Light: Shadows and Reflections

The Sun is the single biggest source of natural light reaching the Earth, but it is far from the only object that produces its own light. Stars, flashes of lightning, a burning flame, and even certain living creatures — like the fireflies that light up on a summer evening — all give off, or emit, light of their own. Objects that produce their own light this way are called luminous objects.

Long before electricity, humans relied on fire — burning wood, oil, wax, or other fuels — as the main source of artificial light after dark. Electric light sources, from old incandescent bulbs to today's far more efficient LED lamps, have largely taken over that role now, lighting homes, streets, and cities through the night.

Not every glowing-looking object actually produces its own light, though. The Moon looks bright in the night sky, but it has no light of its own to give — it is a non-luminous object that simply reflects sunlight falling on it back towards us. The same goes for a steel plate glinting in the sun, or a mirror catching the glow of a lamp: both appear bright only because light from somewhere else is bouncing off them, not because they are producing any light themselves.

Here is a simple way to test whether light can travel around a bend: look at a lit candle flame through a long, flexible hollow tube held perfectly straight, and the flame is clearly visible. Now bend that same tube in the middle, keeping everything else the same, and look again — the flame disappears from view completely, even though the candle hasn't moved an inch.

A similar test uses three identical boxes, each with a small hole punched through its inner tray at exactly the same height. Line all three boxes up so that their three holes fall exactly along one straight line, shine a torch through the first hole, and a clear spot of light can be caught on a screen placed beyond the last box. Nudge just the middle box sideways by even a centimetre, breaking that straight-line alignment of the three holes, and the light spot vanishes from the screen.

Both results point to the same conclusion: light leaving its source does not bend or curve on its own. It travels outward only in straight lines, which is why anything that steps even slightly off that straight path — a bend in a tube, a hole nudged out of line — blocks it from reaching wherever it was headed.

This straight-line behaviour of light has a neat practical consequence. Make a small pinhole in a piece of cardboard, place it between a lit candle and a screen in a dim room, and a surprisingly sharp image of the candle flame appears on the screen — but turned upside down. This happens because light from the top of the flame can only travel in a straight line through the pinhole, which carries it to the bottom of the image on the screen, while light from the bottom of the flame travels straight through to the top of the image — crossing over inside the pinhole and flipping the picture top to bottom. A simple box built around such a pinhole, with a translucent screen fitted inside, is called a pinhole camera, and it always produces this same inverted image of whatever it is pointed at.

Shine a torch at a clean sheet of window glass, and almost all of the light sails straight through, giving a clear, sharp view of whatever is on the other side. Materials that behave this way are called transparent.

Now shine the same torch at a sheet of tracing paper, or a frosted glass panel. Some light does pass through — it is clear that there is a bright room on the other side — but what is seen through it is blurry and unclear, nothing like the sharp view through clean glass. Materials that let some, but not all, light pass through this way, scattering it as it goes, are called translucent.

Finally, shine the torch at a thick wooden board or a sheet of cardboard, and no light gets through at all — not even a dim glow on the other side. Materials that block light completely like this are called opaque.

Most everyday objects are opaque: wood, metal, stone, and thick cardboard, for instance. A good way to test any material is to darken a room, shine a torch at a screen to get a clear, bright spot of light, and then hold different objects between the torch and the screen one at a time, noting whether the spot of light stays fully bright, grows dim, or disappears completely.

Shine a torch on an opaque object, like a closed book standing upright, with a wall or screen behind it, and a dark patch forms on the wall, matching roughly the outline of the book. Since light travels only in straight lines, the book simply blocks the straight paths of light heading towards that patch of wall, leaving it dark while light reaches everywhere else around it. That dark patch is the book's shadow.

To observe a shadow at all, three things need to be present together: a source of light, an opaque object to block that light, and a surface — a screen, a wall, the ground, anything solid — for the dark patch to fall on. Remove any one of the three, and there is simply no shadow to see.

Shadows are not always equally dark, either. A fully opaque object blocks light completely, producing a sharply dark shadow. A translucent object only partly blocks light, so its shadow looks noticeably lighter and greyer rather than fully black. Even some transparent materials can throw a very faint shadow, if they are not perfectly clear.

The size and shape of a shadow is not fixed — it depends on exactly where the object sits between the light source and the surface. Move the object closer to the light source while keeping the light and the screen fixed in place, and its shadow grows larger and often blurrier, because the spreading rays of light now have to travel further past the object before they reach the screen. Move the same object closer to the screen instead, and its shadow shrinks back down and sharpens. One thing that never changes the shadow, though, is the object's own colour: a red ball and a blue ball of the exact same shape and size will cast identically dark shadows, since a shadow is simply a region with no light reaching it at all, with no colour of its own to borrow from the object.

Hold a flat, shiny steel plate or a plane (flat) mirror out in direct sunlight, tilt it carefully, and a bright spot of sunlight can be thrown onto a wall that the Sun itself isn't even shining on directly. Tilt the mirror differently, and that bright spot shifts to a different place on the wall. The mirror hasn't blocked the sunlight the way an opaque object would — it has redirected it, sending it off in a new direction. This change in the direction of light after it strikes a shiny surface is called reflection.

Whenever light reflects off a flat surface like this, it leaves at exactly the same angle, measured from the surface, at which it arrived — a simple, consistent rule that governs every reflection off a flat mirror, however it is tilted.

Look directly into a plane mirror, and a reflection of the face looking into it appears, formed by light bouncing off the mirror and back into the eyes. Place any small object, say a pen, in front of a plane mirror, and it will appear to have an identical twin sitting behind the glass — this apparent second pen is called the image, while the real pen in front of the mirror is the object.

A plane mirror's image has a few distinctive, testable features. Move the pen to different positions in front of the mirror, and its image always matches it in size exactly — a plane mirror never enlarges or shrinks what it reflects. The image also always stays upright, the same way up as the real object, which is described as being erect. Try placing a screen behind the mirror to "catch" this image the way a shadow falls on a screen — it will not work; unlike a shadow cast on a screen, this image is not made of light actually meeting at a point behind the mirror, it only appears as if the pen is sitting there, so no screen anywhere can capture it.

There is one more curious feature. Raising a left hand while looking in a mirror makes the reflection appear to raise what looks like its right hand instead; touching a right ear makes the reflection seem to touch its left ear. This apparent left-right swap is called lateral inversion. It is why the word AMBULANCE is sometimes printed reversed on the front of an ambulance — so that, when this reversed word is itself reflected in the rear-view mirror of a car ahead, the lateral inversion cancels out and the driver reads the word correctly, the right way round.

Two mirrors, carefully angled, can put this bouncing-light property to clever practical use. A periscope uses two plane mirrors set at an angle inside a tube, bouncing light twice so that someone can see over an obstacle or around a corner — exactly how sailors see above the surface from inside a submarine. A kaleidoscope arranges three mirror strips in a triangle, so that light bounces repeatedly between them, multiplying a handful of loose coloured beads or glass pieces into the elaborate, ever-changing patterns seen when looking inside and giving it a twist.