A Light Introduction

Physics
Fox

Summary

Light is what we see. Light comes to our eyes from luminous and illuminated objects.

Luminous objects emit light.

Illuminated objects reflect light.

Seeing things

I'll look at the sun, and I'll look in the mirror
I'm on the right track, yeah I'm onto a winner

Lily Allen

How do we see things? Does the eye have something coming out of it, that ‘feels out’ objects? Sight coming out of eye

That doesn't make sense — otherwise we'd be able to see things equally well all of the time, day or night. Light coming from the sun, bouncing off a mug, and going into the eye

What makes more sense is that there is something that goes into our eyes. We'll call this thing light. The sun creates light, so when the sun is out we can we can see the light from it, as well as light from the sun that has bounced off other objects. (The scientific word for 'bounce' is reflect).

Luminous vs Illuminated

Light is created by some objects, and reflected from other objects. There's a special name we use for each of these types; luminous and illuminated.

Light coming from the luminous sun, reflecting off an illuminated mug, and into the eye

Luminous objects emit (aka. create) light.
Illuminated objects reflect light.

Here's a way to remember it: The spell lumos from Harry Potter makes a wand tip luminous. A wand lighting up at the end

Here are some examples of luminous and illuminated objects: Various luminous objects: sun, light bulb, phone screen, fire, RGB lighting various illuminated objects: The Moon, a book, clothes, cash, a mug

Interactions of Light

Let's summarise the journey light takes to our eyes (from any illuminated object):

Light travels from sun, to mug, to eye. The steps are labelled 1,2,3,4.

Light is created by a luminous object
Light travels through space
Light is reflected off an illuminated object
Light is detected by our eyes (after travelling through space again)

Over the next set of pages, we will understand what happens during steps 2-4. Step 1 is quite complex (and there are many ways it's done), so we'll cover that another time.

Is glow-in-the-dark stuff luminous or illuminated?

Glow in the dark stars: Day (left), night (right)

It's both!

Glow-in-the-dark objects are luminous because they always emit light, but it's such a small amount of light that you can only see it in dark room (hence the name: glow-in-the-dark).

In a bright room, a glow-in-the-dark object reflects light just like any other illuminated object would, so it is also illuminated.

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