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Science in Focus: Shedding Light: Highlights

Workshop 1

shadows: umbra and penumbra | absorption and transmisson

More About Shadows: Umbra and Penumbra


An umbra is a total shadow. It is formed when an object blocks all of the light from a light source. Most shadows, however, have a darker part on the inside and a lighter part around the edges. This lighter part is the penumbra. The penumbra can be formed when light from one source is blocked with light from another source filling in some of the shadow. A penumbra can also be formed when light from a broad source is only partially blocked.



Photo 2

We can use the photon model to explain the umbra/penumbra effect (see below). Penumbra light travels in straight lines. Here we will set up two light sources. Let's use two light bulbs, an object to block the light (like a book), and a screen upon which to view the shadow. The top and bottom regions of the screen in our example receive photons from both bulbs, and therefore are fully illuminated. The middle region represents the area in which photons from both bulbs are blocked by the book. Since no light reaches the screen in this region, a total shadow, or umbra, is formed. In the region directly above the umbra, photons from Bulb 1 reach the screen, but photons from Bulb 2 are blocked by the book. This produces a partial shadow, or penumbra. In the region directly below the umbra, photons from Bulb 2 reach the screen, but photons from Bulb 1 are blocked by the book. Hence another penumbra is formed.

Photo 3

An exciting example of the umbra/penumbra effect occurs during a solar eclipse. The Moon passes between the Earth and the Sun, and forms a shadow on the Earth. Because the Sun is not a point source of light, there will be both umbra and penumbra shadows produced on the Earth. If you are standing in the umbra part of the shadow, you will experience a brief period of total darkness during the day. If you are standing in the penumbra, you will experience a partial eclipse, with the light from the sun dimmer than is typical for that time of day.


Absorption and Transmission of Photons

When a visible light photon hits an object, the energy of that photon can be absorbed by an electron and that photon no longer exists. The electron can then reemit a photon with the same energy -- this is reflection. However, the electron also could reemit photons of lower energy. These lower energy photons are called infrared photons and this form of radiation is not seen by our eyes, but is felt as heat. Since the energy of the photon is conserved, there are more infrared photons emitted then visible light photons absorbed.

Some materials are transparent to radiation. For example clear glass is relatively transparent to visible light. (The actual transmission of light through glass is a complex process.) However, a brick wall is not transparent in visible light, but is transparent to radio wave energy (made up of photons). Radio photons can pass through a brick wall because their energy is so low that they are not absorbed by the electrons.



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