Light Waves

Physics 1 HCreated by Stephanie Ingle

Light• Light is a transverse wave.• Light waves are electromagnetic

waves--which means that they do NOT need a medium to travel.

• Light waves behave like other waves and have the same characteristics such as amplitude, frequency, and wavelength.

Characteristics of Light

• Intensity (brightness) -- represented by amplitude

• Color -- determined by frequency• 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• Radio • Microwaves• Infrared• Visible Light• Ultraviolet• X-rays• Gamma Rays

• 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

Luminous vs Illuminated• Luminous

– a body that emits light

– has luminous flux

• Illuminated–a body that

reflects light–no luminous

flux–does not emit

light of its own

Luminous Flux (P)

• Luminous flux is the rate at which light energy is emitted from the source.

• Equivalent to Power• Measured in lumens (lm)

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)

4PI

4PI

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

bulb

r

24PEr

Reflection & Mirrors

Law of Reflection

• Angles are always measured from the normal, never the surface

• Angle of incidence equal angle of reflectioni = r

ri

incident rayreflected ray

normal

Mirror surface

Types of Reflection

• Regular Reflection– When parallel rays of light fall on a smooth

surface they are reflected parallel from the surface.

• Diffuse Reflection– When parallel rays of light fall on a textured

surface they are reflected in many different directions. They are diffused.

Concave Mirrors• Reflective surface, like inside of a spoon,

forms a “cave”• Parallel rays of light from a far object will

converge at the focal point.• Concave Mirrors also 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 Mirrors• Reflective surface, like back of a spoon or

outside of curve • Parallel rays of light from a far object will

diverge as if they originated at the focal point.• Convex Mirrors also 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

io ddf111

o

i

o

i

dd

hhM

f = focal length

do = object distance

di = image distance

hi = image height

ho = object height

M = magnification

Interpreting CalculationsFocal length (f)

concave or converging, then f is +

convex or diverging, then f is -

Image distance (di)

di is + , then image is real

di is -, then image is virtual

Magnification (M)

M = +, image is erect and virtual

M = - , image is inverted and real

C f

Ray DiagramConcave 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

Image is real, inverted, & reduced

Cf

object

image

Ray DiagramConcave Mirror (object at C)

Draw 2 rays from tip of object:

1) parallel, then through f

2) through f, then parallelImage is real, inverted, & same size

Ray DiagramConcave Mirror (object between f & C)

fC

object

imageImage is real, inverted, & magnified

Draw 2 rays from tip of object:

1) parallel, then through f

2) through f, then parallel

Ray DiagramConcave Mirror (object inside f)

fC object

image

Draw 2 rays from tip of object:

1) parallel, then through f

2) as if it came from the focal point and then parallel

3) extend the reflected rays behind mirror to locate image

Image is virtual, erect, & magnified

Cf

Ray DiagramConvex Mirror

object image

Draw 2 rays from tip of object:

1) parallel, then reflect as

if ray came from focus

2) toward the focal point,

then parallel

3) extend the reflected rays behind the mirror to locate the image

Image is virtual, erect, & reduced