LightLightLightLight Chapters 16 and 18. Light is an electromagnetic wave Electric field waves are...

LLiigghhttChapters 16 and 18Chapters 16 and 18

Light is an electromagnetic wave

• Electric field waves are perpendicular to magnetic field waves.

• Both are perpendicular to the direction the wave is traveling.

• This makes light a transverse wave.

• All electromagnetic waves are caused by vibrating charges.

• Electromagnetic waves can travel through a vacuum

Electromagnetic SpectrumElectromagnetic Spectrum (longest wavelength to shortest wavelength)

1. Radio and TV 2. Microwaves 3. Infrared 4. Visible Light

5. Ultraviolet 6. X-rays 7. Gamma Rays

• Violet light has a wavelength of about 400 nm• Red light has a wavelength of about 700 nm

Different wavelengths of light Different wavelengths of light have different colors.have different colors.

Color SpectrumColor Spectrum (from longest to shortest wavelengths)(from longest to shortest wavelengths)

ROYGBIV 1. Red1. Red

2. Orange2. Orange

3. Yellow3. Yellow

4. Green4. Green

5. Blue5. Blue

6. Indigo6. Indigo

7. Violet7. Violet

Electromagnetic waves

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0% 1. can travel through a vacuum

2. need a medium to travel through

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Which of these electromagnetic waves has the shortest wavelength?

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0% 1. Radio

2. Infrared

3. X-rays

4. Ultraviolet

5. Light

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Which color of light has the highest frequency?

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0%

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0% 1. Blue

2. Green

3. Orange

4. Yellow

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Pathway of Light

• Light travels in a straight line in a vacuum or other uniform medium.

• The straight-line path of light has led to the ray model of light.

• A ray is a straight line that represents the path of a narrow beam of light.

Galileo was first to try to measure Galileo was first to try to measure speed of light.speed of light.

• Before Galileo, everyone thought light had no speed, but instead traveled instantaneously.

• Galileo was the first to hypothesize that light had a finite speed.

• Used lanterns with shutters as first experiment.

• Decided light was too fast to measure.

Olaf Roemer was the second to tryOlaf Roemer was the second to try

• Used time it took Jupiter’s moon Io to eclipse Jupiter.

• Recorded the time it took Io to emerge from behind Jupiter.

• As the Earth moved further and further away from Jupiter, the longer it took.

• He calculated that light took 22 minutes to cross the diameter of the Earth (speed of 220 million meters /sec)

First American Nobel prize First American Nobel prize winner: Albert Michelsonwinner: Albert Michelson

• Developed Earth based techniques to measure the speed of light

• In 1926, successfully measured speed of light.

• Results were within .001 % of currently accepted speed of light

Michelson’s Technique

• His technique was to use two mountain peaks in California 35 km apart, then time how long it took light to bounce back.

• Used an interferometer for timing device

The speed of light is a set valueThe speed of light is a set value

C = 300,000,000 m/s

= 3.00 x 108 m/s = 300,000 km/s = 186,000 mi./s

Speed of light, wavelength of light, Speed of light, wavelength of light, frequency of light relationshipfrequency of light relationship

c = f• c = speed of light (m/s) = wavelength of light (m)

• f = frequency of light (Hz)

Compared to the velocity of radio waves, the velocity of visible light waves is

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0% 1. slower

2. faster

3. the same

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The main difference between a radio wave and light waves is its

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0% 1. Speed

2. Wavelength

3. Both 1 and 2

4. None of the above

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The main difference between a radio wave and a sound wave is its

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2. wavelength

3. energy

4. amplitude

5. basic nature

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Which of the following was not one of Galileo’s assumptions about the speed of

light?

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0% 1. The speed of light can’t be measured.

2. Light is very fast.

3. Light travels instantaneously.

4. None of the above.

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Light takes 1.28 s to travel from the Moon to Earth. What is the distance between them?

