Why Shadows Change Size and How a Pinhole Camera Works

A shadow forms when an opaque object blocks light from a source, creating a dark area on a screen behind it. To form a shadow, three things are needed: a source of light (like the sun or a lamp), an opaque object (such as your hand), and a screen (like a wall or the ground) where the shadow appears. The size of the shadow depends on the distance between the object and the light source. If the object is moved closer to the light source, its shadow becomes larger and more blurred. If the object is moved farther away from the light source, the shadow becomes smaller and sharper. For example, if you hold your hand close to a torch and shine it on a wall, the shadow on the wall will appear big and fuzzy. If you move your hand farther from the torch and closer to the wall, the shadow becomes smaller and clearer. Many students think the shadow is always the same size as the object, but its size changes with distance from the light source.

When an extended (broad) source of light is used, two distinct regions are formed in the shadow: the umbra and the penumbra. The umbra is the darkest part of the shadow where all the light from the source is blocked by the object. The penumbra is the lighter, outer part of the shadow where only a part of the light is blocked, and some light still reaches the screen. If a point source of light (a very small source) is used, only the umbra is formed because all the light is either completely blocked or not blocked at all, so there is no penumbra. For example, when you place a cricket ball in front of a candle (a small point source), you see a sharp, dark shadow (umbra) on the wall. But if you use a tube light (an extended source), the shadow has a dark central part (umbra) and a lighter edge (penumbra). Students often confuse the penumbra as a faint shadow, but it is actually formed due to partial blocking of light from an extended source.

A pinhole camera forms an image by allowing light rays from an object to pass through a tiny hole and project onto a screen inside a dark box. The camera is made by making a small hole in one side of a box and placing a translucent screen on the opposite side. Light travels in straight lines, so rays from the top of the object pass through the pinhole and reach the bottom of the screen, while rays from the bottom reach the top. This causes the image to be real and inverted (upside down). The image is sharp only when the pinhole is very small; if the hole is made larger, the rays from different parts of the object overlap, making the image blurred. For example, you can make a pinhole camera using a shoebox, aluminium foil with a pinhole, and tracing paper as a screen. If you point it at a tree outside, you will see an upside-down image of the tree on the screen. Many students think the image should be upright, but the straight-line travel of light causes the inversion.

A solar eclipse occurs when the moon comes between the sun and the earth, casting a shadow on the earth and blocking sunlight in some areas. This happens only on a new moon day. During a solar eclipse, the umbra of the moon's shadow creates a region of total darkness, while the penumbra creates partial darkness. A lunar eclipse occurs when the earth comes between the sun and the moon, causing the earth's shadow to fall on the moon. This happens only on a full moon day. In a lunar eclipse, the moon passes through the earth's umbra and penumbra, causing it to appear darkened. For example, during a solar eclipse, people in the path of the moon's umbra see the sun completely covered, while those in the penumbra see a partial eclipse. Many students think eclipses can happen any time, but solar eclipses only occur on new moon days and lunar eclipses only on full moon days.