Why You Can't See Around a Corner

Because light travels in a straight line. It cannot bend around an obstacle to reach your eye, so anything the straight path does not reach stays hidden. That single property explains shadows, eclipses and why a bent pipe blocks your view.

This page covers everything in the CBSE Class 7 Science chapter on light and shadows: which objects give out light, how materials differ in letting it through, the experiment proving light travels straight, and what decides a shadow's size and shape.

Luminous objects give out their own light: the Sun, stars, a burning candle, a lit bulb, a firefly.

Non-luminous objects do not. We see them only because light from a luminous object falls on them and bounces off into our eyes: a table, a book, the Moon, you.

Sources are also grouped as natural — the Sun, stars, lightning, fireflies — or artificial, made by people: bulbs, tube lights, candles, torches.

So the Moon looks bright but is non-luminous. It produces no light of its own and simply reflects sunlight, which is exactly why it has phases and why it is dark during a lunar eclipse.

For example, in a completely dark room you cannot see a red ball at all — the ball has not changed, but with no light falling on it there is nothing to reflect to your eye.

Materials are classified by how much light passes through them.

Transparent materials let almost all light through, so you can see clearly through them: clear glass, still water, air.

Translucent materials let some light through but scatter it, so you see only blurred shapes: butter paper, frosted glass, thin cloth, oiled paper.

Opaque materials let no light through at all: wood, metal, cardboard, stone, your own body.

For example, a window pane is transparent, the frosted glass of a bathroom window is translucent, and the wooden door beside it is opaque.

Only opaque objects cast sharp, dark shadows, because only they block the light completely. A translucent object casts a faint, partial shadow, and a transparent one casts almost none.

Note that the same substance can change category with thickness — thick coloured glass may be translucent where a thin sheet is transparent.

Two classic experiments demonstrate rectilinear propagation — the technical name for light travelling in straight lines.

With pipes: look at a lit candle through a straight pipe and you see the flame. Look through a bent pipe and you see nothing, even though the flame is still there. Light will not follow the bend.

With cardboard sheets: take three cardboard sheets with a small hole punched at the same height in each. Line them up so the three holes are exactly in a straight line, place a candle at one end, and you can see the flame through all three. Shift the middle sheet slightly sideways and the flame disappears immediately.

The second experiment is the more convincing, because the only thing that changed was the alignment — nothing was covered.

For example, a torch beam in a dusty room shows a straight visible path, and sunlight entering a dark room through a small gap makes a straight bright streak.

A shadow needs three things together: a source of light, an opaque object to block it, and a screen for the shadow to fall on. Remove any one and there is no shadow.

A shadow is always dark, and it shows only the outline of the object, never its colour or detail — which is why a red ball and a blue ball of the same size cast identical shadows.

The size changes with distance:
- Move the object closer to the light source and the shadow grows larger.
- Move the object closer to the screen, or away from the light, and the shadow grows smaller and sharper.

The shape depends on the direction the light comes from. A cylinder lit from the side casts a rectangular shadow; lit from directly above, it casts a circle.

In real life, your shadow is long in the early morning and evening, when the Sun is low, and short at midday when it is overhead — a shape-and-direction effect you can watch in a single day.