Light

By: Lara Joy Macarine

Other sources of Light

Many substance gain energy and emit light without being heated very much. they do this through a process called luminescence.Some lumminescent materials glow in the long after they have recieved extra energy. They are said to be phosphorescent. Their atoms stay excited for some time before they de-excite and emit light. Certain phospherescent materials are used in the markings that glow on watch faces. Other luminescent materials emit light only during their exposure to exciting energy. They are said to be fluorescent.

Fireflies and a few other types of organisms emit light by a process called bioluminescence. In this process, chemicals within the organisms combine to produce a different chemical that has excited atoms. When the atoms de-excite, they emit photons.

The sun shines because nuclear reactions between hydrogen atoms within its core produce a tremendous amount of energy. Photons and other kinds of particles carry the energy wto the suns surface. At the surface, these particles mexcite atoms that then de-excite by emitting light. The Earth recieve part of the Light .

An aurora such as the northern lights is an emission of light by molecules of air.When high-speed particles arrive at the Earth from large eruptions on the sun, they crash into air molecules. These collisions excite the molecules with extra energy. The molecules then release the energy by giving off light. When the collisions occur at night the light emitted may be bright enough to be seen.

A laser is device that produces a powerful, narrow beam of light in which all the photons have the same energy travel in the same direction. Lasers serve as tools so scientific research surgery, and telephone communications. They also have many industrial and military users

Scientists thought of light as a wave that travels much like a water wave. This idea of light as a wave was popular because it explained experiments in which light created a series of bright and dark lines called an Interference pattern.

They assumed that light waves must also travel through some kind of material, just as water waves travel through water. Although scientists had no evidence of this material, they called it the ether. By the late 1800's, scientists had concluded that light waves consist of regions of force known as electric feilds and magnetic feilds.

A simple moel of light wave begin with a ray (is straight line) that shows the direction of the light's travel. Along the ray and perpendicular (at right angles) to it, short arrows represent the electric feild. Some arrows point upward fom the ray and other arrows point downward from it. They vary lenght so that the overall patternof the tips of the arrows looks like a wave. Arrows representing the magnetic feild also resemble a wave, but these arrrows make right angles to the arrows that represent the elctric feild. These patterns move along the ray. They are the light.

By the early 1900's, exxperiments had shown that scientist finally had to give up the idea of an ether. Many scientist realized that a wave of light, as a regular varying pattern of electric and magnetic feild, can travel through empty spaces.

Light waves resemble other types of waves in some features, including wavelength, frequncy and amplitude. The wavelength is the distance along a straigt line from one crest (peak) of the wave to the next. The frequency of a wave is the number of times each second that crests a stationary checkpoint. The amplitude of a wave is the greatest distance of a crest or trough (low point) from the ray.

Remember! A simple relation exists betweeen a wave's frequency and wavelength.

The Higher the frequency, the shorter the wavelength.A wave energy corresponds to its amplitude. The greater the amplitude, the more energy the wave has. The energy of a light wave also corresponds to its frequency. The wavelength determines the color of the light. Just like a wave.

where c is the speed of flight in m/s

f is the frequency in Hz

is the wavelength in m

fc

Is Light a Wave or a Particle?

The best answer is that light is strictly neither. In some experiments light behaves like a wave, and in others it behaves like a particle.

Unlike other kinds of waves, light waves in a vacuum have one speed, and thats peed is the fastest that anything can travel. Scientist do not understand why this is true. The fact that light in a vacuum has only one speed forms one of the foundations of Einstein's theory of relativity.

When light enters a material, it continually runs into atoms that delay its travel. But between atoms, light travels at its normal speed.

Light is a transvers wave, meaning that the vibrations are perpendicular to the motion and speed of the wave. This is similar to the Slinky held up in the air between you and your friend, where you move the end up and down vibrations in the slinky. If you do this with a set of frequency, you can see standing waves in the Slinky. If you just jerk the end of the Slinky up and down once, you can watch the vibration move from your end to your friends end. This is a moving or propagating transverse wave. So, in this case, the vibration is up and down and the motion are perpendicular to each other.

