Chapter 19 – Optics Jennie L. Borders

Section 19.1 - Mirrors Optics is the study of how mirrors and lenses form images. A ray diagram shows how rays change direction when they strike mirrors and pass through lenses. The incoming ray, called the incident ray, approaches the mirror.

The Law of Reflection The angle of incidence is the angle the incident ray makes with a line drawn perpendicular to the surface of the mirror. The angle of reflection is the angle the reflected ray makes with the perpendicular line. The law of reflection states that the angle of reflection is equal to the angle of incidence.

Ray Diagram surface normal same exit ray angle incident ray reflected ray incident ray

Plane Mirrors A mirror with a flat shiny surface is a plane mirror. When you look into a plane mirror, you see your reversed reflection, a right-left reversed image of yourself. An image is a copy of an object formed by rays of light.

Plane Mirrors To produce your image in a mirror, rays of light strike you and reflect. These reflected rays then strike the mirror and are reflected into your eyes. Your image appears the same distance behind the mirror as you are in front, and the image is right side up.

Plane Mirrors A plane mirror always produces a virtual image. Although you can see a virtual image, this type of image cannot be projected onto any surface. A virtual image is a copy of an object formed at the location from which the light rays appear to come.

Concave Mirrors When the inside surface of a curved mirror is the reflecting surface, the mirror is a concave mirror. The curvature of the reflecting surface causes the rays to come together. The point at which the light rays meet is called the focal point.

Concave Mirrors A real image is the copy of an object formed at the point where light rays actually meet. Unlike a virtual image, a real image can be viewed on a surface such as a screen. Concave mirrors can form either real or virtual images.

Real Image When the object is farther from the mirror than the focal point, the reflected rays meet in front of the mirror, making a real image.

Virtual Image When the object is closer to the mirror than the focal point is the reflected rays spread out and appear to come from behind the mirror, making a virtual image.

Convex Mirrors When the outside surface of a curved mirror is the reflecting surface, the mirror is a convex mirror. The curvature of the convex mirror causes the reflected rays to spread out.

Convex Mirrors Convex mirrors always cause light rays to spread out and can only form virtual images. The image formed by a convex mirror is always upright and smaller than the object.

Section 19.1 Assessment How is the angle of incidence of a light ray related to the angle of reflection? What type of image does a plane mirror form? What types of image can be produced by a concave mirror? A convex mirror? How are real images different from virtual images?

Section 19.1 Assessment If you place an object 10 cm from a particular concave mirror, a virtual image forms behind the mirror. What can you infer about the focal point of the mirror? If you look inside the bowl of a shiny metal spoon, your image is upside down. If you look at the outside of the spoon’s bowl, your image is right side up. Explain.

Section 19.2 - Lenses Light usually travels in straight lines. When light enters a new medium at an angle, the change in speed causes the light to bend, or refract.

Index of Refraction How much the speed of a light ray slows as it enters a new material depends on the material’s index of refraction. The index of refraction for a material is the ratio of the speed of light in a vacuum to the speed of light in the material. A material with a low index of refraction causes light to slow and refract very little.

Index of Refraction

Lenses A lens is an object made of transparent material that has one or two curved surfaces to refract light.

Concave Lenses A concave lens (diverging lens) is curved inward at the center and is thickest at the outside edges. Concave lenses cause incoming parallel rays to spread out, or diverge.

Concave Lenses Concave lenses always cause light rays to spread out and can only form virtual images. The image formed by a concave lens is always smaller than the object.

Convex Lenses A convex lens (converging lens) is curved outward at the center and is thinnest at the outer edges. Convex lenses cause incoming parallel rays to come together, or converge.

Convex Lenses The converging rays meet at a single point, the focal point, on one side of the lens opposite the object. Convex lenses form either real or virtual images. Virtual image

Convex Lenses Real image

Concave vs. Convex

Fiber Optics Light rays are generally unable to exit through the sides of the curving fiber optic strands. Because of this, fiber optics are very useful for carrying information in the form of light.

Total Internal Reflection The critical angle is the angle of incidence that produces an angle of refraction of 90 degrees. Total internal reflection is the complete reflection of a light ray back into its original medium.

Total Internal Reflection Materials that have small critical angles are likely to cause most of the light entering them to be totally internally reflected. Such materials include diamond and the type of glass used in fiber optics.

Section 19.2 Assessment What causes light rays to bend? Why can concave lenses only form one type of image? What type of images are formed by concave lenses? By convex lenses? How is a convex lens different from a concave lens? How are they the same?