CHAPTER 16 LIGHT

CHAPTER 16LIGHT

THE FACTS OF LIGHT

LIGHT IS THE RANGE OF FREQUENCIES OF ELECTROMAGNETIC WAVES THAT STIMULATES THE RETINA OF THE EYE.

THE SHORTEST WAVELENGTHS ARE SEEN AS VILET LIGHT. AS WAVELENGTH INCREASES THE COLORS CHANGE FROM

INDIGO, BLUE, GREEN, YELLOW, ORANGE, AND RED. LIGHT TRAVELS IN A STRAIGHT LINE

KNOWN BECAUSE IF THE LIGHT FROM A FLASHLIGHT IS MADE VISIBLE BY DUST PARTICLES IN THE AIR, THE PATH OF LIGHT IS SEEN TO BE A STRAIGHT LINE.

THE SPEED OF LIGHT

BEFORE 17TH CENTURY, MOST PEOPLE VELIEVED THAT LIGHT TRAVELS INSTANTENOEOUSLY

DANISH ASTRONOMER, OLE ROEMER DISCOVERED THAT IT TOOK 22 MINUTES FOR LIGHT TO CROSS THE DIAMETER OF EARTH’S ORBIT. HE FOUND THIS BY USING CALCULATIONS HE MADE WHEN HE WAS

OBSERVING JUPITER. IN 1926 AMERICAN PHYSICIST MEASEURED THE SPEED OF

LIGHT TO BE 2.997996 ± 0.00004 x 108 FOR MOST CALCULATIONS 3.00 x 108 IS USED

SOURCES OF LIGHT

SUNLIGHT = LUMINOUS BODY A LUMINOUS BODY EMITS LIGHT WAVES

MOONLIGHT = ILLUMINATED BODY AN ILLUMINATED BODY REFLECTS LIGHT WAVES PRODUCED BY AN

OUTSIDE SOURCE

LUMINOUS FLUX

THE RATE AT WHICH VISIBLE LIGHT IS EMITTED FROM A SOURCE IS CALLED THE LUMINOUS FLUX. (P)

THE UNIT OF LUMINOUS FLUZX IS CALLED THE LUMEN (lm)

THE ILLUMINATION OF A SURFACE IS CALLED THE ILLUMINANCE (E) ILLUMINANCE IS MEASURED IN LUMENS PER AQUARE METER (WHICH

IS ALSO CALLED LUX)

LUMINOUS INTENSITY

SOME LIGHT SOURCES ARE SPECIFIED IN CANDELA, OR CANDLE POWER. (cd)

THE LUMINOUS INSTESITY OF A POINT SOURCE IS THE LUNINOUS FLUX THAT FALLS ON 1 m2 OF A SPHERE 1 m IN RADIUS.

THE LUMINOUS INTENSITY IS THE LUMINOUS FLUX DIVIDED BY 4p

HOW TO ILLUMINATE A SURFACE

THERE ARE TWO WAYS TO INCREASE THE ILLUMINATION ON A SURFACE. USE A BRIGHTER BULB MOVE SURFACE CLOSER TO THE BULB

TO FIND ILLUMINANCE USE THE FOLLOWING EQUATION

E = P 4pd2

EXAMPLE PROBLEM

WHAT IS THE ILLUMINATION ON YOUR DESKTOP IF IT IS LIGHTED BY A 1750-LM LAMP THAT IS 2.50M ABOVE YOUR DESK?

KNOWN LUMINOUS FLUX, P=1750 lm D = 2.50m

UNKNOWN ILLUMINANCE, E=?

CALCULATIONS FOR EX. PRBM

E = P 4pd2

E = 1750 lm/ 4 (2.50)p 2 = 22.3 lm/m2 = 22.3 lx

• When white light passes through water or glass it can often separate into a rainbow or spectrum.

• The spectrum can be revealed by passing white light through a prism.

• The colors in the spectrum are arranged in the order red, orange, yellow, green, blue, indigo, and violet.

Additive Color:• White light can be formed by

combining the correct intensities of colored light.

• Primary Colors, or colors that can be mixed in pairs to form three different colors, are blue, red, and green.

• Secondary Colors, or colors formed by combining pairs of primary colors, are yellow, cyan, and magenta.

• Two colors that can be combined to form white light, such as yellow and blue light are called Complementary Colors.