0%

0%

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0% 1. 2.3 x 108 m

2. 3.8 x 108 m

3. 7.7 x 108 m

4. 440 m

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Luminous

• Objects which create light are said to be Luminous

• Examples: Sun, stars, light bulbs

ILLUMINATED

• Objects which reflect light are said to be illuminated

• These objects do not create their own light• examples: Moon, planets, desk, whiteboard,

reflectors, people...

Luminous Flux

• Luminous flux (P) is the rate at which light is emitted from a source.

• The unit of luminous flux is the lumen (lm)

• A typical 100 W light bulb emits approximately 1750 lm.

Illuminance

• Often, we are more likely interested in the amount of illumination an object provides.

• The illumination of a surface is called the illuminance (E).

• Illuminance is the rate at which light falls on a surface.

• Illuminance is measured in lumens per square meter (lm/m2) or lux (lx).

• If the distance of a surface from the point of light is doubled, the illumination is reduced by a factor of 4.

Increasing Illumination

• There are two ways to increase the illumination:1) Use a brighter bulb (increase luminous flux)2) Move the surface closer to the bulb (decrease

the distance)

E = P / 4d2

• E = illumination (lx)E = illumination (lx)• P = luminous flux (lm)P = luminous flux (lm)• d = distance from light source (in meters)d = distance from light source (in meters)

Luminous IntensityLuminous Intensity

• Luminous intensity is measured in units ofLuminous intensity is measured in units of

candelas (cd) candelas (cd) • One candela = # of lumens/4One candela = # of lumens/4ππ

•

Transparent ObjectsTransparent Objects• Allow light to pass through them Allow light to pass through them

undisturbed.undisturbed.

• No trouble identifying objects No trouble identifying objects behind transparent objects.behind transparent objects.

• Examples: glass, transparencies, Examples: glass, transparencies, clear liquidsclear liquids

Translucent ObjectsTranslucent Objects• Light can pass through, but not Light can pass through, but not

clearly.clearly.• Reflect some light, but also allow Reflect some light, but also allow

some light to pass through (transmit)some light to pass through (transmit)• Examples: tissue paper, lampshades, Examples: tissue paper, lampshades,

frosted light bulbs...frosted light bulbs...

Opaque ObjectsOpaque Objects

• Materials which do not allow Materials which do not allow light to pass through.light to pass through.

• Only reflect light.Only reflect light.• Examples: bricks, doors, Examples: bricks, doors,

people...people...

GIVING LIGHT DIRECTION:

POLARIZATION

Polarization of LightPolarization of Light• Unpolarized light vibrates in all directions

in the xyz plane.

• In this illustration the electric field (E) is vibrating on the y-axis, and the Magnetic field (B) is vibrating on the z-axis. The wave is traveling along the x-axis.

Polarization (cont.)Polarization (cont.)

• Polarizers are made of long strands of molecules that are all aligned parallel to each other.

• Look at the blue areas in the illustration.

Polarizers will only transmit light that is vibrating parallel to the direction in which

the polarizer is lined up. Polarizers will not allow light to pass through if the light is

vibrating perpendicular to it.

Polarization (continued)

• Light that does not pass through the polarizing filter is absorbed.

• Since only part of the total amplitude of the wave passes through the filter, the intensity of the light is reduced.

Polarization by Reflection

• Light reflected off surfaces is also partially polarized.

• The light is usually polarized in the same direction as the surface.

• Light reflecting off the road or the surface of a lake is polarized in the horizontal direction.

• Therefore, sunglasses consisting of a vertical polarizing material will help reduce glare produced from this type of reflection.

Polarization Analysis• Suppose light is polarized by passing it through a filter.What happens if a second filter is placed in the path of the

polarized light?• If the polarizing axis of the second filter is parallel to the first,

the light will pass through.• If the polarizing axis of the second filter is perpendicular to the

first, no light will pass through.• To determine the intensity of the light that will pass through a

second filter, Malus’s Law is used.• Malus’s Law states that the intensity of the light coming out of a

second filter is equal to the intensity of the light coming out of the first filter multiplied by the cosine squared of the angle between the two filters.

