"Light energy examples" is a primary and middle-school search: the energy-types unit asks for examples of light energy, of the changes into and out of it, and then (in grade 6 to 8) of the kind of wave light is. This page gives the definition in one paragraph, twenty-five examples sorted by where the energy came from, the energy-change table, the types of waves with light placed among them, and the electromagnetic spectrum table for the older students. The printable has the examples, the conversions, the wave types and practice on two pages.
Download the light energy and types of waves sheet (PDF) (the definition, the 25 examples by source, the energy-change table, the types of waves with examples, the electromagnetic spectrum with wavelengths and uses, and ten practice questions with answers; two pages, free).
What light energy is
Light energy is energy carried by electromagnetic radiation: a wave of electric and magnetic fields that travels through empty space at 299,792,458 metres per second (about 300,000 km/s) and slower through glass, water and air. The eye detects the band of wavelengths from about 400 nanometres (violet) to 700 nanometres (red), which is visible light; the same kind of wave at longer wavelengths is infrared, microwaves and radio, and at shorter wavelengths ultraviolet, X-rays and gamma rays. Physics calls all of it light, or electromagnetic radiation; the energy-types unit in school usually means the visible part.
Light arrives in packets called photons, each carrying an energy E = hf, where h is Planck's constant and f the frequency: a violet photon carries nearly twice the energy of a red one, and an X-ray photon thousands of times more, which is why ultraviolet burns skin and X-rays pass through it. Light energy needs no material to travel through; sound does.
Twenty-five examples of light energy
| Where the energy comes from | Examples |
|---|---|
| Nuclear fusion | 1. the Sun; 2. the stars |
| Burning (chemical, with heat) | 3. a candle flame; 4. a campfire or wood fire; 5. a gas hob's blue flame; 6. fireworks; 7. a match |
| Electricity through a filament or gas | 8. an incandescent bulb; 9. a fluorescent tube; 10. a neon sign; 11. a halogen car headlight; 12. a lightning flash (electricity through air) |
| Electricity through a semiconductor | 13. an LED; 14. a phone, TV or computer screen; 15. a laser pointer; 16. a torch (flashlight) |
| Chemical reactions without burning (chemiluminescence) | 17. a glow stick; 18. luminol at a crime scene |
| Living things (bioluminescence) | 19. a firefly; 20. glowing plankton in a night sea; 21. an anglerfish's lure |
| Very hot matter glowing | 22. molten lava; 23. a red-hot electric hob or a blacksmith's iron |
| The sky | 24. the aurora (borealis and australis); 25. a meteor's streak |
And the light energy the eye cannot see: the infrared from a heater, a remote control or a warm body; the ultraviolet from the Sun or a tanning lamp; the X-rays in a hospital; the microwaves in an oven; the radio waves from a transmitter. All of these are the same kind of energy at other wavelengths.
Energy changes into and out of light
| From | To | Where |
|---|---|---|
| electrical | light | a bulb, an LED, a screen, a laser |
| chemical | light (and heat) | a candle, a fire, a glow stick, a firefly, fireworks |
| nuclear | light (and heat) | the Sun and stars |
| heat (thermal) | light | a glowing filament, lava, a red-hot hob |
| light | chemical | photosynthesis in a leaf; the silver halide in camera film; vitamin D made in skin |
| light | electrical | a solar panel; a camera's sensor; a light-dependent resistor; the retina (to nerve signals) |
| light | heat | a dark road in the sun; a solar water heater; the inside of a parked car |
| light | light | a mirror (reflection); a fluorescent jacket (absorbs ultraviolet, gives out visible) |
A solar panel at 20 percent efficiency turns a fifth of the light energy falling on it into electrical energy and the rest into heat; a leaf stores about 1 percent of the light it receives as chemical energy in sugar; an incandescent bulb turns about 5 percent of its electrical energy into light and 95 percent into heat, an LED about 40 percent, which is why the LED runs cool and lasts.
The types of waves
A wave carries energy from one place to another without carrying the matter along. Waves are sorted two ways.
By the direction of the oscillation:
| Type | The oscillation is | Examples |
|---|---|---|
| Transverse | at right angles to the direction the wave travels | light and every electromagnetic wave; ripples on water (mostly); a shaken rope or string; seismic S waves |
| Longitudinal | along the direction the wave travels (compressions and rarefactions) | sound; a slinky pushed end to end; seismic P waves; ultrasound |
A transverse wave has crests and troughs; a longitudinal wave has compressions (squashed) and rarefactions (stretched). Only transverse waves can be polarised, which is how polarising sunglasses cut glare and how it was proved that light is transverse. (Surface waves on deep water are a mixture: each water particle moves in a circle.)
