Chapter 11: Light: Shadows and Reflections

Objects that give out their own light are called luminous objects. The Sun is the main natural source of light on the Earth; stars, lightning, natural fire and animals such as fireflies also emit their own light. People later made artificial sources: fires fed by oil or wax, then electric bulbs, tube lights and LED lamps. Objects that do not give out light are non-luminous, for example the Moon, a mirror, a book or a table. We see them only because light from some luminous source falls on them and reflects into our eyes. So moonlight is just reflected sunlight. LED lamps use much less power, shine brighter and last longer than old lamps, so they save money and help the environment; a dead LED lamp should be recycled, not thrown in the garbage.

To test how light moves, take three cards with a hole punched at exactly the same height in each, and stand them in a row with a torch at one end and a screen at the other. When the three holes are in one straight line, a bright spot appears on the screen. Move one card a little sideways or up and the spot disappears. In a second test, a candle flame can be seen through a straight flexible pipe but not through a bent pipe (use a candle only under adult supervision). A laser pointer shows the same in water with a drop of milk, but only a teacher should use it and it must never be pointed at anyone's eyes. All these show that light travels in a straight line. A thin beam of light is drawn as a line with an arrow, called a ray.

Materials differ in how much light they let pass. Light passes almost completely through transparent materials such as clear glass, so objects behind are seen clearly. Light passes partially through translucent materials such as butter paper or frosted glass, so objects behind look blurred. Light does not pass through opaque materials such as a wooden board or a steel plate. To test this, shine a torch on a screen in a dim room, predict what each object will do, then place it in the beam and observe. If the observation differs from the prediction, accept the observation and correct the idea.

A shadow is the dark patch where light does not reach because an object blocks it. Since light travels in a straight line, an opaque object in its path stops it. To see a shadow we need a source of light, an opaque object and a screen (a wall, floor or the ground can be the screen). Remove the light, the object or the screen and the shadow is gone. With the torch and screen fixed, moving the object towards the torch makes the shadow larger, moving it towards the screen makes it smaller, and tilting it changes the shape. Opaque objects make dark shadows, translucent ones make lighter shadows, and even some transparent ones make faint shadows. The colour of an opaque object does not change the colour of its shadow. Your shadow is shortest around noon, when the Sun is high. Shadow puppetry, practised in many Indian regions, places flat cut-outs between a light and a screen.

A shiny surface such as a polished steel plate or a plane mirror (a flat, not curved, mirror) can send light in a new direction. If you hold a mirror in sunlight and tilt it, the spot on a wall moves. This change in the direction of light when it falls on a mirror is called reflection of light. To see it clearly, make a thin beam by blocking all but one slit of a comb, shine a torch through the slit along a white paper in a dim room, then place a mirror in the beam; the beam changes its path at the mirror. Before and after reflection the light still travels in straight lines. We see most non-luminous objects because light reflects from them into our eyes. A mirror experiment with sunlight must be done under a teacher, and the reflected beam must never be aimed at anyone's eyes.

The picture we see in a mirror is called an image; the thing in front of it is the object. The image in a plane mirror has these features: it appears behind the mirror, as far behind as the object is in front; it is the same size as the object; it is erect (upright); it cannot be obtained on a screen; and it is laterally inverted, which means left and right appear swapped. Move closer and your image comes closer; move away and it goes farther. Lateral inversion is why AMBULANCE is written reversed on the front of an ambulance: the driver ahead reads it correctly in the rear-view mirror. Letters such as A, H, M, O, T, U and X look the same when flipped left to right. Mirrors were once made by polishing stone or metal; the metal mirror Aranmula Kannadi is still made in Kerala.

A pinhole camera is a device in which light from an object passes through a tiny hole, the pinhole, and forms an image on a screen. Make a small hole in a cardboard, put a lighted candle on one side (under adult supervision) and a screen on the other in a dim room: the flame's image on the screen is upside down, that is, inverted. A portable version is made from two cardboard boxes that slide into each other. The larger box has a small hole in its closed face. The smaller box has a square cut in its closed face, covered with thin translucent paper such as tracing paper, which acts as the screen. Cover your head and the camera with a dark cloth, point the pinhole at a bright distant object such as a tree, and slide the smaller box until a clear image appears. The image shows the colours of the object but is inverted. It is inverted because light travels in straight lines: rays from the top of the object cross at the hole and land at the bottom of the screen.

Light travels straight and mirrors reflect it, so we can build useful items. A simple periscope is made by fixing two plane mirrors, at an angle of about 45°, in a Z-shaped box or tube. Light from an object reflects at the first mirror, travels down the tube and reflects again at the second mirror into the eye, so we can see things that are not visible directly. Periscopes are used in submarines, tanks and soldiers' bunkers. A kaleidoscope is made by joining three equal plane mirror strips in the shape of a triangle and fixing them inside a tube. Place pieces of coloured bangles or beads at one end, cover with tracing paper, and look through the other end. Reflections of reflections make many images that form beautiful patterns, and every turn gives a new pattern. Designers and artists use kaleidoscopes for pattern ideas; thick reflective paper strips can replace mirrors.