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# ZingPath: Properties of Light

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## Properties of Light

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### Lesson Focus

Physical Science

Learners investigate shadow formation by using various light sources.

### Now You Know

After completing this tutorial, you will be able to complete the following:

• Explain that the clarity and size of a shadow changes when the distance between the light source and the object is changed.
• Explain that when light hits an object from a nonpoint source, it forms partial shadows whose densities are different from each other.
• Define the darker part of a shadow from a nonpoint light source as the umbra and the light part as the penumbra.

### Everything You'll Have Covered

Shadows are formed when objects block oncoming light. In this Activity Object, learners discover that several factors affect the size and clarity of shadows, including distance from the light source and the size and type of light source. They investigate shadows formed by point sources and nonpoint sources and learn that when a shadow is formed by a nonpoint source, an umbra and penumbra are created.

A point light source is single, very small one that gives off light in all directions, such as a lit candle. It is a source of light which has no screen or lamp shade to restrict the direction in which the light can travel. As light rays travel from the point source and strike an opaque object, the light rays do not travel through the object. There is an area behind the object where the light rays do not reach, and this is where the dark shadow forms. As the distance between the object and the light source changes, the spread of the light rays changes. When the object is far from the point source, the light rays coming from the source do not spread much. As the object blocks light rays being emitted at smaller angles, a smaller, sharper shadow forms behind it.

A nonpoint light source is one in which light emanates from diffuse sources, such as the Sun or a light bulb with a reflective cone surrounding it. When using a nonpoint light source, two shadows form behind an object with one darker than the other. All points on a nonpoint light source behave like point sources. Light rays do not reach some areas behind the object but reach other areas. The shadow that forms is darker in the middle and not as dark around the edges. The dark middle is called the umbra, and it is where no light reaches. The area that is not as dark is called the penumbra, and it is where some light reaches. The size of the nonpoint light source affects the size of the umbra and penumbra. When the size of the nonpoint source increases, the area of the umbra decreases, and the area of the penumbra increases. Increasing the distance between the object and nonpoint source results in the areas of both the umbra and penumbra decreasing.

### Tutorial Details

 Approximate Time 20 Minutes Pre-requisite Concepts light, light rays Course Physical Science Type of Tutorial Experiment Key Vocabulary eclipse, experiment, light