Subtractive Color:• A dye is a molecule that absorbs

certain wavelengths of light and transmits or reflects others.

• A pigment, like a dye, absorbs some wavelengths of light and reflects others. They, however, are larger and can be seen with a microscope.

• Primary Pigments, pigments that absorb only one primary color from white light and reflect the other two, are yellow, cyan, and magenta.

• Pigments that absorb two primary colors from white light and reflect only one are called Secondary Pigments. These are red, blue, and green.

• If complementary pigments are combined, then all colors will be absorbed and no light will be reflected resulting in black.

• Color spectrums produced in soap bubbles or oily film on water are not the result of separation by a prism or absorption by a pigment.

• It also cannot be explained using a ray model of light. It is instead a result of the constructive and destructive interference of light waves called thin film interference.

• When a light wave strikes the film part of it is reflected and part is transmitted.

• The transmitted wave travels through to the back surface, where, again, part is reflected.

• Different wavelengths being reflected at different points is what results in the spectrum.

• Polaroid material contains long molecules that allow waves of one direction to pass through while absorbing waves vibrating in other directions.• One direction of the Polaroid material is called the polarizing axis and only waves vibrating parallel to the axis can pass through.

• Ordinary light contains electromagnetic waves vibrating in every direction which can be resolved into the two perpendicular components.

• This means that about half of the waves vibrate on one plane while the rest vibrate on the other and only waves in one plane will pass through and the intesity of the light is cut in half.

• The polarizing material produces light that is polarized in a particular plane of vibration and is therefore called a polarizing filter.

• If a second polarizing filter is placed in the path of the polarized light perpendicular to the direction of vibration no light will pass through.

• If it is placed at an angle, however, then the component of light that is parallel to the filter will be transmitted.

• This means that a polarizing filter can be used to determine the orientation of polarization and is therefore often called an analyzer.

• Light can also be polarized by reflection. When looking at reflected light through a polarizer the brightness can change as the polarizer is rotated suggesting that the light is already partially polarized.

LightChapter 16

By: Tony Maruca, Brian Lauff, And Marc Zeno

The Basics of Light

Light- the range of frequencies of electromagnetic waves that stimulates the retina of the eye.

Facts

Light does not travel instantaneously. Light travels fast enough to circle the world seven and a half

times in one second. Light travels at exactly 299,792,458 or 3.00 x 108 m/s in a

vacuum Equation, c=λf Albert A Michelson was the first American to win a Nobel prize

due to his studies with light.

The Wavelength Spectrum

The small band of wavelengths that are visible make up the colors that we see in everyday life.

Equation

Sample Problem

What is the frequency of red light? Solve equation for the variable of frequency

c = λf, f/λ = c/λ so f = c/λ f = 3.00x108/665x10-9 f = 4.5x1014

Sources of Light Light is emitted by a luminous body which gives off the different waves

to a illuminated body which reflects the light. A luminous source for example would be the head lights of a car which

gives off light by using a bulb with a current running through an element. An example of a illuminated body would be a the direction arrows on

warning signs which are designed to reflect a cars headlights to grab the attention of the vehicles driver.

We register light when different wavelengths reach our eyes. The different sensitivities cause us to see different colors.

The Luminous Flux This is the rate at which visible light

is emitted from a source. The illumination of a surface is

called its illuminance. This is also the rate at which the light falls onto a surface.(E)

This is measured in lumens per square meter or lux.

Most light sources are measured in candela, which is the representation of luminous intensity.

Luminous intensity of a point is where the luminous flux falls on a m2 of a sphere one m in radius.

Equation E = P/4d2

Light and Matter Some objects let light travel through

where others will not allow light to travel through at all.

Objects that give no restriction to light are called transparent.

Objects that give some restriction to light are called translucent.

Objects that let no light through are called opaque.

Color is one of the most beautiful things to the human eye. This phenomena is caused when light goes through a spectrum which bends light.

Color can also be created through the mixing of different colors.

There are primary colors and secondary colors. The primary colors are red, green and blue.

Those colors can be mixed together to make the secondary colors which are magenta, cyan and yellow..

Colors can also be created by subtraction of waves. This involves pigments which absorb certain colors. Just like with colors, there are primary pigments and secondary pigments.

Thin-film Interference This is the last way of making light. This is done by using a film which disrupts the

lights waves so that only some of the waves are visible.