Everything you have ever seen reached you as light: light made by a source such as the Sun or a screen, or light that bounced off an object and entered your eye. Light crosses the 150 million km from the Sun in about eight minutes, needs no air or water to travel through, and carries energy that plants turn into food. This unit is about how light behaves, and especially how mirrors and lenses redirect it to form images. First, though: what is light, and how is it made?
The electromagnetic spectrum
| Band | Typical wavelength | Example uses |
|---|---|---|
| Radio waves | metres to kilometres | AM/FM radio, TV broadcasting, MRI |
| Microwaves | about 1 mm to 30 cm | Microwave ovens, cell phones, Wi-Fi, radar |
| Infrared (IR) | about 700 nm to 1 mm | Heat lamps, TV remotes, thermal cameras |
| Visible light | about 400 nm to 700 nm | Seeing; red is longest, violet is shortest |
| Ultraviolet (UV) | about 10 nm to 400 nm | Sterilizing, vitamin D production, causes sunburn |
| X-rays | about 0.01 nm to 10 nm | Medical and dental imaging, airport security |
| Gamma rays | below about 0.01 nm | Cancer radiation therapy, sterilizing food and equipment |
All of these are the same kind of wave travelling at the same speed in a vacuum. What changes is the wavelength (the distance from one crest to the next) and the frequency (how many waves pass a point each second). Short wavelength means high frequency, and high frequency means more energy per packet of light. That is why UV, X-rays and gamma rays can damage living cells while radio waves pass through you harmlessly. A memory aid for the visible colours from longest to shortest wavelength is ROY G BIV: red, orange, yellow, green, blue, indigo, violet.
Light also travels in straight lines in a uniform medium (this is called rectilinear propagation). That is why shadows have sharp edges and why we can draw light as straight arrows called rays, the tool used for the rest of this unit.
Where light comes from
A luminous object makes its own light (the Sun, a candle, a phone screen). A non-luminous object is seen only because it reflects light from a source (the Moon, this page, your hand). Every luminous source turns some other form of energy into light, and the type of source is named after that energy input.
| Source type | How the light is produced | Examples |
|---|---|---|
| Incandescence | An object is heated until it glows; most of the energy comes out as heat (infrared) | The Sun, a candle flame, an old-style filament bulb, a red-hot stove element |
| Electric discharge | An electric current passes through a gas, which then glows | Neon signs, lightning |
| Fluorescence | A material absorbs ultraviolet light and immediately gives off visible light; it stops when the UV stops | Fluorescent tubes and CFLs, highlighter ink under black light |
| Phosphorescence | Like fluorescence, but the material keeps glowing for a while after the energy source is removed | Glow-in-the-dark stars and watch hands |
| Chemiluminescence | A chemical reaction releases energy as light, with little heat | Glow sticks, luminol used at crime scenes |
| Bioluminescence | Chemiluminescence inside a living thing | Fireflies, some deep-sea fish, some fungi and plankton |
| Triboluminescence | Light released when certain crystals are crushed, scratched or rubbed | Wintergreen candies crunched in the dark, sticky tape peeled quickly |
| Light-emitting diode (LED) | Electrons crossing a junction in a semiconductor release energy directly as light (electroluminescence) | Phone screens, traffic lights, LED bulbs |
- Absorbs high-energy light (usually UV) and re-emits visible light
- Glow stops almost instantly when the UV source is turned off
- Fluorescent tubes, highlighters, laundry brighteners
- Also absorbs light and re-emits it as visible light
- Glow continues for seconds to hours after the source is removed
- Glow-in-the-dark toys, emergency exit signs that need no power