Sources and properties of light

Switch off the lamp: the book remains on the table, but we can no longer see it. What must travel from the source to the book and then to the eye for the object to become visible?

We can see a lamp by the light it emits itself. A body that emits its own visible radiation is called a light source. The Sun and stars are natural sources; a lamp and a burning candle are artificial ones. Light carries energy away, so emission requires energy conversion: a lamp uses electrical energy and a candle uses chemical energy. Visible light is part of the electromagnetic radiation we have already met.

Light from the lamp reaches the book. Some is absorbed; some is reflected and scattered in different directions. We see the book when some of this light reaches the eye. Blocking the path from lamp to book or from book to eye changes visibility. An ordinary book is invisible in complete darkness because too little visible light arrives from it, not because it has vanished. The eye receives light; it does not illuminate objects with rays of its own.

The bright Moon may seem to be a source of its own light. Trace the energy, however: sunlight falls on its surface, and some is reflected towards us. In the visible range we therefore treat the Moon mainly as an illuminated body. This does not deny its own thermal radiation in other ranges. We must specify which light we are explaining: ‘we see the body’ does not imply ‘it produces appreciable visible radiation itself’.

Absorbed light no longer travels as the original beam, but its energy is conserved. It can increase a body's internal energy, trigger chemical processes or become electrical energy in a solar cell. Thus an illuminated surface can warm up, and light can cause other changes too. The result depends on how much light arrives, which parts the material absorbs and the forms of energy into which it converts them.

Two lamps may have the same watt rating but different lumen ratings. Watts usually indicate electrical power consumption, not visible output. Luminous flux Φv measures visible radiant power weighted by the human eye's sensitivity to different colours; its unit is the lumen, lm. Different sources convert input energy into visible light with different efficiencies. Electrical power alone is therefore not enough to compare their visible light output.

Take the same luminous flux and focus it into a narrow beam on a small spot, then spread it widely around the room. The total lumens may stay the same while a particular part of the desk receives different illumination. Direction, distance, distribution area and obstacles all matter. Luminous flux characterises the source as a whole; by itself it does not say how much light reaches a chosen surface patch.

A lamp is on in a room and a white sheet of paper lies on the table. Why can the paper be seen from different directions?

Trace the energy's path: the lamp emits its own light and some of it falls on the white paper.

The rough surface reflects light in different directions. Rays from the paper can therefore enter the eyes of observers at different positions.

The paper need not emit appreciable visible light itself. Remove the incident light and other sources, and ordinary paper becomes invisible even though its surface remains.

The paper is visible because it diffusely reflects the lamp's light. Different observers receive different parts of the reflected beam.

To explain visibility, trace the path from source to object to eye.

Light carries energy, and absorption converts it into other forms.

Lumens describe visible light weighted by vision; power consumption in watts and illumination of a particular desk are different characteristics.

To construct shadows, we will represent light propagation with rays.