Light in a Newtonian view Chapter 16

Introducing: light Light is the most important source of information for humans Concept of light rays - there are received and NOT emitted by eyes (seeing is a passive and not an active process) - traveling in straight line - does not necessitate a medium to propagate - it dissipates when traveling in a medium - there are mediums in which they cannot propagate (opaque objects) - it was believed to be tinny particles that travel with very high speed - different in many aspects from sound Light sources: high temperature objects or gases, excited matter point-like (a tiny light-bulb), (a) extended (a neon tube) (b) (each point of it a point-like source) directed sources (reflectors, lasers)

Shadows Consequence of - straight light rays - opaque objects shadows produced by different type of sources point like and directed sources produce clear shadows! extended objects: shadows with diffuse edges concept of umbra and penumbra for shadows of extended objects (penumbra is not due to diffraction, but due to extended sources) eclipses: the most spectacular shadows

The simplest camera: the pinhole camera A camera without lenses a closed box + a hole on one wall + film based on the light rays straight propagation principle very simple (figure) - big hole: each part of the film receives light from many parts of the vase and flowers --> no image recorded - smaller hole: the screen restricts the light so that each part of the film receives light from a small portion of the scene - pinhole: results a sharper but dimmer image (light intensity strongly reduced)

Speed of light extremely large, early attempts to measure the speed of light fail light considered to have infinite speed to measure speed of light easily we need: - very strong sources - long distances - measuring small time intervals wiser approach: Hippolyte Fizeau (1849) speed of light in vacuum: 299,792,458 m/s (~ 3x10 8 m/s) speed of light smaller in air or fluids universal constant in vacuum: does not depend on the relative speed of the source and observer--> special relativity

Colors Color adds beauty to our lives understanding colors involves: physics, chemistry, physiology and psychology Visible light is an electromagnetic wave with frequencies between: Hz The color of the light is determined by the frequencies of the light rays entering our eyes the colors we perceive from objects are determined by two factors: the color present in the illuminating light and the colors reflected by the object Different frequency light rays are interpreted in different ways by our brain, creating the sensation of colors brain has an additive effect: different frequency light rays combine in our eyes, and create sensation of new colors a combination of light beams that produces most of the colors: red, green and blue complimentary colors: combined produce white why the Sun is yellow and the sky blue?

REFLECTION OF LIGHT Some things are visible because they are light sources things can be visible if they reflect light reflection from rough surfaces: diffuse reflection reflection from smooth surfaces: clear reflection laws of reflection: 1.The angle of reflection is equal to the angle of incidence 2. The reflected ray lies in the same plane as the normal and incident ray

Flat Mirrors A smooth reflecting plane surface image of objects produced behind the mirror same size, and same distance from the mirror image has a definite location in space determining the position of the image - seeing is a passive activity - our eye-brain system records only the direction from which the light arrives main method: determining where the reflected light rays coming from different points of the object intersect if the reflected light rays really intersect --> real image if only the inverse direction continuation of light rays intersect -->virtual image

Multiple reflections Reflection from two or more mirrors Used in many devices - Optical illusions - Kaleidoscopes - Periscopes - Retroreflectors

Curved mirrors Concave or Convex mirrors (slice of the surface of a sphere) produce: distorsion, magnification, wide view …. Characteristic points, lines and distances: - center of the sphere (C) - optic axis (the line passing through C and center of the mirror) - focal point (halfway between the mirror and C), light rays coming parallel with the optical axis intersect here - focal distance: length of F-M

Images Produced by Curved Mirrors Concave mirror: two different type of image - for an object at large distance: inverted and reduced in size (real image) - if the object is between C and the mirror: magnified, erect image (virtual image) essential difference between real and virtual images: whether the light actually comes from the image location or only appears to come from there - real image can be projected on a screen - virtual images cannot be projected on a screen Convex mirror: always form reduced size and erect virtual image

Principles for Locating the Image We can predict the position and size of the image by geometrical construction We look at a few special rays, and use the principle: All light rays starting from a given point of the object, are converging in one point (the image point) Consequence: Following two rays are sufficient to construct the image The path of three light rays are easy to construct (we use these) 1. along radius (coming back on itself) 2.parallel to optical axis (through focal point) 3. through focal point (parallel to optic axis ) Additional principle: the image of a point on the optic axis is on the optic axis

Constructing the Image for Curved Mirrors We use the previous principles, and special light rays Concave mirror, virtual image Concave mirror, real image Convex mirror, virtual image Formula f: focal distance i: distance of image o: distance of objects (distances measured from center of mirror) f>0: for concave f<0: for convex i>0 for real image i<0 for virtual image (same for : o) m: magnification m>0 image erect m<0 image reversed

Summary Light travels in straight lines formation of shadows and images understood by straight light rays light travels through vacuum at 299,792,458 m/s we detect different frequency visible electromagnetic radiation as light with different colors, additive effect of colors, complimentary colors light reflects from surfaces the laws of reflection: (1) the angles the incident and reflected rays make with the normal to the surface are equal (2) the reflected ray lies in the same plane as the normal and incident ray mirrors produce real and virtual images light converges to from real images which can be projected light only appears to come from virtual images, which cannot be projected constructing the image for flat and curved mirrors

Home-work assignment Part I. 412/1-2, 5-13 ; 414/46-49 ; 415/50-53, /1-2; 416/19-24; Part II. 412/15; 413/16-23,25; 414/29,31-33,35-37,39; 414/43-44; 415/5; 416/6-17