×sinθ → sinθ = 0.333 → θ ≈ 19.5°. Light bends toward the normal entering a denser medium.
2. What kind of image does a magnifying glass produce when the object is closer than the focal length?
Answer: Virtual, upright, magnified
Inside the focal length, a convex lens produces a virtual image — light only appears to come from it, you can't project it onto a screen.
3. Why does a fibre-optic cable trap light inside its core?
Answer: Total internal reflection — light hits the boundary above the critical angle and reflects back
When light goes from dense to less-dense medium above the critical angle, none of it refracts — all of it reflects back inside.
Last reviewed: May 2026