Light starts as a vibrating magnetic feild. The magnetic feild in a perpendicular direction. The vibrating feild vibration generates a changing electric feild in a perpendicular direction. The vibrating electric feild - that is, in the direction of the roriginal magnetic feild.

These interacting magnetic and electric feilds propagate in the direction perpendicular to both the electric and magnetic feild and spread through spaces as a transverse wave. The successive generate of changing electric and magnetic feilds can only propagate waves at the speed of light, c , which equals 2.998 x 10 m/s in vacuum.

Another term for light, electromagnetic radiation, refers to its generation from alternately oscillating electric feilds. These are the only waves that can travel in a vacuum - sound and water waves need a medium like air or water to transmit the disturbance.

Light is a very small slice of the electromagnetic spectrum, just the wavelengths that our eyes can see, about 380 - 7600nm. Longer wavelengths (lower energy waves) are microwave, TV and radio waves , while shorter wavelengths (higher energgyy waves) are ultraviolet, X-rays and gamma rays.

Electromagnetic Waves

Because light consists of electric and magnetic feilds, it is called an electromagnetic wave. The term light commonly refers to just chose electromagnetic waves that we can see. For light to be visible, it must have a wavelength within a certain narrow range of values called the visible spectrum.

Violet light has the shortest wavelength that is visible. Red light has the longest. Between them lie all the other colors of the spectrum, each with its own wavelength. Seen together at the same time, the colors appear as white light. Sunlight is white because it has all it colors. however, when it passes through a specially shaped transparent solid called a prism, the different colors separate and can be seen.

Waves that have wavelengths slightly too short to be seen are called Ultraviolet Rays. They cause suntan, sunburn, and skin cancer. Waves with somewhat shorter, wavelengths than ultraviolet rays called X-rays. These rays can penetraite a person's body. Doctors and dentists use them to "see" inside the body. Gamma Rays have even shorterwavelengths than X-rays. They result fomm nuclear reactions, such as those in the sun.

Waves with wavelength slightly longer than those of red light are called infrared rays. When you stand in bright sunlight or in front of a fire, you feel warm largely because of the infrared light shinng on you. Microwaves and radio waves have no longer wavelengths than infrared waves.

Sunlight spread into its different colors by a prism and creates a continous spectrum. From violet to red, the spectrum blends smoothly from one color to the next. For example, the yellow comes from sodium atoms. each type of atom can produce only certain colors.

Scientists can learn what kinds of atoms up a light source by observing what colors are present in the light. They direct the light through an instrument called a spectrumeter to separate the colors. The spectrometer may be simple prism or it may be a more complicated device.Each type of atom in the sun's atmosphere absobs certain colors. By nothing whiich colors are removed, scientists are able to determine what kinds of atoms are in teh atmosphere of the sun.

How Light Behaves

The study of light is called optics. By understanding how light behaves, scientists have learned to design a variety of optical instruments that aid in the study of the universe. For example, microscopes enables us to examine extremely small objects, such as single-celled organisms. With telescopes, we can observe distant but very large objects, such as galaxies and planets. Optics also enables us to undersatnd vision, the colors of the sky, the sparkle of a diamond, and many other parts of the everyday world.

Illumination

A body that gives off light is called luminous, like all those glow-in-the-dark toys, and a body that gives off light when it is heated is called incandescent when you can see it reflects light towards your eyes.

The amount of light can be described by giving the radiant flux, the maount of energy radiated in a unit of time by an electromagnetic wave source. The part of this radiation that contributes to our being able to see (the electromagnetic radiation is only the visible range of wavelength) is called the luminous flux, F , and is measured in lumen (lm).

Illuminance, E , is the luminous flux per ssquare meter and is measured in lux.

If you do not have constant illumination in all directions, you can specify the luminous intensity of a source in a particular direction, I , using the unit candela (cd)

Reflection

When ray of light reaches a surface between two types of materials, such as air and glass, several things can happen. Some of the light may reflect from the surface, while some may pass through the surface. The light that enters the second material may refract (cange its direction). In addition, some light may be absorbed by molecules on the surface or within the seconf material.

