Refraction, total internal reflection and lenses

What it shows

When light passes from one medium into another, its speed changes and the ray bends. Snell's law, n₁·sin i = n₂·sin r, gives the angle of refraction. When light travels toward a lower refractive index and the angle of incidence exceeds the critical angle, where sin c = n₂/n₁, no refracted ray exists and all the light is reflected: total internal reflection. The second panel uses three principal rays and the thin-lens equation 1/f = 1/d + 1/d′ to locate the image formed by a converging or diverging lens and describe it as real or virtual, inverted or upright, magnified or diminished.

How to use

In the refraction panel, drag the red light source to change the angle of incidence and read r from the equation line. Press Swap n₁ ↔ n₂ so the light travels from the denser medium, then increase i past the critical angle. In the lens panel, drag the object arrow toward the lens and watch the image change as the object distance passes 2f and f. Press Switch lens type to compare a diverging lens.

Parameters you can change

• Refractive index of the incident medium n₁ 1–2.5
• Refractive index of the refracting medium n₂ 1–2.5
• Angle of incidence i 0–89 °
• Lens type Converging lens, Diverging lens
• Focal length f 5–30 cm
• Object–lens distance d 3–60 cm

Questions to explore

1. Why can total internal reflection not happen when light passes from air into water?
2. What kind of image does a converging lens form of an object placed at d = 2f?
3. Can a diverging lens ever form a real image of a real object?