Light Waves

What is Light?

Light is the range of frequencies of the electromagnetic spectrum that stimulate the retina of the eye.

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Light & Matter

Transparent (Glass): Materials that allow light to pass through without distorting images.

Translucent (Cloudy Glass): Materials that allow light to pass through them, but do not allow them to be seen clearly.

Opaque (Brick): Material in which all light is absorbed or reflected. Not is transmitted through.

Reflection and Absorption of Light and Color

The color observed by any object is the same as that not absorbed by the object. For example, a red block will absorb all colors of the EM visible light spectrum except for red. How does this apply to the clothes we

wear?

Continuous Waves When a wave impacts a boundary, some of the

energy is reflected, while some passes through. The wave that passes through is called a

transmitted wave. A wave that is transmitted through a boundary

will lose some of its energy. Electromagnetic radiation will both slow down and have

a shorter wavelength when going into a denser media. Sound will increase in speed when transitioning into a

denser media. Speed of Light in different mediums

Higher speed

Longer wavelength

Lower speed

Shorter wavelength

Continuous Waves – Higher Speed to Lower Speed Note the differences in wavelength and amplitude between

of the wave in the two different mediums

Displacement

Boundary

Incident + Reflected Wave Transmitted Wave

v1 v2-v1

Note: This phenomena is seen with light traveling from air to water.

Incident + Reflected Wave

Lower speed

Shorter wavelength

Higher speed

Longer wavelength

Transmitted Wave

-v1

v2

v1

Continuous Waves – Lower Speed to Higher Speed Note the differences in wavelength and amplitude between

of the wave in the two different mediums

Displacement

Boundary

Law of Reflection The angle of incidence with respect to the

normal is equal to the angle of reflection.

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Specular & Diffuse Reflection Light incident upon an object with a smooth

surface will create specular reflection. Light incident upon an object with a rough

surface will create diffuse reflection.

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Refraction of Light When light travels through a surface between

two different media, the light will be refracted if the angle of incidence is greater than zero.

If light is passing into a more dense media, it willbend towards a normal with the boundary.

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Law of Refraction (Snell’s Law) The ratio of the sine of the angle of incidence to the angle

of refraction is a constant.

n1 sin1 = n2 sin2

Where:n1, n2 = index of refraction

1 = Angle of incidence

2 = Angle of refraction Note, the incident ray will always

bend towards the normal when transitioning from a material with a lower index of refraction to one with a higher index of refraction.

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Speed of Light and the Index of Refraction

The index of refraction, by definition, is the ratio of the speed of light in a vacuum to the speed of light in a substance.

The index of refraction is always greater than 1.

cn

v

Total Internal Reflection When the angle of incidence is such that the

angle of refraction is equal to 90o, the critical angle (c) has been attained.

All rays will be reflected internally at all angles greater than this angle.

c = sin-1 (n2/n1)

Note: Internal reflection canonly occur if n2 < n1.

Internal Reflection (Use PHET) Application – fiber optic

cable

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1 2

1

2

Homework: Light ILight I: Chapter 16 16, 18, 31, 32, 33, 41  16. Wavelength decreases as freq inc. 18. Screen B has ¼ the illumination of Screen A 31. 7x 10E-7m 32. d = vt = 3E8m/sec * 1.28s = 3.84E8m 33. d = vt d/v = t = 1.5E8km/3E8m/sec = 1.5E11m/3E8m/sec = 500 sec 41.d = vt = 3E8 * 0.1s = 3E7m = 3E4km.

Since we are talking about a round trip time, a distance of 1.5E4km is needed.

Reflection Lab Angles

Angle 1 Angle 2 Angle 3 Angle 4 Angle 5 Angle 6

A 10 50 15 25 20 30

B 30 20 45 25 35 30

C 55 40 30 10 35 45

D 45 65 50 40 10 55

E 40 55 45 60 50 30

F 60 50 65 45 30 60

G 20 30 60 45 55 15

H 50 20 40 60 10 30

Have you ever seen this?

Mirages can happen at any time of the year,but they are more commonly seen during the summer due to the hotter temperatures. The temperature difference between the layers of air is more important to the formation of a mirage than the actual temperature. The greater the difference in temperature the greater the bending effect.

Science (cont.) There are two types of mirages.

Superior and inferior mirages. Mirages are caused by light rays passing through a layer of hot air near the surface. This causes a bending effect. When the light rays are refracted a mirage is created.

Light Passing Through Glass

θ1

θ4

θ3θ2

IncidentRay

ReflectedRay

RefractedRay

Air AirGlass

Note: 1 = 4 2 = 3

The light exiting the glass is parallel to the light entering it.

Start Period 1 here

Sunset is an illusion

Light IIChapter 17 p. 410 5, 6, 8, 10, 115. Angle of incidence is greater than angle of refraction6. Angle of incidence is less than angle of refraction. 8. index of refraction times sine of angle of incidence = index of refraction times sine of angle of refraction10. Angle of incidence that produces an angle of refraction whose sine is 90 degrees.11. Total internal reflection.

Chromatic Dispersion When white light enters a medium, the different

wavelengths that comprise the light will travel at different speeds.

If the angle of incidence is greater than zero, the wave will exhibit chromatic dispersion.

Note: The shorterthe wavelength, thegreater the bending.

What happens tothe frequency? NOTHING

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Diffraction of Light When a wave front is incident on a barrier with an opening,

the wave will spread out after crossing the barrier. This process is called diffraction. Diffraction is an interference phenomena.

As the slit becomes narrower, the amount of diffraction will increase.

As the wavelength of light increases, the amount of diffraction will increase.

Diffraction

Is light a Wave?

Young Double-Slit Experiment: The wave properties of light were first

demonstrated by Thomas Young in 1801. Showed that light undergoes interference in

and diffraction in much the same way that water and sound waves do.

Used a source of monochromatic light so that only one wavelength was chosen. Also used light with no phase difference.

Young Double-Slit Experiment

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Huygen’s Wavelets

Young Double Slit Experiment

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Young Double Slit Experiment

(delta) = r2 – r1

If r2 – r1 is equal to some multiple of , then the image on the screen will be a maximum (constructive interference).

If we assume that D is very big and r1 and r2 are parallel, then the angle between them will be .

= d sin or

m = d sin d

r1

r2

D

Young Double Slit Experiment To determine the distance y between the central and first

maximum, we will again assume that D is very large compared to y or d.

From the diagram, we see thattan = y/D

However, for small angles of , tan = sin Therefore, we

can substitutey/D for sin and get:

d

r1

r2

D

y

m Dy

d

Young Double Slit Experiment

What are the implications of the formula?

As wavelength () and distance (D) to the screen increases, the distance between maximums increases.

As the distance between slits (d) increases, the distance between maximums decreases.

m Dy

d

Key Ideas Transverse waves such as electro-magnetic

radiation do not require a medium. Light Waves travel at different speeds in

different mediums. It slows down when going from air to a liquid or solid.

Waves can interfere with one another resulting in constructive or destructive interference.

The law of reflection states that angle of incident wave equals the angle of the reflected wave.

Key Ideas Snell’s Law / Law of Refraction: A wave will bend

toward the normal when transitioning from a media with a low index of refraction (e.g. air) to a media with a higher index of refraction.

Total internal reflection occurs when the angle of incidence is greater than the critical angle. Consequently, no light will escape.

Diffraction is the spreading out of a wave when it encounters a barrier.

Thomas Young’s double slit experiment showed that light has wave properties similar to water and sound.