Nature of Light Physics 1 Corpuscular Theory of Light Proposed by Isaac Newton in 1600’s Light is...

Nature of Light

Physics 1

Corpuscular Theory of Light• Proposed by Isaac Newton in 1600’s• Light is made of particles called

“corpuscles”• Explained reflection and refraction• Predicted that light traveled faster in water

than in air• This idea proved to be weakness of the

theory• Leon Foucault proved in 1850 that light

traveled faster in air than in water

Wave Theory of Light• Proposed by Christiaan Huygens in 1600’s• Light is made of waves• Explained reflection and refraction• Further confirmed by

– Thomas Young’s (1803) interference experiment

– Augustin Fresnel’s (1814) diffraction experiment

• Widely accepted in late 1880’s after work of Maxwell and Hertz

Electromagnetic Waves

• An electromagnetic (EM) wave is formed when electric and magnetic fields fluctuate together.

Nature of EM Waves

• An EM wave is a transverse wave.

• EM waves can be polarized.

Frequency of EM WavesElectromagnetic Spectrum

Speed of EM Waves

• James Maxwell determined that EM waves propagate through a vacuum at a speed given by

s

m103 8c

Speed of EM Waves

• Like any periodic wave, the speed of an EM wave is related to its frequency and wavelength by the wave equation (v = f).

• Substituting v = c, the wave equation for EM waves is

fc

Reflection(Geometrical Optics)

Physics 1

Wave Fronts and Rays

• EM (light) waves generated from an oscillator can be represented as a series of wave fronts, surfaces of constant phase.

Wave Fronts and Rays

• Radial lines pointing outward from the source and perpendicular to the wave fronts are called rays.

• Rays are convenient for showing the path of a light wave.

Plane Waves

• At large distances, curved wave fronts approach the shape of flat surfaces.

• These flat wave fronts are known as plane waves.

Geometrical Optics

In the study of how light behaves, it In the study of how light behaves, it is useful to use “light rays” and the is useful to use “light rays” and the fact that light travels in straight fact that light travels in straight lines.lines.

In the study of how light behaves, it In the study of how light behaves, it is useful to use “light rays” and the is useful to use “light rays” and the fact that light travels in straight fact that light travels in straight lines.lines.

When light strikes When light strikes the boundary the boundary between two between two media, three things media, three things may happen: may happen: reflection, reflection, refraction, or refraction, or absorption.absorption.

reflection

refraction

absorption

Water

Air

Reflection, Refraction, and Absorption

Water

AirReflection: A Reflection: A ray from air ray from air strikes the strikes the water and water and returns to the returns to the air.air.Refraction: A Refraction: A ray bends into ray bends into the water the water toward the toward the normal line.normal line.

Absorption: A ray Absorption: A ray is absorbed is absorbed atomically by the atomically by the water and does water and does not reappear.not reappear.

reflection

refraction

absorption

The Laws of Reflection

Water

Air1. The angle of 1. The angle of inci- dence inci- dence iiis is equal to the equal to the angle of angle of reflection reflection rr::i = r

i = r

i

N reflectionr

All ray angles are measured with respect to All ray angles are measured with respect to normal normal NN..

2. The incident ray, 2. The incident ray, the reflected ray, the reflected ray, and the normal and the normal NN all lie in the same all lie in the same plane.plane.

3. The rays 3. The rays are are completelcompletely y reversible.reversible.

The Plane Mirror

A mirror is a highly polished surface A mirror is a highly polished surface that forms images by uniformly that forms images by uniformly reflected light.reflected light.Note: images appear to be equi-distant behind mirror and are right-left reversed.

DefinitionsObject distance: The straight-line Object distance: The straight-line distance distance p p from the surface of a mirror from the surface of a mirror to the object. to the object. Image distance: The straight-line Image distance: The straight-line distance distance q q from the surface of a mirror from the surface of a mirror to the image. to the image.

Object distanc

e

Image distanc

e

=

p = q

i = r

ObjecObjectt

ImageImage

pp qq

Real and VirtualReal images and Real images and objects are formed objects are formed by actual rays of by actual rays of light. (Real images light. (Real images can be projected can be projected on a screen.)on a screen.)

Virtual images and Virtual images and objects do not objects do not really exist, but really exist, but only seem to be at only seem to be at a location.a location.

