Natural and artificial sources of light
A luminous object makes its own light. A non-luminous object only reflects light from a source. The Moon is non-luminous: it shines because the Sun lights it.
Natural sources: the Sun and other stars, lightning, fire, and living things such as fireflies and some deep-sea fish. Artificial sources are made by people:
- Filament (incandescent) bulb: a thin wire gets so hot it glows. Most of the energy leaves as heat.
- Fluorescent tube / CFL: an electric current in a gas makes invisible ultraviolet light. A white coating on the glass turns it into visible light (this is luminescence).
- LED (light-emitting diode): current flowing through a tiny semiconductor makes light directly. Little heat, long life.
The share of electrical energy that becomes light is the efficiency: about 5% for a filament bulb, 25% for a fluorescent lamp and 45% or more for a good LED. Numbers differ a little between makers.
Colour rendering index (CRI)
Colours look different under different lamps. The colour rendering index (CRI) is a number from 0 to 100. It tells how truly a lamp shows the colours of objects compared with a natural reference light. Sunlight is 100.
- CRI below 70: colours look dull or wrong (some old street lamps).
- CRI 80 and above: fine for homes and offices.
- CRI 90 and above: shops, art rooms, photo studios, hospitals.
CRI is not the same as warm or cool light. Warm light (about 2700 K, yellowish) and cool light (about 6500 K, bluish) can both have a high CRI. A lamp that spreads its light over many colours renders colours well; a lamp with only a few bright colour lines renders them badly.
Optical fibres
An optical fibre is a glass thread thinner than a hair. It has a centre called the core and a covering called the cladding. The core glass bends light more than the cladding (it has a higher refractive index).
Light enters one end and hits the wall at a small angle to the wall. If the angle with the normal is bigger than the critical angle, all the light is reflected back inside. This is total internal reflection. The light zigzags along the fibre and follows bends with almost no loss.
Uses: internet and phone signals (flashes of light carry data), endoscopes that let doctors look inside the body, decorative lamps. Why better than copper wire: carries much more data, no electrical interference, very little loss over long distance.
Lasers
A laser makes a very special beam. It is single-colour (one wavelength), coherent (all the waves move in step) and highly directional (the beam hardly spreads). The name stands for Light Amplification by Stimulated Emission of Radiation. The full working is in the lesson on lasers.
Uses: barcode scanners, laser pointers, reading and writing discs, sending data through fibres, cutting metal, eye and other surgery, measuring distance. Safety: never point a laser at eyes; even a weak beam can hurt the eye because all its light falls on one tiny spot.
Try it
In the 3D: on the Colour step, drag the CRI slider from 90 to 30 and watch the fruits. On the Laser step, compare the two spots on the wall.
At home: shine a phone torch into one end of a clear water stream or a bent clear plastic rod in a dark room. The other end glows. Also look at the packet label of an LED bulb: find the watts, lumens and CRI.
Key formulas and definitions
- Efficiency = (light energy out ÷ total energy in) × 100%
- Energy (kWh) = power (kW) × time (h)
- Critical angle: sin C = n(cladding) ÷ n(core) (for glass to air, sin C = 1/n)
- CRI scale: 0 (very poor) to 100 (same as natural reference light)
Worked examples
1. A 60 W filament bulb gives out 3 W of light. Find its efficiency.
Efficiency = 3 / 60 × 100 = 5%. The other 95% leaves as heat.
2. An LED of 9 W gives the light of a 60 W filament bulb. Both run 5 hours a day for 30 days. How much energy is saved?
Time = 5 × 30 = 150 h. Difference in power = 60 − 9 = 51 W = 0.051 kW. Energy saved = 0.051 × 150 = 7.65 kWh.
3. Glass of refractive index 1.5 is in air. Find the critical angle.
sin C = 1 / 1.5 = 0.667, so C ≈ 41.8°. Light hitting the glass–air surface at more than this angle is totally reflected.
4. In a fibre the core has n = 1.50 and the cladding n = 1.45. Does a ray hitting the wall at 80° to the normal stay inside?
sin C = 1.45 / 1.50 = 0.967, so C ≈ 75°. 80° is bigger than 75°, so the ray is totally reflected and stays inside.
Common mistakes
- Saying the Moon is a light source. It only reflects sunlight.
- Thinking CRI means warm or cool colour. CRI is about how true the colours look, not how yellow or blue the light is.
- Believing a bright bulb is an efficient bulb. A hot filament bulb is bright but wastes about 95% of the energy as heat.
- Saying light "leaks" through the fibre wall. Inside the fibre it is totally reflected, but only if the angle is bigger than the critical angle.