# Light: Brightness and Colour

> What makes light bright or dim, where colours come from, and why a red shirt looks red.

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

### Story time: The stage lights
Chloe was helping with the school musical, working the stage lights. In rehearsal, the lead actor wore a white shirt and a red scarf.

When Chloe switched on only the red spotlight, the white shirt turned red, and the red scarf seemed to vanish into it. When she switched to the green spotlight, the shirt glowed green, and the scarf turned **black**.

The scarf had not changed at all. Only the light hitting it had. By the end of the night, Chloe understood colour better than anyone in the cast.

**In short:** Brightness depends on how powerful a light source is and how far away you are: double the distance and the light is a quarter as bright. White light is a mix of all colours, and an object looks the colour it reflects while absorbing the rest.

## Sources of light

- **Luminous** objects **make their own light**: the Sun, a candle flame, a light bulb, a phone screen.
- **Non-luminous** objects do not make light. We see them because they **reflect** light into our eyes: the Moon, a book, your friend's face.

Light travels in **straight lines** called **rays**. That is why shadows have sharp edges and why you cannot see round corners. Light travels at about $3 \times 10^8$ m/s, the fastest speed in the universe.

## Brightness

**Brightness** depends on how much light energy reaches your eye each second. Two main things affect it:

1. **The power of the source.** A 10 W LED is brighter than a 3 W one. For a light wave, a bigger **amplitude** means more energy, so brighter light.
2. **The distance from the source.** The further away, the dimmer it looks, because the light spreads out over a bigger area.

### Key idea: The inverse square law
Light spreads out in all directions. If you move **twice** as far from a lamp, the same light is spread over **four** times the area. So it looks **one quarter** as bright.

$$\text{brightness} \propto \frac{1}{d^2}$$

- 2 × the distance: $\tfrac{1}{4}$ as bright
- 3 × the distance: $\tfrac{1}{9}$ as bright
- 10 × the distance: $\tfrac{1}{100}$ as bright

### Worked example: Reading by torchlight
You read a book 1 m from a torch. You move the book to 3 m away. How bright is the page now?

Distance tripled, so brightness becomes $\dfrac{1}{3^2} = \dfrac{1}{9}$ of what it was.

Brightness is also why stars look so different. A huge, powerful star far away can look dimmer than a small star nearby.

## White light and the spectrum

In 1666, Isaac Newton shone sunlight through a glass prism. White light came in. A rainbow came out. White light is actually a **mixture of all the colours**.

The colours of the **visible spectrum**, in order:

**Red, Orange, Yellow, Green, Blue, Indigo, Violet** (remember **ROY G BIV**)

*Diagram: Visible spectrum from red at about 700 nanometres to violet at about 400 nanometres*
*Each colour is light of a different wavelength.*

**Colour depends on wavelength** (and frequency). Red light has the **longest** wavelength and lowest frequency. Violet has the **shortest** wavelength and highest frequency.

A prism splits white light because each colour refracts by a slightly different amount. Violet bends the most, red the least. This is **dispersion**. Raindrops do the same, which makes rainbows.

## Why objects have colour

An object's colour depends on which colours it **reflects** and which it **absorbs**.

- A **red** shirt **reflects red** light and **absorbs** all the other colours.
- A **white** object **reflects all** colours.
- A **black** object **absorbs all** colours (which is why black clothes get hot in the sun).

Back to Chloe's stage. Under **green** light, the red scarf has no red light to reflect. It absorbs the green. Nothing comes back, so it looks **black**.

### Watch out: The colour is not "in" the object
A red apple is not giving out red light. It is reflecting the red part of the light that falls on it. In a dark room, it has no colour at all.

## Coloured filters

A **filter** lets **its own colour through** and **absorbs the rest**.

- A **red** filter lets red light pass and blocks everything else.
- White light through a red filter comes out **red**.
- Red light through a **blue** filter: **nothing** comes out (black).

## Mixing coloured light

The **primary colours of light** are **red, green and blue** (RGB). Every screen you look at makes all its colours from tiny red, green and blue dots.

| Mix | Result |
|---|---|
| Red + green | **Yellow** |
| Red + blue | **Magenta** |
| Green + blue | **Cyan** |
| Red + green + blue | **White** |

This is **additive** mixing. It is different from mixing paints, where the primaries are different and mixing everything gives a muddy dark colour.

**Mind map**

```text
Light
  Sources
    Luminous: makes light
    Non-luminous: reflects
    Travels in straight lines
  Brightness
    More power = brighter
    Bigger amplitude
    Inverse square law
  Colour
    White = all colours
    ROY G BIV
    Red: long wavelength
    Violet: short wavelength
  Objects
    Reflect their colour
    Absorb the rest
    Filters pass own colour
  Mixing light
    Red, green, blue
    R + G = yellow
    All three = white
```

### Year 11: Intensity
Physicists measure brightness as **intensity**, the power per unit area (W/m²):

$$I = \frac{P}{4\pi r^2}$$

for a source that radiates equally in all directions. Comparing two distances:

$$\frac{I_1}{I_2} = \frac{r_2^2}{r_1^2}$$

The intensity of a wave is proportional to the **square of its amplitude**: $I \propto A^2$. Double the amplitude and the intensity is four times bigger.

### Year 12: Light as photons
Light also behaves as a stream of particles called **photons**. Each photon's energy depends only on its frequency:

$$E = hf = \frac{hc}{\lambda}$$

with Planck's constant $h = 6.63 \times 10^{-34}$ J s. So a violet photon carries more energy than a red one.

In the **photoelectric effect**, light knocks electrons out of a metal. Bright red light may release none, while dim ultraviolet light releases some instantly. **Brightness** sets the **number** of photons. **Colour** (frequency) sets the **energy of each**. Einstein won his Nobel Prize for explaining this.

## Quick summary

- Luminous objects make light. Non-luminous objects reflect it.
- Brightness depends on source power (amplitude) and distance.
- Inverse square law: double the distance, one quarter the brightness.
- White light is a mix of all colours: ROY G BIV. Red has the longest wavelength, violet the shortest.
- Objects reflect their own colour and absorb the others. Filters let their own colour through.
- Primary colours of light: red, green, blue. All three together make white.

## Quiz

Try these questions from the chapter quiz.

1. Which of these is luminous?
   - A. The Moon
   - B. A mirror
   - C. A candle flame
   - D. A white wall
   - Answer: C. A candle flame. A candle flame makes its own light. The others only reflect light.

2. You move twice as far from a lamp. It now appears:
   - A. Half as bright
   - B. One quarter as bright
   - C. Twice as bright
   - D. The same
   - Answer: B. One quarter as bright. Inverse square law: 1 ÷ 2² = ¼.

3. Which colour of visible light has the longest wavelength?
   - A. Red
   - B. Green
   - C. Blue
   - D. Violet
   - Answer: A. Red. Red is at the long wavelength end of the spectrum.

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