Class 7 Science Chapter 11: Light: Shadows and Reflections Notes

What is this chapter about?

On a holiday in the Western Ghats, Keshav watches hundreds of fireflies flash in the dark. He learns that these insects talk to one another with light, and that they are disappearing because of light pollution, shrinking forests and too many tourists. On the bus home he looks at the moonlit hills and wonders: does the Moon make its own light? And why do the headlight beams look like straight lines? This chapter answers such questions: where light comes from, how it travels, and what happens when it meets objects and mirrors.

Sources of light

The Sun is our main source of natural light: it emits, or gives out, its own light. Stars, lightning, natural fire and some animals such as fireflies also emit light. People first made light with fire, then with lamps burning oil, wax or gas, and today mostly with electric lamps. Objects that emit their own light are luminous; objects that do not are non-luminous. The Moon is non-luminous: moonlight is sunlight reflected by the Moon. Modern LED lamps need far less electricity, shine brighter and work for longer, but at the end of their life they must be recycled, not thrown in the garbage.

Light travels in a straight line

Shine a torch through holes in three matchboxes standing in a line, and a bright spot appears on a screen; move one box slightly and the spot disappears. A candle flame shows at the end of a straight pipe but never at the end of a bent one. A laser beam in water with a drop of milk shows a straight path too. All these show that light travels in a straight line.

Transparent, translucent and opaque

Light passes almost completely through transparent materials such as clear glass, partly through translucent ones such as tracing paper, and not at all through opaque ones such as cardboard. Read more in how shadows are formed.

Shadows

When an opaque object stands in the path of light, it blocks the light, and a dark patch, the shadow, forms on the surface behind it. To see a shadow we need a light source, an opaque object and a screen, such as a wall or the ground. The shadow’s shape, size and sharpness change with the positions of the object, the light and the screen, but its colour does not change with the object’s colour. Translucent objects give lighter shadows. India’s many forms of shadow puppetry, such as Tholu Bommalata in Andhra Pradesh and Ravana Chhaya in Odisha, use this idea.

Reflection and plane mirrors

A shiny plate or a mirror can throw a spot of sunlight onto a shaded wall: it changes the direction of light. This is the reflection of light. A flat, or plane, mirror forms an image that is the same size as the object, erect (upright), as far behind the mirror as the object is in front, and cannot be caught on a screen. It is also laterally inverted: left and right swap, which is why AMBULANCE is painted reversed on ambulances. See reflection and plane mirrors.

Pinhole camera, periscope and kaleidoscope

In a pinhole camera, light from an object passes through a tiny hole and forms an image on a screen. The image is inverted, upside down, because rays from the top and bottom of the object cross at the hole. Read the pinhole camera. Mirrors also make useful toys and tools: a periscope uses two plane mirrors to see over walls, and a kaleidoscope uses three to make endless patterns. Every answer is in the NCERT solutions for Chapter 11.