Physics Chp t 17
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Transcript of Physics Chp t 17
Unit 6, Chapter 17
CPO ScienceFoundations of Physics
Unit 6: Light and Optics
17.1 Reflection and Refraction
17.2 Mirrors, Lenses, and Images
17.3 Optical Systems
Chapter 17 Light and Color
Chapter 17 Objectives1. Describe the functions of convex and
concave lenses, a prism, and a flat mirror.
2. Describe how light rays form an image.
3. Calculate the angles of reflection and refraction for a single light ray.
4. Draw the ray diagram for a lens and a mirror showing the object and image.
5. Explain how a fiber-optic circuit acts like a pipe for light.
6. Describe the difference between a real image and a virtual image and give an example of each.
Chapter 17 Vocabulary Terms lens mirror prism optics geometric
optics specular
reflection diffuse converging diverging law of reflection normal line ray diagram magnification object
index of refraction
focal point focal length optical axis light ray magnification critical angle Snell’s law real image virtual image chromatic
aberration refraction fiber optics dispersion
magnifying glass
spherical aberration
reflection diffraction telescope focus total internal
reflection resolution pixel image focal plane thin lens
formula
17.1 Reflection and Refraction
Key Question:How do we describe
the reflection and refraction of light?
*Students read Section 17.1 AFTER Investigation 17.1
17.1 Reflection and Refraction 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.
17.1 Reflection and Refraction
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.
17.1 Reflection and Refraction 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.
17.1 Reflection
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.
17.1 Reflection 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.
Law of Reflection
The angle of incidence equals the angle of reflection.
The incident ray strikes the mirror.
The reflected ray bounces off.
17.1 Law 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.
30o 30o
17.1 Refraction
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.
17.1 RefractionWhen 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.
17.1 Index of refractionThe ability of a material to bend rays of light is
described by the index of refraction (n).
17.1 Snell's law of refraction Snell’s law is the relationship between the
angles of incidence and refraction and the index of refraction of both materials.
ni sin i= nr sin r Index of
refraction of refractive material
Angle of incidence (degrees)
Angle of refraction (degrees)
Index of refraction of
incident material
17.1 Calculate the angle of refraction
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.
17.1 Dispersion and prisms
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.
17.1 Dispersion and prisms
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.
17.2 Mirrors, Lenses, and Images
Key Question:
How does a lens or mirror form an image?
*Students read Section 17.2 AFTER Investigation 17.2
17.2 Mirrors, Lenses, and Images
We see a world of images created on the retina of the eye by the lens in the front of the eye.
17.2 Mirrors, Lenses, and Images
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.
17.2 Mirrors, Lenses, and Images 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.
The virtual image in a flat mirror is created by the eye and brain.
17.2 Mirrors, Lenses, and Images 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.
17.2 The image formed by a 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.
17.2 The image formed by a lens A converging lens can also form a real image. In a real image, light rays from the object
actually come back together.
17.2 Drawing ray diagrams
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:
1. A light ray passing through the center of the lens is not deflected at all (A).
2. A light ray parallel to the axis passes through the far focal point (B).
3. A light ray passing through the near focal point emerges parallel to the axis (C).
17.3 Optical Systems
Key Question:
How are the properties of images determine?
*Students read Section 17.3 AFTER Investigation 17.3
An optical system is a collection of mirrors, lenses, prisms, or other optical elements that performs a useful function with light.
Characteristics of optical systems are:— The location, type, and magnification of the
image.— The amount of light that is collected.— The accuracy of the image in terms of
sharpness, color, and distortion.— The ability to change the image, like a
telephoto lens on a camera.— The ability to record the image on film or
electronically.
17.3 Optical Systems
17.3 The sharpness of an image Defects in the image are called aberrations
and can come from several sources.
— Chromatic aberration is caused by dispersion, when different colors focus at different distances from the lens.
17.3 The sharpness of an image
— Spherical aberration causes a blurry image because light rays farther from the axis focus to a different point than rays near the axis.
17.3 The sharpness of an image
— Diffraction causes a point on an object to focus as a series of concentric rings around a bright spot.
17.3 Thin lens formula The thin lens formula is a mathematical way to
do ray diagrams with algebra instead of drawing lines on graph paper.
1 + 1 = 1do di
df
focallength (cm)
Image distance(cm)
Objectdistance
(cm)
17.3 Use the thin lens formula
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.
17.3 Image relay A technique known as image relay is used to
analyze an optical system made of two or more lenses.
Application: The Telescope