We use light to see!

Visible light is the part of the electromagnetic spectrum that our eyes can see:

It is only a small part of the full spectrum, isn't it?

Visible Spectrum

Visible Light: the wavelengths that are visible to most human eyes.

The main colors, in order, go "Roy G Bv": Red Orange Yellow Green Blue Violet
light spectrum

As we see on this beautiful rainbow:

Light has wavelengths of about 380 nm to 750 nm

nm means nanometer, one billionth of a meter.

Example: red light has a wavelength of about 700 billionths of a meter. Small!

Definitions vary, but here is a rough guide:

Color Wavelength Range (nm)
Red 620–750
Orange 590-620
Yellow 570-590
Green 495-570
Blue 450-495
Violet 380-450

The frequency of red light is about 400 THz (and for violet is about 800 THz)

THz means teraHertz, a trillion cycles per second

So red light vibrates at about 400 million million cycles per second. Fast!

Higher frequency (with shorter wavelength) has more energy:

red lower energy, blue higher energy

Speed of Light

Light travels at almost 300,000,000 meters per second (to be exact: 299,792,458 meters per second) in a vacuum.

That is 300 million meters every second, or:

At that speed light travels:

Distance   Time
1 meter in 3.3 ns (3.3 billionths of a second)
Around the Earth's equator in 134 ms (134 thousandths of a second)
From Earth to Moon in 1.3 s
Surface of Sun to Earth in about 8 minutes

It is so fast, but still takes about 8 minutes from the surface of the Sun to the Earth.

The symbol for this speed is c:

c = 300,000,000 m/s

Light Can Travel Slower

We really shouldn't call it the speed of light, firstly because it applies to the whole electromagnetic spectrum, and gravity waves, and more! (Maybe we could call it "Max Speed"!)

But also because light only travels that speed in a vacuum! It can travel slower ...

Medium Speed
million m/s
Vacuum 299.8
Air 299.7
Ice 228
Water 225
Ethanol 220
Glass 205
Olive oil 204
Diamond 123

Wavelength and Frequency are Linked

The Wavelength and Frequency are related:

Frequency = Velocity Wavelength

Wavelength = Velocity Frequency

Assuming the light is in a vacuum, the velocity is the speed of light: 3 × 108 m/s

Let's try a simple example (not a wavelength of light):

Imagine a very long wavelength of 75,000 km

wavelength vs frequency

Frequency = 300,000 km/s 75,000 km

Frequency = 4 /s

= 4 Hz

We can fit only 4 of those wavelengths in 300,000 km, so it vibrates only 4 times in 1 second.

So the frequency is 4 Hz (4 per second)

Example: Blue light has a wavelength of about 480 nm (480 × 10-9 m)

So the frequency is:

Frequency = 3 × 108 m/s 480 × 10-9 m

Frequency = 6.25 × 1014 /s

= 6.25 × 1014 Hz

Which is 625 TeraHertz

Light Travels in Straight Lines

Light travels in a straight line until its hits something (or unless it's path is changed by different densities or gravity).

light beams forest
Light from the Sun streams across the road.
The shadows also show that light travels in straight lines.

light beam
This light spreads out a little and is scattered by the atmosphere.

laser beams
Laser beams making straight lines.


refraction plastic block


Light behaves as a wave, so it can:


Light also behaves as packets of energy called Photons.

So it is like a particle and also like a wave. This is called the "wave-particle duality".



Einstein wrote:

"It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either."



Intensity is power per area, usually in Watts per square meter:

Intensity = W/m2

Example: Sun on a small 100 square meter house

About 150 to 300 watts of energy are received from the Sun per square meter.

Let's choose the smaller number:

Intensity = 150 W/m2

How much Power is that over the whole roof?

Power = 150 W/m2 × 100 m2

Power = 15,000 W

So a small house gets about 15 kilowatts on it's roof, which is several times more than a household uses.

But that is only while the Sun shines, and only about 20% can be captured by typical solar panels

But that is still lots of energy from the Sun.

Inverse Square

brightness decreases by the square of the distance Inverse Square: when one value decreases as the square of the other value.

Example: light and distance

The further away we are from a light, the less bright it is.

inverse square law: distance=1 area=1 intensity=1, distance=2 area=4 intensity=0.25, distance=3 area=9 intensity=0.111...

The brightness decreases as the square of the distance. Because the light is spreading out in all directions:


Light is normally free to vibrate in any direction at right angles to its path.

But polarized light vibrates in one plane only:

unpolarized vs polarized
Light gets partly polarized when it
bounces off surfaces like water or glass.

Polarizing lenses can block light from that plane, to cut down on reflected light and make it easier to see into water:

polarized picture of water
Without and with a polarizing lens

Fiber Optics

Light, and infrared, can be sent along fiber optic cables, carrying information coded into the wavelength.

fiber optic
Fiber optic cables

The light stays inside because of a special property of refraction: when the refractive index is lower on the outside, and the angle is not too steep, the light beam has total internal reflection on the inside:

fiber optic bounce inside
Light bounces off the walls inside the cable

Fiber optic cables are much better than electrical wires: