Unit 6, Chapter 17. CPO Science Foundations of Physics. Unit 6: Light and Optics. Chapter 17 Light and Color. 17.1 Reflection and Refraction 17.2 Mirrors, Lenses, and Images 17.3 Optical Systems. Chapter 17 Objectives.
Foundations of Physics
Chapter 17 Light and Color
How do we describe the reflection and refraction of light?
*Students read Section 17.1 AFTER Investigation 17.1
The overall study of how light behaves is called optics.
The branch of optics that focuses on the creation of images is called geometric optics, because it is based on relationships between angles and lines that describe light rays.
A lens is an optical device that is used to bend light in a specific way.
A converging lens bends light so that the light rays come together to a point.
A diverging lens bends light so it spreads light apart instead of coming together.
Mirrors reflect light and allow us to see ourselves.
A prism is another optical device that can cause light to change directions.
A prism is a solid piece of glass with flat polished surfaces.
Images appear in mirrors because of how light is reflected by mirrors.
The incident ray follows the light falling onto the mirror.
The reflected ray follows the light bouncing off the mirror.
In specular reflection each incident ray bounces off in a single direction.
A surface that is not shiny creates diffuse reflection.
In diffuse reflection, a single ray of light scatters into many directions.
The incident ray strikes the mirror.
The reflected ray bounces off.
The angle of incidence equals the angle of reflection.
A light ray is incident on a plane mirror with a 30 degree angle of incidence.
Sketch the incident and reflected rays and determine the angle of reflection.
Light rays may bend as they cross a boundary from one material to another, like from air to water.
This bending of light rays is known as refraction.
The light rays from the straw are refracted (or bent) when they cross from water back into air before reaching your eyes.
When a ray of light crosses from one material to another, the amount it bends depends on the difference in index of refraction between the two materials.
The ability of a material to bend rays of light is described by the index of refraction (n).
Angle of refraction
Angle of incidence
ni sinQi= nr sin Qr
Index of refraction of incident material
Index of refraction of refractive material
A ray of light traveling through air is incident on a smooth surface of water at an angle of 30° to the normal.
Calculate the angle of refraction for the ray as it enters the water.
When white light passes through a glass prism, blue is bent more than red.
Colors between blue and red are bent proportional to their position in the spectrum.
The variation in refractive index with color is called dispersion.
A rainbow is an example of dispersion in nature.
Tiny rain droplets act as prisms separating the colors in the white light rays from the sun.
How does a lens or mirror form an image?
*Students read Section 17.2 AFTER Investigation 17.2
We see a world of images created on the retina of the eye by the lens in the front of the eye.
Objects are real physical things that give off or reflect light rays.
Images are “pictures” of objects that are formed in space where light rays meet.
The most common image we see every day is our own reflection in a mirror.
The image in a mirror is called a virtual image because the light rays do not actually come together.
Light rays that enter a converging lens parallel to its axis bend to meet at a point called the focal point.
The distance from the center of the lens to the focal point is called the focal length.
The optical axis usually goes through the center of the lens.
A lens can form a virtual image just as a mirror does.
Rays from the same point on an object are bent by the lens so that they appear to come from a much larger object.
A converging lens can also form a real image.
In a real image, light rays from the object actuallycome back together.
A ray diagram is the best way to understand what type of image is formed by a lens, and whether the image is magnified or inverted.
These three rays follow the rules for how light rays are bent by the lens:
A light ray passing through the center of the lens is not deflected at all (A).
A light ray parallel to the axis passes through the far focal point (B).
A light ray passing through the near focal point emerges parallel to the axis (C).
How are the properties of images determine?
*Students read Section 17.3 AFTER Investigation 17.3
1 + 1 = 1
Calculate the location of the image if the object is 6 cm in front of a converging lens with a focal length of 4 cm.
A technique known as image relay is used to analyze an optical system made of two or more lenses.