17.3 Optical Systems 1

Chapter 17 Objectives Describe the functions of convex and concave lenses, a prism, and a flat mirror. Describe how light rays form an image. Calculate the angles of reflection and refraction for a single light ray. Draw the ray diagram for a lens and a mirror showing the object and image. Explain how a fiber-optic circuit acts like a pipe for light. Describe the difference between a real image and a virtual image and give an example of each. 2

Chapter 17 Vocabulary Terms angle of refraction chromatic aberration converging lens critical angle diffuse reflection dispersion diverging lens eyepiece fiber optics focal length focal plane focal point focus geometric optics image image relay incident ray index of refraction law of reflection lens magnification magnifying glass mirror normal line object objective optical axis optics prism ray diagram real image reflected ray refracting telescope Snell’s law specular reflection spherical aberration telescope thin lens formula total internal reflection virtual

Inv 17.3 Optical Systems Investigation Key Question: How are the properties of images determine? 4

17.3 Optical Systems 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 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 Thin lens formula 1 + 1 = 1 do di df The thin lens formula is a mathematical way to do ray diagrams with algebra instead of drawing lines on graph paper. 1 + 1 = 1 do di df Object distance (cm) focal length (cm) Image distance (cm)

Locating an image You are asked for the image distance. 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. You are asked for the image distance. You are given focal length and object distance. Use thin lens formula 1 + 1 = 1 do di df Solve for 1 = 1 - 1 = 3 - 2 = 1 di = 12 cm di 4 6 12 12 12

17.3 Changing the size of an image A technique known as image relay is used to analyze an optical system made of two or more lenses.

17.3 Recording images There are two basic techniques for recording images. Film records an image by using special inks that respond to light. A digital camera uses a tiny sensor called a CCD.

17.3 Recording images There are separate light sensors for red light, blue light, and green light. A color image is recorded as a table of numbers. Each point on the image has three numbers corresponding to the amount of red light, blue light, and green light.

17.3 Recording images The resolution of a digital camera is the number of points, called pixels, that can be recorded by the CCD. A 2-megapixel camera stores 2 million pixels per image. Since each pixel is three numbers, a 2-megapixel image requires 6 million stored numbers.

17.3 The Telescope When people think of a telescope, most of them think of a refracting telescope. An astronomical refracting telescope is constructed of two converging lenses with different focal lengths. The lens with the longest focal length is called the objective and the shorter-focal-length lens is the eyepiece.

17.3 The Telescope The image from this refracting telescope is inverted which is usually fine for looking at objects in space.

17.3 The Telescope The design of the terrestrial telescope sets the lenses a distance apart equal to the sum of their focal lengths. This design aids viewing animals or birds right side up.

Newtonian Reflecting Telescope The most successful reflecting- telescope design is called a Newtonian telescope, after Sir Isaac Newton, who designed and built the first one. The larger the diameter of the objective of a telescope, the more light it can gather to form an image. The Hubble telescope is unique because it orbits well above Earth’s atmosphere so there is no distortion from atmospheric refraction.