Light

What is Light? Is light a particle or a wave? Yes – exhibits behaviors of BOTH

Light Can Act Like a Wave In 1801 Thomas Young an English scientist did the Double slit experiment. Passed a beam of light through two narrow openings and projected it onto a screen. He found the light produced a striped pattern which meant the light was constructively and destructively interfering. This meant that light is composed of waves.

Light can Behave Particle Other observations indicated that light can also act like a particle: When light hits metal it knocks electrons off the surface. They found that red light cannot knock electrons off metal no matter how bright it is. If light were a wave then the brighter light should have more energy. Photons are light particles that contain certain amounts of energy based on their frequency and wavelength. Blue light has a higher frequency and shorter wavelength thus contains more energy than red light.

What is Light? The “particle” of light is called the photon. It has no mass and no charge, but it does carry energy. For ‘optics’ lessons, we will focus on light as a wave What type of wave is light? Electromagnetic

Exhibits Behaviors of Waves Reflection Refraction Diffraction

Reflection Law of reflection Angle of ‘incidence’ is equal to angle of ‘reflection’ (as measured from the ‘normal’ line)

More on Reflection

Reflection is Important Allows us to see as light bounces off objects Important with mirrors … more to come

EX 1 Sitting in her parlor one night, Gerty sees the reflection of her cat, Whiskers, in the living room window. If the image of Whiskers makes an angle of 40° with the normal, at what angle does Gerty see him reflected? Solution: Because the angle of incidence equals the angle of reflection, Gerty must see her cat reflected at an angle of 40°.

EX 2 Answer: 40°

Electromagnetic waves What are the characteristics of electromagnetic waves? Travels at “speed of light” c = 3x108 m/s Transverse wave Requires no medium (can travel via a vacuum)

Electromagnetic Wave Relationships Frequency and wavelength are inversely related via wave speed And for electromagnetic waves, velocity is equal to the speed of light (c)

EX 3 What is the frequency of infrared light that has a wavelength of 8x10–6 m?

How Frequency/Wavelength relates to color

Wave Characteristics Wavelength and Frequency for electromagnetic waves determines the ‘type’ of wave

The Color of light is Determined By its Frequency and Wavelength

Only red light is reflected Seeing color The color an object appears depends on the colors of light it reflects. For example, a red book only reflects red light: Homework White light Only red light is reflected

How The Eye Works Rods and Cones Rods (~120 million) detect B&W images Cones (~6 million) detect color. Each cone has sensitivity to Red, Green OR Blue

Green, Red and Blue Cones Cones have a sensitivity to Green, Red and Blue Note that to a lesser extent, our cones can detect other colors

Primary Colors Primary colors of light are Red, Green and Blue Additive Colors Mixing lights adds colors Adding primary colors gives ‘secondary’ color in between Adding all three primary colors gives white

Secondary Colors Secondary colors are Magenta, Yellow and Cyan Colors that absorb (subtractive) colors are pigments Absorbs a primary color, reflects two (2) primary colors Primary “paint” colors (pigments) Adding secondary colors gives ‘primary’ color in between Mixing all secondary colors gives Black

MIRRORS

Reflection Definitions Object Distance: the distance from the mirror to the object. Positive ‘in front of mirror’ Image distance: the distance from the mirror to the image. Nature of image real - inverted and able to projected on a screen; in front of mirror virtual - right-side-up and NOT able to be projected on a screen; in ‘back’ of mirror Size of Image enlarged – image is larger than object reduced – image is smaller than object true – image is same size as object Orientation of Image upright – image points in same direction as object inverted - image points in opposite direction as object

Reflection on Plane Mirror Image forms where all rays converge Images are: Virtual Upright Same Size

(more) Reflections Definitions Focal Point: The point where parallel rays meet (or appear to meet) after reflecting from a mirror. The distance from the focal point to a mirror is called the focal length. The focal length of a converging mirror is always positive (in front of the mirror) The focal length of a diverging mirror is always negative

Law of Reflection Reflected Rays bounce off at the same angle as the Incident ray (as measured from the normal) Image formation is where the rays cross i.e. Our eyes perceive the image at this location

Mirror Types Plain/Flat Converging/Concave Diverging/Convex

Mirror Types

Reflected Rays Parallel Rays go through focal point Lines through focal point reflect parallel Rays through center reflect at incoming angle

Optics Refresher Ray Model of Light Light is either absorbed or reflected When it is reflected, it is reflected in all directions (expanding outwardly like a sphere – similar to sound) We can think of this as individual ‘rays’ of light travelling in straight lines in all directions

Reflection Reflection Incident ray Reflected ray When light bounces off a surface. Rough surfaces reflect light rays in many directions. Diffuse reflection Causes a blurry image or no image. Incident ray Ray hitting the surface Reflected ray Ray bouncing off the surface

Reflection Normal Angle of Incidence Angle of reflection An imaginary line that is perpendicular to the surface the light is reflecting from. Angle of Incidence Angle between incident ray and the normal. Angle of reflection Angle between the reflected ray and the normal. The angle of incidence equals the angle of reflection.

About Diverging (convex) Mirror These characteristics always true for diverging mirrors. Virtual image Upright Smaller

Terms of Mirrors Principal Axis (A) Optical axis Center of Curvature (C) - Center of sphere Radius of Curvature (R) – distance of C to mirror Focal Point (F) Focal Length ( f ) R=2*f

Equations for Curved Mirrors f = focal length do = distance to object di = distance to image All distances are positive in front of mirror and negative behind mirror. Magnification hi = height of image ho = height of object

Understanding Image d + Real - virtual f Concave convex m Upright inverted >1 enlarged <1 reduced =1 Same size h

EX 4 Wendy the witch is polishing her crystal ball. It is so shiny that she can see her reflection when she gazes into the ball from a distance of 15 cm. a) What is the focal length of Wendy‘s crystal ball if she can see her reflection 4.0 cm behind the surface? b) Is this image real or virtual?

EX 5

EX 6 Answer: a) -10 cm b) smaller

EX 7 Answer: -13.3 cm

EX 8 Answer: a) -36 cm b) Yes, it would change the image distance

Drawing Ray Diagrams Helps predict characteristics of the image

Concave Mirror

Image is virtual, upright and reduced Convex Mirrors Image is virtual, upright and reduced

Convex Mirror Ex 1 (diverting) Draw a ray parallel to the principal axis. Where this ray crosses the mirror, draw a reflected ray through the focal point. Draw a ray toward the focal point. Where this ray hits the mirror, draw a reflected ray parallel to the principal axis Where the two reflected rays cross, draw the reflected image. Optional: Draw the midpoint ray through the center of the mirror. This ray will reflect at the same angle as it came in (law of reflection)

Ex 2 Concave (converging) Mirror

Ex 3 Concave (converging) Mirror When an object is located at the focal point of a concave mirror NO image will form.

Ex 4 Concave (converging) Mirror