Chapter 18: Refraction and Lenses Glencoe Physics Chapter 18: Refraction and Lenses

Refraction Refraction of light involves the bending of light waves Laws of Refraction 1. As light enters a medium of greater optical density, the refracted ray is bent toward the normal to the surface at the point of entry. 2. As light enters a medium of lesser optical density, the refracted ray is bent away from the normal to the surface at the point of entry

Index of Refraction The constant known as index of refraction, "n”, depends on the speed of light through a substance. It has no units. It is the ratio of the speed of light in a vacuum to the speed of light through the substance. Page 486 shows a table of indexes of refraction of various materials.

Index of Refraction and the Speed of Light Refraction occurs because the speed of light changes with media. The index of refraction is a measure of the amount of bend in light for a particular substance c n = index of refraction n = ------ vs = speed of light in substance Vs c = speed of light in vacuum speed of light in a vacuum = 3 X 108 m/s

Sample Problem The index of refraction of water is 1.33. Calculate the speed of light through the water.

Snell's Law Snell was a Dutch scientist who studied the refraction of light and determined a consistent behavior. Snell's Law - Light refracts in such a way that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given material.

Mathematical Expression of Snell's Law sin i n = index of refraction n = ----------- i = angle of incidence sin r r = angle of refraction Light is incident on a quartz crystal at an angle of 300. Find the angle of refraction of the light if the light is propagating from air into the quartz. Assignment Page 487, 1-5

Thin Lenses A lens is a transparent object having non-parallel sides. The two major types of lenses are: convex lens - converging lens - causes parallel rays to converge - thick in the middle concave lens - diverging lens - causes parallel rays to diverge - thin in the middle

Types of Converging and Diverging Lenses 1. double concave 2. plano concave 3. convexo concave 4. concavo convex 5. plano convex 6. double convex

Converging Lenses Draw a convex lens and label F, 2F, di, do, and the image if the object is located beyond 2F. ------------------------------------------------axis

Lens Equations The same equations which are used with mirrors are used with lenses.

Virtual Images Formed by a Converging Lens Describe the image formed by a convex lens if the object is located at F. Draw a diagram in your description. --------------------------------------------------------axis Image is ......... Draw a diagram of an object located between F and a convex lens. Describe the image.

Sample Problem An object 2cm tall is located 12cm from a convex lens of 18cm focal length. Draw and locate the image and find the size. ------------------------------------------------------axis

Virtual Images Formed by Concave Lenses A divergent or concave lens is thin in the middle. Images formed by diverging lenses are always virtual and upright. The following diagram will illustrate this fact. -------------------------------------------------------axis Assignment: Page 496, 15-19, 497, 20-24

Optical Phenomena 1. Total Internal Reflection - This effect is actually caused by refraction. Light incident upon a transparent object is refracted internally and the result is an apparent reflection of light toward the source. The angle of incidence which causes this phenomenon is known as the critical angle. Examples: The legs of someone sitting on the side of a pool appear to be higher than the person to someone under the water looking upward.

Optical Phenomena 2. The Puddle Effect - Air above a hot road surface is more dense both physically and optically. Light incident on the area of hot air will refract in a manner consistent with reflection from a smooth surface;i.e. , a pool of water. Example: While driving down a dry road in warm weather, a mirage of a pool of water will appear ahead of a driver. The "pool" will eventually disappear as the driver gets closer to the mirage. Also desert mirages.

Optical Phenomena 3. Why is the sky blue? You may think of the atmosphere as a giant prism. As light is incident upon the atmosphere above us, the blue portion of the visible spectrum is scattered across the sky, thus creating a blue sky. 4. Why is the sunset red? As the sun approaches the horizon, the red portion of the visible spectrum is refracted toward the earth as the blue portion of the spectrum is refracted away from the earth's surface. This accounts for the red and orange light surrounding a sunrise.

Optical Phenomena 5. What causes a rainbow? As light passes through a prism the result is a colored spectrum. Rain drops assume a prismatic shape as gravity acts upon them during their fall to the earth's surface. As light passes through the drops, it is refracted in a manner consistent with a prism. This results in a rainbow.

Optical Devices The most common types of optical devices involve human vision. The two most prominent problems are nearsightedness and farsightedness. A nearsighted person cannot see distant objects. This can be corrected by using concave lenses. The concave lens focuses the image on the retina instead of in front of the retina.

Refracting Telescopes The image seen in a refracting telescope is virtual, enlarged, and inverted. The telescope is a system of two convex lenses. Construct a diagram which illustrates the optical system of a refracting telescope. Your diagram should include the object, a real image, and a virtual image. -------------------------------------------------------axis

Chromatic Aberration In many optical devices, refraction occurs at the edge of a lens. This refraction creates a spectrum of colors. This is known as chromatic aberration. Chromatic aberration cannot be eliminated but the lens can be coated around the outer edge. This coating has a different index of refraction which interferes with the spectrum of color. This will correct the chromatic aberration and the corrected lens is an achromatic lens.