Virtual images are on Virtual images are on the opposite side of the the opposite side of the mirror from the mirror from the incoming rays.incoming rays.

Real object

Virtual image

Light rays

No light

Image of a Point Object

Plane mirror

Real object

p

Image appears to be at same Image appears to be at same distance behind mirror regardless of distance behind mirror regardless of viewing angle.viewing angle.

Image appears to be at same Image appears to be at same distance behind mirror regardless of distance behind mirror regardless of viewing angle.viewing angle.

qVirtual image

q = p

Image of an Extended Object

Plane mirror

p q

Image of bottom and top of guitar Image of bottom and top of guitar shows forward-back, right-left shows forward-back, right-left reversals.reversals.

Image of bottom and top of guitar Image of bottom and top of guitar shows forward-back, right-left shows forward-back, right-left reversals.reversals.

q = p

Virtual image

Terms for Spherical MirrorsA spherical mirror A spherical mirror is formed by the is formed by the inside (concave) inside (concave) or outside or outside (convex) surfaces (convex) surfaces of a sphere.of a sphere.A concave A concave spherical mirror is spherical mirror is shown here with shown here with parts identified.parts identified.

Concave Mirror

Radius of curvature RVertex V

Center of Curvature CThe axis and linear The axis and linear

aperture are aperture are shown.shown.

Linear aperture

V

C

R Axis

The Focal Length f of a Mirror

axis

ri

R

Incident parallel ray

f

The focal length, f

The focal The focal length length f f is: is:

2

Rf

The focal length f is equal to half the radius RThe focal length f is equal to half the radius R

Since Since ii = = rr, we , we find that find that FF is is mid- way mid- way between between VV and and CC; we find:; we find:

C VF

Focal point

For objects lo- For objects located at cated at infinity, the infinity, the real image real image appears at the appears at the focal point focal point since rays of since rays of light are light are almost almost parallel.parallel.

For objects lo- For objects located at cated at infinity, the infinity, the real image real image appears at the appears at the focal point focal point since rays of since rays of light are light are almost almost parallel.parallel.

The Focus of a Concave MirrorThe focal point The focal point FF for a concave mirror is the for a concave mirror is the point at which all parallel light rays point at which all parallel light rays converge.converge.

axis

Incident parallel Rays

CF

Focal Focal pointpoint

2

Rf

The Focus of a Convex MirrorThe focal point for a convex mirror is the The focal point for a convex mirror is the point point F F from which all parallel light rays from which all parallel light rays diverge.diverge.

axisC F

RR

Incident Rays

Reflected Rays

Virtual focus; reflected rays diverge.

2

Rf

Image Construction:

Ray 1: A ray parallel to mirror axis Ray 1: A ray parallel to mirror axis passes through the focal point of a passes through the focal point of a concave mirror or appears to come concave mirror or appears to come from the focal point of a convex mirror.from the focal point of a convex mirror.


C F

Convex mirror

Object

C F

Concave mirror

Object

Ray 1

Ray 1

Image Construction (Cont.):

Ray 2: A ray passing through the focus Ray 2: A ray passing through the focus of a concave mirror or proceeding of a concave mirror or proceeding toward the focus of a convex mirror is toward the focus of a convex mirror is reflected parallel to the mirror axis.reflected parallel to the mirror axis.

Ray 2: A ray passing through the focus Ray 2: A ray passing through the focus of a concave mirror or proceeding of a concave mirror or proceeding toward the focus of a convex mirror is toward the focus of a convex mirror is reflected parallel to the mirror axis.reflected parallel to the mirror axis.

Concave mirror

C FRay 2

Ray 1

Image C F

Convex mirror Ray

2

Ray 1

Image

Image Construction (Cont.):

Ray 3: A ray that proceeds along a Ray 3: A ray that proceeds along a radius is always reflected back along its radius is always reflected back along its original path.original path.


C F

Convex mirror

Concave mirror

C F

Ray 2

Ray 1

Ray 3

Ray 3

C F

Ray 2

Ray 1

Image

The Nature of ImagesAn object is placed in front of a concave An object is placed in front of a concave mirror. It is useful to trace the images as mirror. It is useful to trace the images as the object moves ever closer to the vertex the object moves ever closer to the vertex of the mirror.of the mirror.

