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Propagation of Light - Physics Form 1 Coursework e-Content CDs



By the end of this topic you should be able to:

  • Describe the formation of shadows
  • Describe the formation of eclipses


Rectilinear Propagation of Light

In this demonstration, a ray of light is directed through holes in pieces of cardboard.

What conclusion can you make about the behaviour of light?

Light travels in a straight.

Formation of Shadows

Shadows are formed when light is partially or totally blocked by an object.
An object such as an oiled paper which blocks some rays of light but allows some rays to pass through it forms a partial shadow called penumbra . Such an object is said to be translucent. An object such as cardboard which blocks all the rays of light directed to it and therefore forms a total shadow is said to be opaque. A total shadow is also called the umbra.

An object such as glass which allows most light to pass through and therefore forms no shadow  is said to be transparent

Formation of shadows is another indication that light travels in a straight line.


Formation of Shadows from a point Source

A uniformly dark shadow forms on the screen since all the rays are blocked by the object.

A point source can be obtained by illuminating a small hole on a cardboard with a powerful beam of light from a torch.

Formation of Shadows from an Extended Source

Shadows formed by extended (broad) sources of light have both umbra and penumra.

Estimating the height of a tall building using the shadow of a short tree




Formation of Eclipses

Due movements of the moon, it sometimes comes between the Sun and the earth. When this happens, it blocks the sun's rays and prevents them from reaching part of he earth. We say the earth is eclipsed from the Sun. On other occasions, the moon falls behind the earth; so it (the moon) is eclipsed from the Sun. Consequently, there are two main types of eclipse, namely: solar eclipse and lunar eclipse. These events are demonstarted in the section that follows.

Solar Eclipse (corona type)

Solar eclipse occurs when the moon moves to a position between the Sun and the earth and the three lie in a straight line. When this happens, an observer on the earth may see partial disappearance of the Sun, as at A and C, or total disappearance.

Solar Eclipse (annular type)

Annular eclipse is a special case of solar eclipse which occurs when the moon, revolving in an elliptical orbit, moves slightly farther from the earth but still in the same line with the Sun and the earth. When this happens the umbra of the moon does not reach the earth and only the penumbra of the moon is cast on the earth. The Sun is not fully eclipsed by the moon but appears as a ring as observed from the earth.


Lunar Eclipse

Lunar eclipse occurs when the earth is between the Sun and the moon. The moon therefore receives no light to reflect to the earth; so it cannot be seen.

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