• Example: If light passes through two filters at 45o angles to each other, the light coming out of the second filter will have 50% of the intensity of the light that came out of the first.I2 = I1cos2θ I2 = 100cos2(45o) = 50

Polarized Light and 3-D Viewing

• Vision in 3 dimensions depends on the fact that both eyes give impressions simultaneously, each eye viewing a scene from a slightly different angle.

• The combination of views in the eye-brain system gives depth.• A movie can be seen in 3-D when the left eye sees only the left

view and the right eye sees only the right view. • This is accomplished by projecting the pair of views through

polarization filters at right angles to each other. The overlapping pictures look blurry to the naked eye.

• To see in 3-D, the viewer wears polarizing eyeglasses with the lens axes also at right angles. In this way, each eye sees a separate picture just as in real life.

• The brain interprets the two pictures as a single picture with a feeling of depth.

3-D Viewing

Light reflected from a lake surface is polarized ____.

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0% 1. vertically

2. horizontally

3. randomly

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In order for sunglasses to be effective in reducing glare produced from a road, the

glasses should be polarized ___.

0%

0%

0% 1. vertically

2. horizontally

3. Both vertically and horizontally

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Glasses used for 3-D viewing are polarized _____.

0%

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0% 1. vertically

2. horizontally

3. Both vertically and horizontally

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If two Polaroid filters are held with their polarization axes at right angles to each other, the amount of light transmitted

compared to when their axes are parallel is ___.

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0%

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0% 1. twice as much

2. the same

3. half as much

4. zero

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The amount of light that gets through Polaroid filters at 25o, compared to the amount that

gets through the 45o Polaroids is

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0% 1. less

2. more

3. the same

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Making Colors by Addition of Light

Primary Light Colors

• There are three primary light colors

• Red, blue, green

• All three colors added together make white light

• Combinations of any of these two produce secondary colors.

Light Color by Addition(secondary colors of light)

• Red + Blue = Magenta

• Blue + Green = Cyan

• Red + Green = YellowYellow

Light color addition (cont.)• Color monitors and TV screens use this

principle

• By varying the intensity of the three colors, any pixel can have any color possible.

Complementary Colors

• Complementary colors are two colors which combine to produce white light.

• Yellow light is the complementary color to

blue light.

• Cyan and red are complementary colors.

• Magenta and green are complementary colors.

The three primary colors of light are

0%

0%

0%

0%

0% 1. red, yellow, and green

2. red, yellow, and blue

3. red, green, and blue

4. yellow, green, and blue

5. yellow, cyan, and red

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When red and green light shine on a white sheet of paper, the resulting color is

0%

0%

0%

0%

0% 1. blue

2. cyan

3. green

4. yellow

5. magenta

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When red and blue light shine on a white sheet, the resulting color is

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0%

0%

0%

0% 1. blue

2. cyan

3. green

4. yellow

5. magenta

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Complementary colors are two colors that

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0%

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0% 1. look good together

2. are primary colors

3. are next to each other in the visible spectrum

4. produce white light when added together

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The complementary color of blue is

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0%

0%

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0% 1. red

2. cyan

3. green

4. yellow

5. magenta

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Colors seen on TV result from color

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0% 1. addition

2. subtraction

3. None of the above

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Shadows

• A shadow is formed where light rays cannot reach.

• Sharp shadows are produced by a small light source nearby or by a larger source far away. This total shadow is called an umbra.

• A partial shadow is called a penumbra and appears when some of the light is blocked, but other light from another source fills in. Penumbras often appear somewhat blurry and are caused by a large light source nearby or a smaller source far away.

The shadow produced by an object held close to a piece of paper in sunlight will

be ____.

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0% 1. fuzzy

2. sharp

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A total shadow is called a(n) ____.

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0% 1. umbra

2. penumbra

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A stage performer stands where beams of red and green light cross. What is the color of her

shirt under this illumination?

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0%

0%

0% 1. red

2. green

3. yellow

4. white

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What are the colors of the shadow that the stage performer casts where both shadows

overlap?

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0%

0%

0% 1. red

2. green

3. yellow

4. black

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If the red light is on the audience’s left and the green light is on their right, what color will the shadow to

the left of the performer appear to them?

