Light Waves, Mirrors and Reflection Physics 1 PAP

Light Light is a transverse wave. Light waves are electromagnetic waves--which means that they do NOT need a medium to travel. Light can behave as a wave or a particle. We will treat light as a wave and so it will have the wave characteristics such as amplitude, frequency, and wavelength.

Characteristics of Light Intensity (brightness) -- represented by amplitude Color -- determined by frequency or wavelength Wave speed - depends on the medium Light waves as well as ALL Electromagnetic waves travel with a speed of 3.0 x 108 m/s in a vacuum.

Characteristics of Electromagnetic Waves Made up of 2 components electric field & magnetic field The electric and magnetic fields are perpendicular to each other. A changing electric field will create a magnetic field and a changing magnetic field will create an electric field; therefore the wave propagates itself through space without need of a medium.

Electromagnetic Waves All of these follow the same rules as Light and travel at the same speed. They are listed in order of increasing frequency and energy and decreasing wavelength Light is the visible part of the spectrum Radio Microwaves Infrared Visible Light Ultraviolet X-rays Gamma Rays (highest f, E, shortest ƛ) To help you remember the order: “Red Martians invade Venus using x-ray guns”

Luminous vs Illuminated a body that reflects light no luminous flux does not emit light of its own Ex: moon Luminous a body that emits light has luminous flux Ex: light bulb, sun

Luminous Flux (P) Luminous flux is the rate at which light energy is emitted from the source. Equivalent to Power Measured in lumens (lm)

Illuminance (E) Amount of light that falls on a surface Intensity of light at any given distance from source measured in lux lux = lumen/m2 r bulb

Luminous Intensity (I) The amount of light (luminous flux) that falls on one square meter at a distance of 1 meter from the source. Equivalent to Intensity at r = 1 m Measured in candelas (cd)

Reflection & Mirrors

Law of Reflection i incident ray reflected ray normal Plane (flat) mirror surface r Angles are always measured from the normal, never the surface Angle of incidence equal angle of reflection i = r

Types of Reflection Specular or Regular Reflection Diffuse Reflection When parallel rays of light fall on a smooth surface they are reflected parallel from the surface. Forms a clear image. (Plane mirror) Diffuse Reflection When parallel rays of light fall on a textured surface they are reflected in different directions. They are diffused. Image not as clear. (lab table)

Real versus Virtual Images Real Image – an image formed when rays of light actually intersect at a single point. It is always inverted and image can be seen on paper or a screen. Virtual Image – an image formed by light rays that only appear to intersect at a point. The light rays seem to diverge from the point where the image is formed. It is always erect or upright and can not be seen on a screen.

Plane (Flat) Mirror Forms an upright, virtual image Image is backwards (reversed from left to right) Image distance from the mirror is equal to object distance from the mirror (di = do) Image size is equal to object size (hi = ho) Magnification (M) = 1

Concave or Converging Mirrors Reflective surface is “caved in” or like the inside of a spoon Parallel rays of light (from a far object) will converge at the focal point in front of a concave mirror so they are called “converging mirrors” Focal point is half the distance from the center of curvature (C) to the mirror f = R/2, where R is radius of curvature

Convex or Diverging Mirrors Reflective surface is curved out or like the back of a spoon Parallel rays of light (from a far object) will diverge as if they originated from a focal point behind the mirror so they are called “diverging mirrors” Focal point is half the distance from the center of curvature (C) to the mirror f = R/2, where R is radius of curvature

Calculations f = focal length do = object distance di = image distance hi = image height ho = object height M = magnification Note: All distances are measured from the center of the mirror along the principal axis. All heights are measured from the principal axis.

magnification formula can be written as 3 separate equations:       Note: the negative sign on di is to make the sign of M consistent with signs for hi and di (which are opposites)

Interpreting Calculations with signs Focal length (f) concave or converging mirror: f is + (in front of mirror) convex or diverging mirror: f is - (behind the mirror) Image distance (di) di is “+” in front of mirror and the image is real di is “-” behind the mirror and the image is virtual Magnification (M) M = +, image is erect (+ hi) and virtual M = - , image is inverted (- hi) and real

Concave Mirror (object beyond C) Ray Diagram Concave Mirror (object beyond C) Draw 2 rays from tip of object: 1) parallel, then through f 2) through f, then parallel The image is formed where the reflected rays intersect. object Image is real, closer, inverted, & smaller image C f

Concave Mirror (object at C) Ray Diagram Concave Mirror (object at C) Draw 2 rays from tip of object: 1) parallel, then through f 2) through f, then parallel object Image is real, inverted, & same size and distance C image f

Concave Mirror (object between f & C) Ray Diagram Concave Mirror (object between f & C) f C object image Draw 2 rays from tip of object: 1) parallel, then through f 2) through f, then parallel Image is real, inverted, larger, & farther away

Ray Diagram Concave Mirror (object inside f) Draw 2 rays from tip of object: parallel, then through f as if it came from the focal point and then parallel extend the reflected rays behind mirror to locate image image C object f Image is virtual, erect, larger, & behind the mirror

Ray Diagram Convex Mirror Draw 2 rays from tip of object: parallel, then reflect as if ray came from focus toward the focal point, then parallel extend the reflected rays behind the mirror to locate the image image object f C Image is virtual, erect, smaller, & behind the mirror