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When light waves, which travel in straight lines, encounter any substance, they are either reflected, absorbed, transmitted, or refracted. This is illustrated in figure 2-2. Those substances that transmit almost all the light waves falling upon them are said to be transparent. A transparent substance is one through which you can see clearly.

Clear glass is transparent because it transmits light rays without diffusing them (view A of figure 2-3). There is no substance known that is perfectly transparent, but many substances are nearly so. Substances through which some light rays can pass, but through which objects cannot be seen clearly because the rays are diffused, are called translucent (view B of figure 2-3). The frosted glass of a light bulb and a piece of oiled paper are examples of translucent materials. Those substances that are unable to transmit any light rays are called opaque (view C of figure 2-3). Opaque substances either reflect or absorb all the light rays that fall upon them.

Figure 2-2. - Light waves reflected, absorbed, and transmitted.

Figure 2-3. - Substances: A. Transparent; B. Translucent; and C. Opaque.

All substances that are not light sources are visible only because they reflect all or some part of the light reaching them from some luminous source.

Examples of luminous sources include the sun, a gas flame, and an electric light filament, because they are sources of light energy. If light is neither transmitted nor reflected, it is absorbed or taken up by the medium. When light strikes a substance, some absorption and some reflection always take place. No substance completely transmits, reflects, or absorbs all the light rays that reach its surface.

Q.3 When light waves encounter any substance, what four things can happen?
Q.4 A substance that transmits almost all of the light waves falling upon it is known as what type of substance?
Q.5 A substance that is unable to transmit any light waves is known as what type of substance?

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