Sources and rectilinear propagation of light

Introduction to light

Light is a form of energy that helps our eyes to see things. Imagine you are in a room at night and NEPA takes the light; you cannot see anything until you strike a match or turn on your phone torch. This energy travels as a wave.

Simply put, light is the energy that makes objects visible to our eyes.

Sources of light

We can group things that give off light into two main types: Luminous and Non-luminous objects. Luminous objects are like a burning candle or the Sun; they make their own light. Non-luminous objects do not make light; they only reflect light from other sources into our eyes, like a textbook or a plastic bucket.

Rectilinear propagation of light

If you shine a torch in a dark, dusty room, you will see the light moving in a straight line. If you place an obstacle like a piece of cardboard in front of it, the light cannot bend around it to show what is behind. This straight-line movement is why we have shadows.

In simple terms, light does not bend corners; it only moves straight.

Rays and beams

To show how light moves on paper, we use lines with arrows. One single line is a ray. When many rays move together, we call it a beam. Think of a single strand of hair versus a whole bunch of hair.

There are three types of beams you must know:

Parallel beam: The rays are like the two sides of a straight road; they never meet.

Convergent beam: The rays come from different directions and meet at one point, like a magnifying glass focusing sunlight.

Divergent beam: The rays start from one point and spread out, like the light coming out of a car headlamp or a flashlight.

Shadows

A shadow is formed because light travels in straight lines and cannot pass through solid (opaque) objects. If the light source is very small (a point source), the shadow is sharp and dark. If the source is large, the shadow has a dark center called the Umbra and a partial outer shadow called the Penumbra.