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0%

0%

0% 1. red

2. green

3. yellow

4. black

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Making Colors by Subtraction

Color by Subtraction of Light

• Not only can objects reflect and transmit light, but they can also absorb light.

• Colored filters are filters that transmit certain wavelengths of visible light while absorbing others.

We can only see reflected or transmitted

colors

ALL OTHER COLORS ARE ABSORBED

Color by Subtraction (continued)

• The blue filter absorbs red and green, the red absorbs blue and green, and the green absorbs blue and red.

• The cyan filter absorbs red light, the magenta absorbs green light, and the yellow absorbs blue light.

Color by Subtraction (continued)

• When cyan and yellow are mixed, green results.

• When cyan and magenta are mixed, blue results.

• When yellow and magenta are mixed, red results.

Dyes

• A dye is a molecule that absorbs certain wavelengths of light and transmits or reflects others.

• When white lights falls on the red block in figure 16-13 on page 441, the dye molecules absorb the blue and green light and reflect the red. Therefore the block appears to our eyes to be red.

• Observe the changes in the appearance of the color as the light color changes.

Primary Pigments

• Pigments are like dyes except they are larger and can be seen with a microscope.

• Pigments absorb and reflect light rather then illuminate it.

• Primary pigments absorb only one primary color of light.

• The primary pigments are magentamagenta, cyancyan, and yellowyellow

• Mixtures of paints from these primary color pigments can produce any color imaginable.

Secondary Pigments

• A pigment that absorbs two primary colors of light and reflects one is a secondary pigment.

• The secondary pigments are red, green, and blue.

• Note: The primary pigment colors are the secondary light colors and the secondary pigment colors are the primary light colors.

Complementary Pigments

• Red pigment is complementary to cyan.

• Blue pigment is complementary to yellow.

• Green pigment is complementary to magenta.

• Complementary pigments are those pigments that, when mixed, result in a black color.

The cyan color of ocean water is evidence that the water absorbs ___

light.

0%

0%

0%

0%

0% 1. red

2. magenta

3. yellow

4. green

5. blue

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When magenta and cyan light are mixed, the resulting color is

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0%

0%

0% 1. blue

2. red

3. green

4. black

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The mixing of cyan and yellow light to produce the color green is an example of

color by

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0%

0% 1. addition

2. subtraction

3. None of the above. This is not possible.

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A sheet of red paper will look black when illuminated with ___ light.

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0%

0%

0% 1. red

2. yellow

3. magenta

4. cyan

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The color of an opaque object is determined by the light that is

0%

0%

0%

0% 1. transmitted

2. absorbed

3. reflected

4. All of the above.

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What color will a yellow banana appear to be when illuminated with

blue light?

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0%

0%

0%

0% 1. Red

2. Green

3. Yellow

4. White

5. Black

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What color will a yellow banana appear to be when illuminated with

green and red light?

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0%

0%

0% 1. Yellow

2. Blue

3. White

4. Black

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The primary pigment are

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0%

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0% 1. red, blue, and green

2. red, yellow, and green

3. cyan, magenta, and yellow

4. red, blue, and yellow

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Complementary pigments are two pigments that result in a ____ color when

mixed.

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0%

0%

0% 1. white

2. black

3. brown

4. rainbow

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The pigment complementary to magenta is ____.

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0%

0%

0% 1. red

2. blue

3. yellow

4. green

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BEHAVIOR OF LIGHT:

REFRACTION

Refraction

• Refraction is the change in direction, or bending of a wave, at the boundary between two media.

• The beam in the first medium is called the incident ray.

• The beam in the second medium is called the refracted ray.

Note that when the light beam goes from air to glass at an angle, it is bent toward the normal. In this case the angle of incidence is larger than the angle of refraction. In such a case, the new medium is said to be optically dense.

Refraction (continued)

• When a light strikes a surface along the perpendicular, the angle of incidence is zero, and the angle of refraction will also be zero.

• The refracted ray leaves perpendicular to the surface and does not change direction.

