In this topic you will learn about how light travels and how it affects matter.

Light is a form of energy. Sources of light include the Sun, the stars, light bulbs, and candles. Light travels in straight lines from its source. Light passes through some materials but not through others. For example, light passes through a glass window but not through a brick wall. A brick is opaque. Opaque materials do not allow any light to pass through them. Light rays that do not pass through a material reflect off it. Reflect means to bounce off a surface.

When light rays reflect off a surface, they change the direction they are traveling in. You see objects because light reflected from the object enters your eyes. A mirror is very smooth and shiny. Light rays are reflected from a mirror. When you stand in front of a mirror, you see a face just like yours. This is your reflection. Light rays that are reflected from surfaces that are dull or rough are reflected in many different directions. You don't see a reflection when light is reflected in many different directions.

Light rays can change direction when they move from one material to another. For example, when light rays pass from air to water or from water to air, they bend. A spoon in a glass of water looks bent because the light rays are bent.

Light can change matter. A camera uses light to make photographs. The film inside the camera is coated with chemicals that change when light shines on them. Powerful beams of light, called lasers, can be strong enough to cut steel. Lasers are also used to read special codes of information on items at the supermarket.