Rectilinear Propagation of Light
Senior 1 · Physics
Rectilinear Propagation of Light
Lesson 1 of 5
22 min
At night, with the electricity off, you cannot read the book in your hands until someone lights a candle or switches on a phone torch. The book has not changed; what was missing is light reaching it and bouncing from it into your eyes. In the morning, when sunlight comes through a gap in the curtains or the iron-sheet roof, you can see its path in the dust: a sharp-edged shaft running in a straight line from the gap to the floor.
Definition
A luminous object gives out its own light: the Sun, a candle flame, a lamp. A non-luminous object gives out no light of its own; we see it only because it reflects light from a luminous source into our eyes: the Moon, this page, your friend's face.
Light sources are natural (the Sun, stars, lightning, fireflies) or artificial (made by people: candles, lamps, torches, screens).
| Luminous (natural) | Luminous (artificial) | Non-luminous |
|---|---|---|
| the Sun and other stars | a candle or a kerosene lamp | the Moon and the planets |
| lightning | an electric bulb, an LED torch | books, walls, clothes, people |
| fireflies | a phone or television screen | a mirror (it only reflects light) |
| glowing lava, as in Nyiragongo volcano | a car headlamp | clouds, which are lit by the Sun |
Figure 1.1: The flame is seen through the holes only when all three holes lie on one straight line from the flame to the eye.
Light from the flame reaches the eye only along the straight line through the holes. Light travels in straight lines: this is the rectilinear propagation of light.
Important
Safety. Keep hair, sleeves and paper away from the candle flame, and stand the candle in a tin lid of sand. Never look directly at the Sun, even for a moment, and never through binoculars or a camera: it can blind you.
Definition
Rectilinear propagation of light: in a uniform material, light travels in straight lines.
A ray is the path along which light travels; it is drawn as a straight line with an arrow showing the direction. A beam is a bundle of rays.
Figure 1.2: The three kinds of beam.
| Beam | What the rays do | Example |
|---|---|---|
| parallel | stay the same distance apart | sunlight reaching the Earth; a laser pointer |
| converging | come together towards a point | sunlight after passing through a magnifying glass |
| diverging | spread out from a point | light from a candle, a bulb or a torch |
Figure 1.3: What happens when light meets three kinds of material.
| Material | What light does | Can you see clearly through it? | Examples |
|---|---|---|---|
| transparent | passes straight through | yes | clear glass, clean water, air, clear plastic |
| translucent | passes through but is scattered | no, only light and blurred shapes | frosted glass, tracing paper, oiled paper, thin cloth, the plastic of a water tank |
| opaque | does not pass through | no | wood, metal, brick, cardboard, your body |
Light travels through empty space at about , three hundred thousand kilometres every second, and almost as fast through air. Nothing travels faster. For a trip at a steady speed, .
Worked example 1: sunlight's journey
The Sun is about from the Earth. How long does its light take to reach us?
That is about 8.3 minutes: you always see the Sun as it was 8 minutes ago.
Worked example 2: how far away is the storm?
During a storm over Huye, Keza sees a lightning flash and hears the thunder later. Sound travels at about . How far away was the lightning?
The light reaches her almost at once (in about a hundred-thousandth of a second), so the delay is the time the sound takes:
Warning
Common mistakes. (1) Calling the Moon a source of light: it is non-luminous and only reflects sunlight. (2) Thinking that the eyes send out rays to see things: light travels FROM the object INTO the eye, so arrows on rays always point towards the eye. (3) Confusing translucent (light passes, but blurred) with transparent (you see clearly).
Tip
Transparent: light goes trans (across) clearly. Opaque: light is stopped. Translucent sits in between: "light gets through, but the picture doesn't".
To show that light travels in straight lines (c4science). Watch the candle and the three cards: the flame is visible only while the holes are exactly in line.
Practice
Key points
Sources and credits
The experiment write-up, the diagrams, the tables, the worked examples and the practice exercises are written by Ganzaa.