A transparent material lets light rays pass through it without mixing them up. You can see through such material. A translucent material also allows rays to pass through it, but it mixes them up so that you cannot see clearly through the material. An opaque material blocks all light.

The reflection of the ray from a surface resembles the bounce a pool ball takes at the edge of a pool table. Imagine a line perpendicular to the reflecting surface. Such line is usually called the normal. The angle between the path of an incoming ray and the normal is called the angle of incidence.

The reflected makes the same angle to the normal ray as the incoming ray, but on the other side of the normal. Reflection works this way even when it involves rough surfaces. Wherever a ray reflects from a surface, it has an equal angle to the normal at that spot as it had before.

Waves behave according to the Law of Reflection, which states that the angle of reflection equals the angle of incidence. Angle of incidence is usually defined as the angle of the incoming wave relative to the normal angle, which is perpendicular to the reflecting surface. The angle of reflection is also relative to "normal".

A wave Reflects when some otrthe entire wave bounces back from a boundary between two different media.If you shine a flasjlight directly at a flat miror, the light bounces back to travel in exactly the opposite direction.

Is you have a nice, clean, flat, shiny mirror, the light will still be recognizable as the beam from the flashlight. This is called a specular reflection. The diffuce reflection the reflected light goes in many different directions and is so much more spread out.

Refraction

Waves that do not reflect back from a boundary but travel into the new medium instead are said to refract.

When light passes through a surface, its speed changes. This happens because the light must travel through a different kind of molecule than it passedcthrough before. For example, if light passes from air into glass, it slows because the glass molecules are more densely packed than the air molecules. If the light enters at any angle except a right angle, the change in the lights speed changes its direction of travel. In other words, the light refracts.

Snell's Law

Snell's Law gives a relationship between indices of refraction, the angles of incidence and refraction, and speeds in this problem.

We have a relation

This result is known as Snell's law after its discoverer, the seventeenth-century Dutch astronomer WillebrordSnell.To observe refraction, place a pencil in a glass of water and then look at the pencil from top and one side. The pencil appears bent at the water surface. The light from the top part of the pencil comes directly to your eyes. The rays from the bottom part pass through the surface between the water and the air. There the rays refract, and so they seem to have come from a pencil bottom bent from the pencil top.

v

Index of Refraction

The ratio between the the speed of light and it speed in a particular medium. The greater the index of refraction, the greater the extent to which a light beam is reflected on entering or leaving the medium. The symbol for the index of refraction is , so that

c v

n

vcn /

List of values of n for a number of substance

Substance n

Air 1.0003

Benzene 1.5

Carbon Disulfide 1.63

Diamond 2.42

Ethyl Alcohol 1.36

Glass, crown 1.52

Glass, flint 1.63

Ice 1.31

Lucite and plexiglass 1.51

Quartz 1.46

Water 1.33

Zircon 1.92

1 2

The index of refraction of any material is always compared to a vacuum, which is given as n=1. (Air's index is 1.0003 and for most purposes can be rounded to 1.0). Water has an index equal tp 1.333, and most optical glass has an index of about 1.5 to 1.6.

it is easy to write Snell's Law in terms of the indexes of refraction n and n of two successive media. This is usually written in the form

n 1 isin n2 rsin

Absorption

Opaque materials absorb certain colors of lights. For instance, a red book cover exposed to white light looks red because molecules on its surface absorb all other colors in the light. Transparent materials also absorb certain colors if they contain dyes or pigments.

Scattering describes what happens when light rays strike atoms, molecules, or other individually, tiny particles. These particles send the rays of light off in new directions that is, they cause the rays to scatter.

On a clear day, the ocean appears blue because of two processes: 1. the ocean's surface reflects some of the blue light from the

sky toward the observer. 2. light coming directly from the sun enters the water. The

water molecules then scatter more blue rays toward the observer than they do the other colors in sunlight.