An object is placed in front of a concave An object is placed in front of a concave mirror. It is useful to trace the images as mirror. It is useful to trace the images as the object moves ever closer to the vertex the object moves ever closer to the vertex of the mirror.of the mirror.We will want to locate the image and answer three questions for the possible positions:

3. Is it enlarged, diminished, or the same size?

2. Is the image real or virtual?

1. Is the image upright or inverted?

Object Outside Center C

Concave mirror

C FRay 3

Ray 2

Ray 1

1. The image is 1. The image is inverted; i.e., inverted; i.e., opposite of the opposite of the object orientation.object orientation.

2. The image is real; 2. The image is real; i.e., formed by i.e., formed by actual light rays in actual light rays in front of mirror. front of mirror.

3. The image is 3. The image is reduced in size; i.e., reduced in size; i.e., smaller than the smaller than the object.object.

Image is located between C and F

Image is located between C and F

Object at the Center C

C F

Ray 2

Ray 1

1. The image is 1. The image is inverted; i.e., inverted; i.e., opposite of the opposite of the object orientation.object orientation.


3. The image is the 3. The image is the same size (true) as same size (true) as the object.the object.

Image is located at C, inverted.

Image is located at C, inverted.

Ray 3

Object Between C and F1. The image is 1. The image is

inverted; i.e., inverted; i.e., opposite of the opposite of the object orientation.object orientation.


3. The image is 3. The image is enlarged in size; i.e., enlarged in size; i.e., larger than the larger than the object.object.

Image is outside of the center C

Image is outside of the center C

CF

Ray 1

Ray 3

Ray 2

Object at Focal Point

Image is located at infinity (not formed).

Image is located at infinity (not formed).

C

F

Ray 3

Reflected rays are parallel

When the object When the object is located at the is located at the focal point of the focal point of the mirror, the image mirror, the image is not formed (or is not formed (or it is located at it is located at infinity).infinity).

When the object When the object is located at the is located at the focal point of the focal point of the mirror, the image mirror, the image is not formed (or is not formed (or it is located at it is located at infinity).infinity).

The parallel reflected The parallel reflected rays never cross.rays never cross.

Ray 1

Object Inside Focal Point1. The image is 1. The image is

upright; i.e., same upright; i.e., same orientation as the orientation as the object.object.

2. The image is 2. The image is virtual; that is, it virtual; that is, it seems to be seems to be located behind located behind mirror.mirror.3. The image is 3. The image is enlarged; bigger enlarged; bigger than the object.than the object.

Image is located behind the mirror

Image is located behind the mirror

C

FUpright and enlarged

Virtual image

Observe the Images as Object Moves Closer to Mirror

Concave mirror

C FRay 3

Ray 2

Ray 1

C F

Ray 2

Ray 1

Ray 3

CF

Ray 1

Ray 3

Ray 2

C

F

Ray 3

Reflected rays are parallel

Ray 1

C

FUpright and enlarged

Virtual image

Convex Mirror Imaging

C F

Convex mirror Ray

2

Ray 1

Image

All images are upright, virtual, and All images are upright, virtual, and diminished. Images get larger as object diminished. Images get larger as object

approaches.approaches.

All images are upright, virtual, and All images are upright, virtual, and diminished. Images get larger as object diminished. Images get larger as object

approaches.approaches.

C F

Convex mirror

Ray 1

2

Image gets larger as object

gets closer

Converging and Diverging MirrorsConcave mirrors and Concave mirrors and converging parallel converging parallel rays will be called rays will be called converging mirrors converging mirrors from this point onward.from this point onward.

Convex mirrors and Convex mirrors and diverging parallel diverging parallel rays will be called rays will be called diverging mirrors diverging mirrors from this point from this point onward.onward.

CF

Converging Mirror

Concave

C F

Diverging Mirror

Convex

Image Construction Summary:



Ray 2: A ray passing through the focus Ray 2: A ray passing through the focus of a concave mirror or proceeding of a concave mirror or proceeding toward the focus of a convex mirror is toward the focus of a convex mirror is reflected parallel to mirror axis. reflected parallel to mirror axis.

Ray 2: A ray passing through the focus Ray 2: A ray passing through the focus of a concave mirror or proceeding of a concave mirror or proceeding toward the focus of a convex mirror is toward the focus of a convex mirror is reflected parallel to mirror axis. reflected parallel to mirror axis.



Nature of Light Physics 1 Corpuscular Theory of Light Proposed by Isaac Newton in 1600’s Light is...

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