Snell’s Law

• When light passes from one medium to another, it may be reflected and refracted.

• The degree to which it is bent depends on the angle of incidence and the properties of the medium.

• Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

• For light going from the vacuum into another medium, the constant, n, is called the index of refraction.

Equation for Snell’s Law

• Snell’s law is written as:n = sin θi / sin θr

where θr is the angle of refraction, n is the index of refraction, and θi is the angle of incidence.

• In the more general case, the relationship can be written as:

ni sin θi = nr sin θr (ni is the index of refraction of the medium in which the

incident ray travels (the first medium) and nr is the index of refraction of the medium in which the refracted ray moves (the second medium)

http://www.sciencejoywagon.com/physicszone/otherpub/wfendt/refraction.htm

Example Problem

• A laser beam in air is incident upon ethanol at an angle of incidence of 37.0o. What is the angle of refraction?

ni = 1.0003

θi = 37.0o

nr = 1.36

θr = x

ni sin θi = nr sin θr

1.0003 ( sin 37.0) = 1.36 x

x = 26.3o

Index of Refraction and the Speed of Light

• The index of refraction is also a measure of how fast light travels when it passes into a medium from a vacuum.

• Index of Refraction nsubstance = c / vsubstance

Example Problem

• What is the speed of light in chloroform?

• nsubstance = c / vsubstance

• 1.51 = 3.00 x 108 /(x)

• x = 1.99 x 108 m/s

Total Internal Reflection

• When a ray of light passes from a more optically dense medium into air, the angle of refraction is greater than the angle of incidence.

• Total internal reflection occurs when light passes from a more optically dense medium to a less optically dense medium at an angle so great that there is no refracted ray.

•Critical angle (θc) – when the incident angle causes the refracted angle to lie along the boundary of the substance (θr = 90o)

•The critical angle is unique to the substance.

•Any ray that is greater than the critical angle can’t leave the substance-all of the light is reflected resulting in total internal reflection.

Equation for Determining the Critical Angle

sin θc = 1.00/nr

Example Problem: What is the critical angle for crown glass?

sin θc = 1.00/1.52

θc = 41.1o

What is the critical angle for water?

sin θc = 1.00/1.33

θc = 48.8o

Applications of Reflected and Refracted Light

Effects of Refraction and Total Internal Reflection• An object may be located at a

greater depth than it appears.

• A submerged object near the surface may appear to be inverted.

• An object near the surface of a pool may not be visible to an observer standing near the edge.

• The effects of total internal reflection are applied in the field of fiber optics.

Atmospheric Refraction

• Mirages, floating images that appear in the distance, are due to the refraction of light in the Earth’s atmosphere.

• On hot days, there is a layer of hot air in contact with the ground and cooler air above it.

• Because light travels faster in the hot air than the cooler air above it, there is a gradual bending of the light rays. This can produce an inverted image to an observer just as if it were reflected from a surface of water.

• www.astronomycafe.net/weird/lights/mirgal.htm

Atmospheric Refraction (continued)

• A similar situation occurs when driving along a hot road that appears to be wet.

• Light from the sky is being refracted through a layer of hot air.• www.dewbow.co.uk/glows/mirage4.html • Mirages are formed by real light and can be photographed (they are

not “tricks of the mind”.)• When the sun sets, you actually see the sun for several minutes after it

has sunk below the horizon because the light is refracted by the Earth’s atmosphere. The same thing occurs at sunrise.

• http://science.howstuffworks.com/mirage2.htm

Dispersion of Light

• The separation of light into its spectrum is called dispersion.

• Red light is bent the least, while violet light is bent the most.

• This means that the index of refraction depends on the color, or wavelength, of light.

• The index of refraction for red light is smaller than it is for violet.

• www.dewbow.co.uk/glows/mirage4.html

BEHAVIOR OF LIGHT:

REFLECTION

The law of reflection states that the angle an incoming light ray makes with the normal (angle of incidence) is equal to the angle the outgoing light ray makes (angle of reflection).

SEEING THE LIGHT!!