Chapter 13 Properties of Light: Reflection and Mirrors Herriman High Honors Physics

Reflection  When a wave reaches a boundary between two media and is turned back – this is reflection Total reflection –boundary with a more rigid medium Partial reflection – less rigid medium. This can be analyzed using the ray model of light, i.e. representing a light wave with a ray. Herriman High Honors Physics

Law of Reflection  The law of reflection states that the angle of incidence = angle of reflection Incident ray Reflected ray Angle of incidence Angle of reflection Herriman High Honors Physics

Mirrors  Plane mirrors are flat mirrors which form a virtual image. A virtual image is one which forms on the opposite side of the mirror. Images are the same size as the object and the same vertical orientation. Herriman High Honors Physics

Image in a Plane Mirror  Seeing yourself in a plane mirror requires that the mirror be at least ½ the height of the object  You see your head looking straight across  You see your feet looking down, since the ray passes through the mirror at ½ the distance between you and the image so the mirror, by similar triangles must be ½ your height. Herriman High Honors Physics

Plane Mirrors: Things to Remember  Images are virtual – i.e behind the mirror  Images are the same distance from the mirror as the object  Images are upright  Images are the same size as the object.  Images are inverted right to left. Herriman High Honors Physics

Curved Mirrors  Vocabulary Principal Axis – line perpendicular to mirror passing through the center of curvature and the focal point. Center of Curvature – the geometric center of the sphere from which the mirror was formed Focal length – ½ the center of curvature Image Distance – distance from the mirror to where the image is formed Object Distance – distance from mirror to object. Herriman High Honors Physics

Curved Mirrors  Any light ray which approaches the mirror parallel to the principal axis will be reflected through the focal point.  Any light ray which passes through the focal point will be reflected parallel to the principal axis. Herriman High Honors Physics

Curved Mirrors: Concave  Concave Mirror Curves inward Close objects Form a virtual image which is larger than the object Objects further away form real images which are cast on a screen Herriman High Honors Physics

Curved Mirrors: Convex  Convex Mirror Curves outward Close objects form a virtual image which is smaller than the object These mirrors are used to give a wide field of view Herriman High Honors Physics

Image Object Relationships  f = focal length  d i = distance to image  d 0 = distance to object  h i = height of image  h 0 = height of object  For convex mirrors, f is negative Herriman High Honors Physics

Sample Problem  If an object which is 3 cm in diameter is placed 10 cm from a convex mirror with a focal length of 5 cm, where will the image appear, how large will it be, and will it be upright or inverted? Herriman High Honors Physics

Solution The image appears on the opposite side of the mirror and is 1/3 as large and is upright. Herriman High Honors Physics

The Colors of Light  Light is defined as the frequencies of electromagnetic radiation which stimulate the human retina.  Light ranges in color from: Red - lowest frequency/longest wavelength Orange Yellow Green Blue Indigo Violet – highest frequency/shortest wavelength

White Light and the Color Black  White light contains all of the colors (frequencies) of visible light.  Black is the absence of light.  Vision is the process of seeing reflected light. The color that you see is reflected from an objects – all other wavelengths are absorbed.

White Light and the Color Black  White light contains all of the colors (frequencies) of visible light.  Black is the absence of light.  Vision is the process of seeing reflected light. The color that you see is reflected from an objects – all other wavelengths are absorbed.

Color by Transmission  The color of a transparent object depends upon the color it transmits.  The material which selectively absorbs colors of light is called a pigment.  Absorbed light warms the object – hence darker colors tend to get warm in sunlight and lighter colors tend to remain cooler.

Mixing Light  When red, blue and green light (primary colors) are mixed, new colors are formed (secondary colors).  All three colors form white light. Red and Green – yellow Red and Blue – magenta Green and Blue - Cyan

Mixing Pigments  When you mix pigments (paints) you add by subtraction – that is each pigment you add to your mixture subtracts a color of light.  Primary pigments Magenta Yellow Cyan  Secondary Mixtures Yellow and Cyan – Green Yellow and Magenta – Red Magenta and Cyan - Blue

Chapter 14 Properties of Light: Refraction and Lenses Herriman High Honors Physics

Refraction  Refraction is the bending of light at a boundary.  If the speed in the new media is less than the speed in the old, the wave bends toward the normal  If the speed in the new media is more than the speed in the old, the wave bends away from the normal. Sidewalk Grass Herriman High Honors Physics

Snell’s Law and the Index of Refraction  How much light bends at a boundary is given by Snell’s Law or the Index of refraction: Another way that this law is written is: n 0 sin i = n sin r Where n 0 is the index for the material you are moving out of and n is the index for the material you are going into. Herriman High Honors Physics

Sample Problem  What is the speed of light in crown glass if its index of refraction is 1.52?  What is the angle of refraction for a wave in crown glass if its angle of incidence is 30°. Assume that it is coming from air into a cube of the glass. Herriman High Honors Physics

Solution n 0 sin i = n sin r 1 sin 30° = 1.52 sin r 1 sin 30°/1.52 = sin r r = 19.73° Herriman High Honors Physics

Converging and Diverging Lenses  Converging lenses are convex, or curve outward.  Diverging lenses are concave, or curve inward. Herriman High Honors Physics

Sign Conventions for Lenses  Lenses use the same equations as mirrors  The following rules apply to lenses: The focal length is positive for a converging lens and negative for a diverging lens. Object distance is positive Image distance is positive if it is on the opposite side of the lens from the object and negative if it is on the same side as the object. The height of the object is always taken to be positive. The image height is positive if it is upright and negative if it is inverted. Herriman High Honors Physics

Sample Problem  If an object which is 2 cm in diameter is placed 12 cm from a convex lens with a focal length of 5 cm, where will the image appear, how large will it be, and will it be upright or inverted? Herriman High Honors Physics

Solution The image appears on the opposite side of the lens and is 0.7 x as large and is inverted. Herriman High Honors Physics

The Eye  The lens of a human eye focuses an image on the retina at the rear of the eye.  If the lens forms an image in front of the retina, then a person is said to be nearsighted  If the lens form an image at the rear of the retina (behind it) then the person is said to be farsighted. Herriman High Honors Physics

Corrective Lenses  Since a nearsighted person’s eye forms the image too soon, the lens of their eye has too much convergence, hence a diverging lens, (concave) is used to separate the rays of light coming from the object so that the image will converge on the retina.  Since a Farsighted person’s eye forms the image after the retina, a converging lens, (convex) is used to help the rays of light converge sooner, forming the image on the retina. Herriman High Honors Physics