Define and use the terms normal, angle of incidence and angle of refraction
3.2.2 Refraction of Light 📐
Key Terms
| Term | Definition |
|---|---|
| Normal | A straight line that is perpendicular to the surface at the point of incidence. Think of it as the “ground line” when a ball bounces off a wall. |
| Angle of Incidence | The angle between the incident ray and the normal. It’s like the angle at which a skateboarder lands on a ramp. |
| Angle of Refraction | The angle between the refracted ray and the normal. Imagine the skateboarder sliding off the ramp at a new angle. |
How Light Changes Direction
When light moves from one medium to another (e.g., air to water), it bends because its speed changes. The relationship is described by Snell’s law:
$$n_1 \sin \theta_1 = n_2 \sin \theta_2$$
Where $n_1$ and $n_2$ are the refractive indices of the first and second media, $\theta_1$ is the angle of incidence, and $\theta_2$ is the angle of refraction.
Practical Example
- Imagine a laser pointer shining through a glass of water. The light ray hits the glass surface at $30^\circ$ to the normal.
- Water’s refractive index is about $1.33$, while air is $1.00$.
- Using Snell’s law, calculate the angle of refraction:
$$\sin \theta_2 = \frac{n_1}{n_2} \sin \theta_1 = \frac{1.00}{1.33} \sin 30^\circ = 0.375$$
$$\theta_2 = \sin^{-1}(0.375) \approx 22^\circ$$
So the light bends towards the normal, ending up at about $22^\circ$.
Quick Check Questions
- What is the normal? 🤔
- How do you measure the angle of incidence? 📏
- What happens to the angle of refraction when light enters a denser medium? 📘
Summary
Remember: the normal is the “reference line”; the angle of incidence is measured from this line to the incoming ray; the angle of refraction is measured from the normal to the outgoing ray. By keeping these angles straight, you can predict how light will bend at any interface.
Revision
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