chapter 14. characteristics of light section 14.1

Chapter 14

Characteristics of LightCharacteristics of Light

Section 14.1

Electromagnetic WavesElectromagnetic Waves

Light is made of electromagnetic waves.Take a prism and break up white light into a rainbow

like band of colors. These are all in the visible spectrum.

Red, orange, yellow, green, blue, indigo and violet.ROY G BIV


The spectrum also includes non-visible electromagnetic waves, such as x-rays, microwaves, radio waves, and radiation.

Because they all are electromagnetic waves they all have similar properties.


Electromagnetic waves are transverse waves consisting of oscillating electric and magnetic fields at right angles to each other.

Oscillate: to have a periodic vibration


Electromagnetic waves vary depending on frequency and wavelength

All electromagnetic waves move at the speed of light


We will use 3.00 X 108 m/s as the speed of light, c.

The wave speed equation is:

c = f Speed of light = frequency X wavelength

Sample ProblemSample Problem

The AM radio band extends from 5.4 X 105 Hz to 1.7 X 106 Hz. What are the longest and shortest wavelengths in this frequency range?

f 1 = 5.4 x 105 Hz f 2 = 1.7 x 106 Hzc = 3.0 x 108 m/sc = f= c/ f

1 = 5.6 x 102 m

2 = 1.8 x 102m

Light travels in straight lines.Light travels in straight lines.

Light travels in straight lines.• Show the laser on the wall. Put an index card in

the beam. This shows that the light is traveling in a straight line, but you can only see it when it hits something.

• Put some chalk dust in the beam to show it is continuous.

Brightness decreases by the square of the distance form the source

• Show how the size of the dot the laser makes gets bigger as it gets further from the source.

Laser

The brightness of light is inversely proportional to the square of the distance from the light source.Ex. If you move twice as far away from the light source, ¼ as much light falls on the book.

Flat mirrorsFlat mirrors

Section 14.2

Reflection of LightReflection of Light

Reflection – the turning back of an electromagnetic wave at the surface of a substance

Clear vs. Diffuse ReflectionClear vs. Diffuse Reflection

Specular reflection: light reflected from smooth shiny surfaces

In specular reflection the incoming and reflected angles are equal (=’)

Diffuse reflection: light is reflected from a rough textured surface

Diffuse Reflection: Diffuse Reflection: Definition, Examples and Definition, Examples and

SurfacesSurfaces

http://education-portal.com/academy/lesson/diffuse-reflection-definition-examples-surfaces.html#lesson




Part 2 - ReflectionPart 2 - ReflectionReflection from a mirror:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

Reflection of LightReflection of Light

Angle of incidence – the angle between a ray that strikes a surface and the normal to that surface at the point of contact.

Angle of reflection – the angle formed by the line normal to a surface and the direction in which a reflected ray moves

Normal is a line perpendicular to the reflection surface.

The Law of ReflectionThe Law of Reflection

Angle of incidence = Angle of Angle of incidence = Angle of reflectionreflection

In other words, light gets reflected from a surface at THE SAME ANGLE it hits it.

The same !

!!

Reflection: Angle of Reflection: Angle of Incidence and Curved Incidence and Curved

SurfacesSurfaces

http://education-portal.com/academy/lesson/reflection-definition-angles-of-incidence-diffuse-reflection.html#lesson

Drawing a Reflected Drawing a Reflected ImageImage

Use ray diagrams to show image location

We will find the virtual image (the image formed by light rays that only appear to intersect)

Drawing a Reflected Drawing a Reflected ImageImage

Draw the object in front of the mirrorDraw a ray perpendicular to the mirror’s surface. Because this

is 0 from normal, the angle is the same from the mirror to the virtual object

Draw a second ray that is not perpendicular to the mirror’s surface from the same point to the surface of the mirror.

Next, trace both reflected rays back to the point from which they appear to have originated, that is, behind the mirror. Use dotted lines when drawing lines that that appear to emerge from behind the mirror. The point at which the dotted lines meet is the image point.

Flat MirrorsFlat Mirrors

Image is VIRTUAL, UPRIGHT, UNMAGNIFIED

Chapter 14Chapter 14

14.3 Concave Mirrors

Spherical MirrorsSpherical MirrorsA spherical mirror has the shape of part of

a sphere’s surface. The images formed are different than those of flat mirrors.

Concave mirrors were silvered on the inside of the sphere and convex mirrors were silvered on the outside of the sphere.

Concave Spherical Mirror Concave Spherical Mirror

An inwardly curved, mirrored surface that is a portion of a sphere and that converges incoming light rays.

Concave Spherical Concave Spherical MirrorsMirrors

• Principle axis - the line passing through the center of the sphere and attaching to the mirror in the exact center of the mirror

• Center of curvature - the point in the center of the sphere from which the mirror was sliced (C)

• Vertex - the point on the mirror's surface where the principal axis meets the mirror (A)

• Focal point - midway between the vertex and the center of curvature (F)

• Radius of curvature - the distance from the vertex to the center of curvature (R)

• Focal length - the distance from the mirror to the focal point, one-half the radius of curvature (f)

• http://www.youtube.com/watch?v=np8lENrge0Q

• http://www.youtube.com/watch?v=jrje73EyKag

http://www.youtube.com/watch?v=np8lENrge0Q

http://www.youtube.com/watch?v=np8lENrge0Q

http://www.youtube.com/watch?v=jrje73EyKag

http://www.youtube.com/watch?v=jrje73EyKag


The light bulb is distance p away from the center of the curvature, C. Light rays leave the light bulb, reflect from the mirror and converge at distance q in front of the mirror. Because the reflected light rays pass through the image point, the image forms in front of the mirror.


