Light Light and Matter Prisms Colors Lenses Light

Light

Light and Matter Prisms Colors Lenses

Light and Matter What you see depends on the amount of light in the room and the color of the objects. For you to see an object, it must reflect some light back to your eyes. Remember reflection occurs when a light wave strikes an object and bounces off. Objects can absorb light, reflect light, and transmit light (allow light to pass through them). The type of matter in an object determines the amount of light it absorbs, reflects, and transmits.

Light and Matter Opaque material only absorbs and reflects light (no light passes through it). As a result, you cannot see the candle inside.

Light and Matter Materials that allow some light to pass through them are described as translucent. You cannot see clearly through translucent materials.

Light and Matter Transparent materials transmit almost all the light striking them, so you can see objects clearly through them. Only a small amount of light is absorbed and reflected.

Prisms Refraction causes a prism to separate a beam of white light into different colors. Remember refraction is caused by a change in the speed of a wave as it passes from one material to another. How does the bending of light create these colors? It occurs because the amount of bending usually depends on the wavelength of the light.

Prisms Wavelengths of visible light range from the longer red waves to the shorter violet waves. White light, such as sunlight, is made up of this whole range of wavelengths. The animation shows what happens when white light passes through a prism.

Prisms The triangular prism refracts the light twice – once when it enters the prism and again when it leaves the prism and reenters the air. Because the longer wavelengths of light are refracted less than the shorter wavelengths are, red light is bent the least. As a result of these different amounts of bending, the different colors are separated when they emerge from the prism. Which color of light would you expect to bend the most?

Rainbows Does the light leaving the prism remind you of a rainbow? Like prisms, rain droplets also refract light. The refraction of the different wavelengths can cause white light from the Sun to separate into the individual colors of visible light.

Rainbows In a rainbow, the human eye usually can distinguish only about seven colors clearly. In order of decreasing wavelength, these colors are red, orange, yellow, green, blue, indigo, and violet.

Colors Why do some apples appear red, while others look green or yellow? An object’s color depends on the wavelengths of light it reflects. Remember white light is a blend of all colors of visible light.

Colors When a red apple is struck by white light, it reflects red light back to your eyes and absorbs all of the other colors. The figure shows white light striking a green leaf. Only the green light is reflected back to your eyes.

Colors Although some objects appear to be black, black isn’t a color that is present in visible light. Objects that appear black absorb all colors of light and reflect little or no light back to your eye. White objects appear to be white because they reflect all colors of visible light.

Lenses What do your eyes have in common with cameras, eyeglasses, and microscopes? Each of these things contains at least one lens. A lens is a transparent material with at least one curved surface that causes light rays to bend, or refract, as they pass through. The image that a lens forms depends on the shape of the lens. Lenses can be convex or concave.

Convex Lenses Convex lenses are thicker in the middle than at the edges. A convex lens focuses light at a focal point. The focal length of the lens depends on the shape of the lens. If the sides are less curved, light rays are bent less. As a result, lenses with flatter sides have longer focal lengths.

Forming Images with a Convex Lens The type of image a convex lens forms depends on where the object is relative to the focal point of the lens. When the candle is more than two focal lengths away from the lens, its image is real, reduced, and upside down. A real image is an image formed by light rays that converge to pass through the place where the image is located.

Forming Images with a Convex Lens When the candle is between one and two focal lengths from the lens, its image is real, enlarged, and upside down.

Forming Images with a Convex Lens When the candle is less than one focal length from the lens, its image is virtual, enlarged, and upright. A virtual image is an image formed by diverging light rays that is perceived by the brain, even though no actual light rays pass through the place where the image seems to be located.

Concave Lenses A concave lens is thinner in the middle and thicker on the edges. Light rays that pass through a concave lens bend outward away from the optical axis. The image formed is always virtual, upright, and smaller than the actual object is. Concave lenses are used in some types of eyeglasses and some telescopes.

Lenses and Eyesight Light enters your eye through a transparent covering on your eyeball called the cornea. The cornea causes light rays to bend so that they converge. The light then passes through an opening called the pupil. Behind the pupil is a flexible convex lens. The lens helps focus light rays so that a sharp image is formed on your retina. The retina is the inner lining of your eye. It has cells that convert the light image into electrical signals, which are then carried along the optic nerve to your brain to be interpreted.