1. Light Waves, Mirrors and Reflection
Physics 1 PAP

2. 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.

3. 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.

4. 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.

5. 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”

6. 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

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

8. 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

9. 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)

10. Reflection & Mirrors

11. 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

12. 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)

13. 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.

14. 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

15. 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

16. 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

17. 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.

18. 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)

19. 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

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

21. 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

22. 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

23. 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

24. 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