Why Some Things Glow and Others Don’t: The Science of Light

Luminous bodies are objects that produce their own light, while non-luminous bodies do not emit light but can be seen when light falls on them. Light sources can also be classified as natural (occurring in nature) or artificial (made by humans). Luminous bodies include the Sun and a glowing bulb, as they emit light themselves. Non-luminous bodies, like a book or a chair, are visible only when light from a luminous source shines on them.

Natural sources of light are those found in nature, such as the Sun and fireflies. Artificial sources are man-made, like electric bulbs and candles. For example, during the day, sunlight (a natural source) lets us see everything around us. At night, we use a tube light (an artificial source) to light up a room. A common mistake is to think that all bright objects are luminous, but a shiny steel plate is not a source of light—it just reflects light from another source.

We see non-luminous objects because light from a luminous source falls on them, reflects, and then enters our eyes. The reflected light carries information about the object’s shape, color, and texture, enabling us to see it.

A ray of light is a straight line showing the path along which light travels. A beam of light is a group of rays traveling together. For example, sunlight streaming through a small hole in a curtain forms a visible narrow beam, and the individual straight lines within it are rays. In real life, when you look at a cricket ball on the ground, you see it because sunlight (or another light source) bounces off the ball and into your eyes. Remember, we cannot see light itself unless it enters our eyes or is scattered by dust or fog. This is why we see a beam of light in a smoky room but not in clear air.

Light travels in a straight line, a property known as rectilinear propagation of light. This can be demonstrated with a simple experiment using three cardboards, each with a small pinhole, and a candle. Place the cardboards in a straight line and the candle at one end. If you look through the holes, you will see the candle flame only when all the holes are perfectly aligned, showing that light moves straight from the candle to your eye. If you shift one cardboard, the flame disappears from view, proving that light does not bend around obstacles. In real life, shadows are formed because light cannot bend around objects, so areas behind the object do not receive light. Some students mistakenly think light can curve around corners, but this experiment shows it always travels straight unless reflected or refracted.

Materials are classified based on how much light they let pass through: transparent materials allow almost all light to pass through, so you can see clearly through them; translucent materials let some light pass, but you cannot see clearly; opaque materials do not let any light pass through. For example, clear glass and clean water are transparent—you can see objects on the other side easily. Frosted glass and oiled paper are translucent—they allow light but blur the image. Wood and metal are opaque—no light passes through, so you cannot see through them. In daily life, window panes are transparent, bathroom windows are often translucent for privacy, and doors are opaque. A common confusion is thinking all thin or light-colored materials are transparent, but only those that let light pass without scattering are truly transparent.