By what they travel through:
| Type | Needs a medium? | Examples |
|---|---|---|
| Mechanical | yes: a solid, liquid or gas | sound, water waves, waves on a string, seismic waves |
| Electromagnetic | no: they cross empty space | radio, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays |
Sound cannot cross a vacuum (no sound in space); light can, which is why we see the Sun and hear nothing from it. All electromagnetic waves travel at the same speed in a vacuum, 3 × 10⁸ m/s; sound in air travels at about 343 m/s, nearly a million times slower, which is why the lightning is seen before the thunder is heard.
The wave equation links them all: speed = frequency × wavelength, v = fλ. For light, c = fλ: red light at 700 nm has a frequency of 3 × 10⁸ ÷ 7 × 10⁻⁷ = 4.3 × 10¹⁴ Hz.
The electromagnetic spectrum
| Band | Wavelength | Where it is used or found |
|---|---|---|
| Radio | over 1 m (down to about 1 mm for the whole radio band) | broadcasting, Wi-Fi, mobile phones, radar |
| Microwaves | 1 mm to 1 m | ovens, satellite links, phones |
| Infrared | 700 nm to 1 mm | heaters, remote controls, thermal cameras, night vision |
| Visible | 400 nm (violet) to 700 nm (red) | seeing; red 620 to 750, orange 590 to 620, yellow 570 to 590, green 495 to 570, blue 450 to 495, violet 380 to 450 nm |
| Ultraviolet | 10 nm to 400 nm | sunburn, vitamin D, sterilising lamps, fluorescent inks |
| X-rays | 0.01 nm to 10 nm | medical imaging, airport scanners |
| Gamma rays | under 0.01 nm | radioactive decay, cancer treatment, sterilising instruments |
Across the table the wavelength shrinks, the frequency rises and the photon energy rises with it, which is why the right-hand end is the dangerous end.
Where it sits in school
Light as a form of energy, with examples and the energy changes, is grade 3 to 5 in the United States (NGSS 4-PS3 and 4-PS4), Year 3 and 6 in England (Light), Year 4 to 6 in Australia and CBSE Class 4 to 6; transverse and longitudinal waves, mechanical and electromagnetic, are grade 6 to 8 (MS-PS4), Key Stage 3 and GCSE, Year 8 and 9, and CBSE Class 8 and 9 (Sound; Light); the spectrum and E = hf are GCSE and A level, grade 11 and 12 and Class 12. The common errors: calling sound a transverse wave, saying light needs air, and listing heat as light energy (a heater's infrared is; its warmth is not).
Practice: ten questions
- Name five sources of light energy, one from each of: burning, electricity, a living thing, a chemical reaction without burning, and the sky.
- What energy change happens in a solar panel? In a leaf? In a candle?
- Is light a transverse or a longitudinal wave? Give one piece of evidence.
- Why can light cross space but sound cannot?
- Give two examples of light energy the eye cannot see.
- Which travels faster, light or sound, and by roughly how much?
- Put in order of increasing wavelength: X-rays, radio, visible light, ultraviolet, infrared.
- Which colour of visible light has the most energy per photon, red or violet?
- A wave on a slinky made by pushing one end: transverse or longitudinal?
- Why does an LED bulb stay cool while an old filament bulb gets hot?
Answers: 1) Candle; LED; firefly; glow stick; aurora (any correct set). 2) Light to electrical; light to chemical; chemical to light and heat. 3) Transverse; it can be polarised. 4) Light is electromagnetic and needs no medium; sound is mechanical and needs matter. 5) Infrared from a heater; ultraviolet from the Sun (or X-rays, microwaves, radio). 6) Light, about a million times faster (3 × 10⁸ against 343 m/s). 7) X-rays, ultraviolet, visible, infrared, radio. 8) Violet (higher frequency, E = hf). 9) Longitudinal. 10) The LED turns most of its electrical energy into light; the filament bulb turns 95 percent into heat.
Download the light energy and types of waves sheet (PDF)
Where to get help
A 1-on-1 online science class with a tutor who holds a master's degree in science, for grades 4 to 10 in the US curriculum, Key Stage 2 and 3 and GCSE in England, the Australian years and CBSE Class 4 to 10: energy types with real objects on the shared screen, the two kinds of wave drawn and acted out with a rope and a slinky, then the spectrum and the exam questions for the older ones. The recording and notes after every class, the same tutor every week, US$10 to $13 an hour. The physics page has the courses; the free 30-minute first class is a real lesson, usually on the homework the student brings.
Questions parents ask
1What is light energy?
2What are 10 examples of light energy?
3What energy changes involve light?
4Is light a transverse or a longitudinal wave?
5What are the types of waves?
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