If you were to place a sheet of paper at the image point, you would see a clear, focused image of the light bulb (a real image). If the paper was placed in front of or behind the image point, the image would be unfocused.


Real image – an image formed when rays of light actually intersect at a single point

Focal length – equal to half the radius of curvature of the mirror.


Mirror equation: 1/p + 1/q = 2/R

1 + 1 = 2 .

Object distance Image distance radius of curvature

Or: 1/p + 1/q = 1/f

1 + 1 = 1 .

Object distance Image distance focal length


Object and image distances have a positive sign when measured from the center of the mirror to any point on the mirror’s front side.

Distances for images that form on the backside of the mirror always have a negative sign.


The measure of how large or small the image is with respect to the original object is called the magnification of the image.

M = h’/h = -(q/p)

Magnification = image height = image distance

object height object distance


For spherical mirrors, three reference rays are used to find the image point. The intersection of any two rays locates the image. The third ray should intersect at the same point and can be used to check the diagram.

Rules for drawing reference Rules for drawing reference raysrays

Ray Line drawn from object to mirror

Line draw from mirror to image after reflection

1 Parallel to principal axis Through focal point F

2 Through focal point F Parallel to principal axis

3 Through center of curvature C

Back along itself through C

Ray 1Ray 1

Ray 2Ray 2

Ray 3Ray 3

All three rays togetherAll three rays together

Spherical Mirrors - Spherical Mirrors - ConcaveConcave

Image is REAL, INVERTED, and DEMAGNIFIED !!!

C F

Concave Spherical MirrorConcave Spherical Mirror

When an object changes its location in relation to the mirror, its image changes in location, and form.

Concave Spherical MirrorConcave Spherical Mirror

Object’s distance

Type of Image Location of Image

Greater than focal length

Real and inverted

In front of mirror

At the focal length

Image is infinitely away from mirror and can’t be seen

Between focal point and mirror’s surface

Virtual and upright

Behind mirror

Distance greater than focal Distance greater than focal lengthlength

Distance = focal lengthDistance = focal length

Between focal length and Between focal length and mirrormirror

Spherical Mirrors – ConcaveSpherical Mirrors – ConcaveObject Inside the Focal Object Inside the Focal

PointPoint

Image is VIRTUAL, UPRIGHT, and MAGNIFIED

C F

M= -(q/p)We have p, but not q, so we need another

equation to find q.1/p + 1/q = 1/fWe have p and f, so we can solve for q.1/q = 1/f – 1/p

1/q = 1/f – 1/pSubstitute:(1/10 cm) – (1/30 cm) = 1/qSolve:0.06667 cm = 1/qq= 15 cm

Now with q we can substitute into the original formula and solve.

M= -(q/p)M= -(15 cm/30cm)

M= -0.50This means that the image is smaller than

the object and inverted. Therefore it is a real image.

Spherical Mirrors - Spherical Mirrors - ConvexConvex

Convex spherical mirror:An outwardly curved, mirrored surface that is a portion of a sphere and that diverges incoming light rays

The focal point and center of curvature are situated behind the mirror.


Convex mirrors take the objects in a large field of view and produce a small image, but give a the observer a complete view of a large area.

Examples:In stores, the passenger’s side of a car


Image is VIRTUAL, UPRIGHT, and DEMAGNIFIED

C F

ColorColor

White light is not a single color; it is made up of a mixture of the seven colours of the rainbow.

We can demonstrate this by splitting white light with a prism:

This is how rainbows are formed: sunlight is “split up” by raindrops.

Wavelengths of LightWavelengths of Light

Red Light – nm

Green Light - nm

Blue Light - nm

Adding colorsAdding colorsWhite light can be split up to make separate

colors. These colors can be added together again.

The primary colors of light are red, blue and green:Adding blue and

red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white again

Adding red and green makes yellow

Seeing colorSeeing colorThe color an object appears depends on the

colors of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is

reflected

A white hat would reflect all seven colors:

A pair of purple pants, in addition to being ugly, would reflect purple light

(or red and blue, as purple is made up of red and blue):

Purple light

White

light

Using colored lightUsing colored light

If we look at a colored object in colored light we see something different. For example, consider the outfit below – I mean, from a physics standpoint, not as a fashion choice:

White

light

Shorts look blue

Shirt looks red

In different colours of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Using filtersUsing filtersFilters can be used to “block” out different colors of

light:

Red Filte

r

Magenta

Filter

chapter 14. characteristics of light section 14.1

Documents

reflection surface

light source

brightness of light

f speed of light

white light

reflection of lightreflection

radio waves

transverse waves