Properties of Light

ELECTROMAGNETIC WAVES

• Light is an example of an electromagnetic wave.

• It requires no medium through which to travel.

VISIBLE LIGHT

History

• Isaac Newton knew that light traveled in straight lines and cast shadows.

• Other scientists discovered that light was made from a stream of tiny particles or traveled as waves.

• Olaus Roemer estimated the speed of light in the late 1600’s, but his value was low.

• William Herschel, 1799, discovered that different colors of light have different temperatures. He discovered that red was the hottest color while violet was the coolest.

• Ultraviolet was discovered by Wilhelm Ritter in 1801.

• In 1972, the National Institute of Standards and Technology discovered that the actual speed of light was 299,792,458 meters per second.

• Albert Einstein proved that light moves fastest in a vacuum. However, the speed of light varies depending on the medium through which it travels.

TRANSVERSE NATURE

Polarization

• Polarization results when we separate electromagnetic waves by the plane in which they are vibrating.

• By taking only those waves which vibrate in one plane, we cut down the amount of energy.

Waves and Rays

Geometrical Optics

• We assume that light follows straight-line paths (rays)

• Changes occur when a ray hits a boundary

– rays may bounce off (reflection)

– rays may bend into the other medium (refraction)

– rays may be absorbed (light energy thermal energy)

• Diffraction and interference are phenomena best described by light waves (not rays)

Doppler Shift

Doppler Shift

Reflection

Incident vs Reflected Rays

• Incident Ray – leaves the object and strikes the mirror

• Reflected Ray – leaves mirror and strikes your eye

• The reflected ray is on the line of sight from the image to your eye.

Angles are Measured from the Normal

In geometrical optics, angles are always measured

with respect to the normal of the interface

60o

30on

orm

al

Incident ray

interface

Reflection

• The law of reflection is always valid! The angle of incidence is always equal to the angle of reflection. This is why “rough” surfaces reflect light in all directions

Specular Reflection

Diffuse Reflection

Refraction

Refraction of Light Beam

Refraction

• Refraction occurs when a wave changes speed as it passes from one medium to another with different characteristics.

• Electromagnetic waves travel at 3 x 108 m/s (c) in a vacuum

Transmission Across a Boundary

• The only time a wave can be transmitted across a boundary, change its speed, and still not refract is when the wave approaches boundary in a direction which is perpendicular to the boundary.

Consider a light ray which traverses a thick slab

– ray bends towards the normal upon entering the glass

– ray bends away from the normal when it exits from the glass

– exiting light ray is at same angle as original ray, but is shifted over to one side

Why refraction happens?

• Because speed of light changes.

• Because light is a wave.

• Because its wavelength changes in different medium.

Fata Morgana

Halos

Speed Change

• A ray of light crossing the boundary from a fast medium to a slow medium bends toward the normal.

• A ray of light crossing the boundary from a slow medium to a fast medium bends away from the normal

REFRACTION

Index of Refraction

Every substance has an optical density called the substance's index of refraction (n).

The index of refraction indicates is how well light passes through the substance (vs ) compared to how fast light travels through a vacuum (c).

c

vs

speed of light in vacuumspeed of light in medium

n

Index of Refraction

• Index of Refraction is a measure of optical density

• Represented by n• The higher n is, the more optically dense the

material and the slower light travels in the material

Laws of refraction

• Index of refraction depends on:– The medium– The wavelength (or

color of the light)

Indices of Refraction

Refraction is Dispersive

Light of different frequencies is refracted by different amounts

Rainbows are a result of dispersion

Table of Indices

Blue Yellow Red

(486.1 nm) (589.3 nm) (656.3 nm)Crown Glass 1.524 1.517 1.515

Flint Glass 1.639 1.627 1.622

Water 1.337 1.333 1.331

Cargille Oil 1.53 1.52 1.516Carbon

Disulfide 1.652 1.628 1.618

Material

Snell’s Law• Snell’s Law relates the index of refraction

to the angles of incidence and refraction.

ni sin i = nr sin r

ni is the refractive index of the medium the light is leaving,

i is the incident angle between the light ray and the normal to the medium to medium interface,nr is the refractive index of the medium the light is

entering,r is the refractive angle between the light ray and the normal to the medium to medium interface.

Question 1(1) aim directly at

the image

(2) aim slightly above

(3) aim slightly below

To shoot a fish with a gun,

should you aim directly at

the image, slightly above, or

slightly below?

Answer 1Due to refraction, the image

will appear higherhigher than

the actual fish, so you

have to aimaim lowerlower to

compensate.

(1) aim directly at the image

(2) aim slightly above

(3) aim slightly below

Question 2To shoot a fish with a laser gun, should you aim

directly at the image, slightly above, or slightly

below?

(1) aim directly at the image

(2) aim slightly above

(3) aim slightly below

Answer 2

light from fishlaser beam

(1) aim directly at the image

(2) aim slightly above

(3) aim slightly below

The lightlight from the

laser beam will

also bendbend when it

hits the air-water

interface, so aimaim

directlydirectly at the fish at the fish.

Total Internal Reflection

Internal Reflection

• All rays reflect internally, but the top three rays reflect only a small percentage internally; most energy leaves the prism.

• The fourth and fifth rays are reflected 100 % internally

Total Internal Reflection

• Total internal reflection is a special case of reflection created when when a light passes from a more optically dense medium to a less dense one at an angle such that there is no refracted ray.

Fiber Optics

• Total internal reflection is used to send signals through fiber optic cables.

Total internal reflection• There is a critical angle for

every boundary• When light hits the

boundary at this angle it is no longer refracted, but reflected!

Remember !! this only works when the incident medium has the higher index of refraction.

WHY?

Critical Angle

• The critical angle is the angle of incidence which causes the angle of refraction to be 90°, so Snell’s law can be written as

n1sin c = n2 sin 90o

• The higher the index of refraction the lower c

Critical Angle

• When you exceed the critical angle, you

have total internal reflection

Diffraction

• Diffraction is the change in direction of a wave as it encounters the edge of a barrier.

Wavelength vs Diffraction

• Shorter wavelengths bend the least when encountering the edge of a barrier.

• They are in contact with the edge for a shorter period of time.

Double-Slit Interference

Brightfringes

Thomas Young (1802) used double-slit interference to prove the wave nature of light.

• Remember that white light contains

all the colors of the

s p e c t r u m

each color in the spectrum has a different wavelength and so bends at a different rate.

The Visible Spectrum