# Reflection and Refraction

> Why mirrors work, why a straw looks bent in water, and how light changes direction at a boundary.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/reflection-and-refraction/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The bent spear
On a school trip to the coast, Ava watched an old fisherman catch fish with a spear in the shallows. She tried too. She aimed straight at a fish she could clearly see and stabbed. Missed. Again. Missed.

The fisherman laughed kindly. "Aim **below** where you see it."

She tried it. Splash. Got one.

The fish was never where it looked. Light from the fish bent as it came out of the water into the air, and her eyes followed the bent line back to the wrong spot. That bending is called **refraction**.

**In short:** Reflection is a wave bouncing off a surface, and the angle of incidence equals the angle of reflection. Refraction is a wave changing direction because its speed changes as it enters a new material: it bends towards the normal when it slows down.

## Reflection

**Reflection** is when a wave **bounces off** a surface. Mirrors, echoes and seeing most everyday objects all depend on reflection.

Some key words:
- **Incident ray**: the ray coming in.
- **Reflected ray**: the ray going out.
- **Normal**: an imaginary line drawn at **90°** to the surface where the ray hits. Always measure angles from the normal, not from the surface.
- **Angle of incidence** ($i$): between the incident ray and the normal.
- **Angle of reflection** ($r$): between the reflected ray and the normal.

### Key idea: The law of reflection
$$\text{angle of incidence} = \text{angle of reflection} \qquad i = r$$

The incident ray, the reflected ray and the normal all lie in the same flat plane.

*Diagram: Ray diagram of reflection off a flat mirror with equal angles of incidence and reflection measured from the dashed normal*
*Angles are measured from the normal. Here i = r.*

**Smooth vs rough surfaces**
- A **smooth** surface (mirror, still water) reflects all rays the same way. You get a clear image. This is **regular (specular) reflection**.
- A **rough** surface (paper, a wall) scatters rays in all directions. No image forms. This is **diffuse reflection**. It is why you can see a page from any angle.

**Image in a flat mirror**: the same size as the object, the same distance behind the mirror as the object is in front, upright, and **laterally inverted** (left and right swapped). That is why AMBULANCE is printed backwards on the front of ambulances.

## Refraction

**Refraction** is when a wave **changes direction** as it passes from one medium into another, because its **speed changes**.

Light travels fastest in a vacuum (and air). It slows down in water and slows even more in glass.

### Key idea: The refraction rules
- Light going into a **denser** medium (air to glass) **slows down** and bends **towards** the normal.
- Light going into a **less dense** medium (glass to air) **speeds up** and bends **away from** the normal.
- Light hitting the boundary **along the normal** (at 90° to the surface) goes **straight through** without bending, but still changes speed.

### Exam tip: Think of a car driving onto sand
Picture a car driving at an angle from a road onto sand. The front wheel that hits the sand first slows down, while the other wheel is still fast on the road. The car swings round. Light does the same thing at a boundary.

**Effects of refraction**
- A straw in a glass of water looks **bent** at the surface.
- Pools look **shallower** than they really are.
- Ava's fish looked higher than it was.
- **Lenses** in glasses, cameras and microscopes bend light to focus it.
- A **prism** splits white light into a rainbow, because each colour bends by a slightly different amount. This is called **dispersion**.

### Watch out: Frequency stays the same
When a wave refracts, its **speed** and **wavelength** change. Its **frequency does not change**. The source sets the frequency.

**Mind map**

```text
Light at boundaries
  Reflection
    Bounces off
    i = r
    Measure from normal
    Regular vs diffuse
    Mirror image reversed
  Refraction
    Changes speed
    Changes direction
    Into denser: towards normal
    Into less dense: away from normal
  Effects
    Bent straw
    Shallow pool
    Lenses
    Prism rainbow
```

### Year 11: Snell's law and total internal reflection
The **refractive index** of a material tells you how much it slows light:

$$n = \frac{c}{v}$$

Water: $n \approx 1.33$. Glass: $n \approx 1.5$. Diamond: $n \approx 2.4$.

**Snell's law** links the angles on both sides:

$$n_1 \sin\theta_1 = n_2 \sin\theta_2$$

Going from a dense to a less dense medium, if the angle is bigger than the **critical angle** $\theta_c$, no light gets out. It all reflects back. This is **total internal reflection**.

$$\sin\theta_c = \frac{n_2}{n_1}$$

Optical fibres carry internet data as light bouncing along inside thin glass, using total internal reflection. Diamonds sparkle because their critical angle is small (about 24°).

### Year 12: Diffraction and the wave model of light
Waves also **diffract**: they spread out when they pass through a gap or round an edge. The effect is biggest when the gap is about the same size as the wavelength.

In Young's double slit experiment, light through two narrow slits makes a pattern of bright and dark bands. Only waves can interfere, so this proved light behaves as a wave. Bright fringes appear where:

$$d\sin\theta = m\lambda$$

with $d$ the slit spacing and $m = 0, 1, 2, \dots$

## Quick summary

- **Reflection**: a wave bounces off a surface. Angle of incidence = angle of reflection.
- Measure angles from the **normal**, a line at 90° to the surface.
- Smooth surfaces give clear images. Rough surfaces scatter light (diffuse reflection).
- **Refraction**: a wave changes direction because its speed changes in a new medium.
- Into a denser medium: slows down, bends towards the normal. Out: speeds up, bends away.
- Frequency never changes on refraction. Speed and wavelength do.

## Quiz

Try these questions from the chapter quiz.

1. A ray hits a mirror with an angle of incidence of 35°. The angle of reflection is:
   - A. 55°
   - B. 35°
   - C. 70°
   - D. 145°
   - Answer: B. 35°. By the law of reflection, i = r.

2. The normal is a line:
   - A. Along the surface
   - B. At 90° to the surface
   - C. Parallel to the incident ray
   - D. At 45° to the surface
   - Answer: B. At 90° to the surface. The normal is perpendicular to the surface at the point where the ray hits.

3. A ray strikes a mirror at 20° to the surface. The angle of incidence is:
   - A. 20°
   - B. 70°
   - C. 40°
   - D. 90°
   - Answer: B. 70°. Angles are measured from the normal: 90° − 20° = 70°.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/reflection-and-refraction/#quiz