Light, Images, and Shadows Chapter 27.4—27.8 Notes

Light and Transparent Materials Remember that light is the energy carried in an electromagnetic wave, and it is generated by vibrating electric charges When light strikes matter (anything that has mass and volume), electrons in the matter are forced to vibrate How a material responds to light depends on the frequency of the light and the natural frequency of the electrons in the material Light will pass through materials whose atoms absorb the energy and immediately reemit it as light

Light and Transparent Materials Materials that transmit light are transparent—glass and water are examples of transparent materials In a vacuum, recall that light travels at 300,000,000 m/s In water, light travels at 75% of its speed in a vacuum In glass, light travels at about 67% of its speed in a vacuum In diamond, light can only travel at about 40% of its speed in a vacuum

Light and Opaque Materials Materials that absorb light without reemission, and therefore do not allow any light to pass through them, are called opaque Wood, stone, and people are examples of opaque materials In opaque materials, when light energy comes into contact with the material, the energy is turned into random kinetic energy Internal energy for the object The material becomes slightly warmer

What We Can See We can only see light that is transmitted or reflected—not absorbed Therefore, we only see light that passes through transparent materials, not opaque materials There are two types of reflection: specular reflection and diffuse reflection Specular reflection is the reflection of light off of a smooth surface; light keeps a similar pattern Diffuse reflection is the reflection of light off of a rough surface; light is scattered Specular Diffuse

Images Images we see are dependent on the path light takes If light can take a similar path, then the image will be preserved Transparent materials allows most light to pass through and a clear image will be seen If light is scattered (sent in all directions), then no image will be seen, but light can still be seen Translucent materials (like a cloudy piece of plastic) will allow some light to pass through, but no image will be seen Opaque materials allow no light to pass through and no image will be seen

Depth Perception and 3-D Effect Depth perception is dependent on the fact that you have two eyes—you receive slightly different visions from each eye because the light is received from different angles Your brain merges the two visions into one 3-D vision If someone has lost vision in one eye, then he or she has lost their depth perception For example, if the left eye sees the left side of a bee, and the right eye sees the right side of a bee, then the bee will appear to be right in front of you If both eyes can only see one side of the bee, then the bee must be far away

Shadows A thin beam of light is called a ray—any beam of light can be thought of as a bundle of rays Shadows are formed in areas where light cannot reach it because an opaque object is in the light’s path Two parts of a shadow: the umbra and the penumbra The umbra is the sharp, black center of the shadow The penumbra is the fuzzy, gray outer portion of the shadow

Shadows cause Eclipses When the Earth’s shadow falls on the moon, we will observe a lunar eclipse When the moon’s shadow falls on the Earth, we will observe a solar eclipse If you stand in the umbra of the shadow, you will see the entire eclipse If you stand in the penumbra of the shadow, you will only see a partial eclipse

Lunar Eclipse: Earth falls between the moon and the sun

Solar Eclipse: moon falls between the Earth and the sun NASA Eclipses