Light · 16.1
The Nature of Light
Light is an electromagnetic wave, but for mirrors and lenses a simpler picture works: rays drawn at right angles to wavefronts, travelling in straight lines until something bends them.
Build the model
Connect the measurement to the mechanism.
The ray model treats light as straight lines showing the direction of energy flow. It explains shadows, pinhole images, and eclipses, and it sets up reflection and refraction. Intensity falls with the square of distance from a point source because the energy spreads over a growing sphere.
- Simple definition
- A light ray is a straight line marking the direction light travels; a beam is a bundle of rays.
- Example
- A laser pointer's spot lands exactly along the straight line from the aperture — light travels in straight lines through uniform air.
Light crosses a metre in about three nanoseconds — the fastest anything can travel.
In vacuum; slower in any medium
Twice as far from a small source means the same energy spread over four times the area — a quarter of the brightness.
For a point source radiating evenly
A wavefront is a surface of constant phase; a crest is one possible constant-phase surface. Rays are arrows drawn at right angles to those surfaces, showing where the wave is going.
Two views of the same wave
Straight-line evidence
Sharp shadows, pinhole cameras, and eclipses all follow from rectilinear propagation — light does not bend around large objects noticeably.
When rays suffice
If the objects are much larger than the wavelength, ray optics predicts mirrors, lenses, and prisms accurately. Diffraction only matters near the wavelength scale.
Luminous or lit
Sources like the Sun and lamps emit light; everything else is seen by reflected light — which is why colour depends on what a surface reflects.
Change one variable at a time
Make the relationship visible.
Double the distance and the same light covers four times the area — intensity quarters.
Intensity I = P/4πr²1.99 W/m²
At double the distance0.5 W/m²
Catch the common trap
Explain before calculating.
Moving three times farther from a small lamp changes its brightness by what factor?
Choose an answer to test the model.
Worked examples
State the rule, substitute, then check units.
EasyHow long does light take to travel 1.0 km?
- t = d/c = 1000 ÷ 3 × 10⁸.
- t ≈ 3.3 μs.
Answer≈ 3.3 μs
MediumSunlight takes 8.3 minutes to reach Earth. Estimate the Earth–Sun distance.
- t = 8.3 × 60 = 498 s.
- d = ct = 3 × 10⁸ × 498.
- d ≈ 1.5 × 10¹¹ m — one astronomical unit.
Answerd ≈ 1.5 × 10¹¹ m
HardA lamp delivers 2.0 W/m² of light intensity at 3.0 m. At what distance is the intensity 0.50 W/m², and what is it at 1.5 m?
- I ∝ 1/r²: quartering intensity needs double distance → 6.0 m.
- Halving the distance to 1.5 m quadruples the intensity.
- I = 8.0 W/m².
Answer0.50 W/m² at 6.0 m; 8.0 W/m² at 1.5 m
ChallengingA pinhole camera forms a 5.0 cm image of a 10 m tall tree on a screen 20 cm behind the pinhole. How far away is the tree, and why does shrinking the pinhole sharpen then blur the image?
- Similar triangles: d(tree)/h(tree) = d(screen)/h(image) → d = 10 × 0.20 ÷ 0.050.
- d = 40 m.
- A smaller hole narrows each ray bundle (sharper) until the hole nears the wavelength scale — then diffraction spreads the light and blurs it again.
Answer40 m; geometry sharpens, diffraction eventually blurs