Unit 8 The Study of Light

Properties Of Light Because light is a periodic wave, it possesses the characteristics of all periodic waves: Reflection Refraction Interference Diffraction Visible light is part of the electromagnetic spectrum of waves. Electromagnetic waves are transverse and have a constant speed in space.

COMPARISON OF LIGHT AND SOUND Characteristics Light Sound Sim Dif Type of wave Speed of wave Medium Reflection, Refraction and Diffraction Polarization X Transverse Longitudinal 3 x108 m/s 330 m/s X Not needed Needed X All three occur All three occur X Occurs Doesn’t Occur X

“Let there be LIGHT” I. Properties of Light A. General Info Light is classified as an electromagnetic wave. This means its speed in air is 3 x 108 m/s All electromagnetic waves travel at the same speed. They differ in wavelength and frequency It is possible to change either the speed or wavelength of an electromagnetic wave, but not the frequency.

Calculating the Speed of Light The first experiment Galileo flashed his lantern, and the assistant was supposed to open the shutter to his own lantern as soon as he saw Galileo's light. Galileo would then time how long it took before he saw the light from the other hilltop.

The History of the Speed of Light The quest for how fast light moves was continued in 1675 when Olaus Roemer used an eclipse of the moon of Jupiter to measure the speed of light. In the nineteenth century, Albert Michaelson used sunlight and rotating mirrors to obtain more precise measurements.

The Actual Speed of Light As a result of the experiments and Einstein’s special theory of relativity, we know that the speed of light in a vacuum is constant under all circumstances. It has been set at a value of 2.9979x108 m/s The letter c is used to represent the speed of light in a vacuum. The speed of light can change depending on the nature of the medium.

Calculations B. Calculating the speed of Electromagnetic Radiation 1.) Speed in air Use the formula v = d/t to calculate speed, distance, or time for any electromagnetic wave traveling in air. 2.) To calculate frequency or wavelength use: c= fλ c= speed of light f= frequency λ= wavelength

Sample Problem!!! 1.) It takes a light ray 10s to reach your eye. How far did the light ray travel?

Another Sample! 1.) Infrared rays have a frequency of 1 x 1012 Hz. What are their wavelength?

ELECTROMAGNETIC SPECTRUM MORE FACTS of LIGHT Type of Wave: Speed of Wave: Does light carry energy? Give Examples. ELECTROMAGNETIC SPECTRUM Light is a transverse wave. The wave moves perpendicular to the direction of wave motion. Light travels at a speed of 3.0 x108 m/s. Yes. Sunlight from the Sun carries most of the energy for the Earth. Tanning, photosynthesis, solar power, solar heating, etc. Speed of EM Waves: All electromagnetic waves travel at the speed of light. (3.0 x108 m/s)

More on wavelength and frequency… C. Wavelength and Frequency of Electromagnetic Radiation For all electromagnetic radiation, it should be noted that as the frequency of the wave increases, its wavelength decreases. The chart below ranks the types of electromagnetic radiation.

Color Wavelength (m) Avg Frequency (Hz) Violet 4.0 - 4.2 x 10-7 Blue Green 4.9 - 5.7 x 10-7 Yellow 5.7 - 5.9 x 10-7 Orange 5.9 - 6.5 x 10-7 Red 6.5 - 7.0 x 10-7 7.3x1014 6.6x1014 5.6x1014 5.2 x1014 4.8x1014 4.4x1014 Speed of a wave: v = fλ

There is a need for SPEED D. Changes in Speed and Wavelength As an electromagnetic wave changes media, its speed and wavelength will change. The amount of the change depends on the optical density of the medium. The optical density refers to how difficult it is for a wave to travel through a medium. The more optically dense, the more difficult for the wave to get through.

More Notes Optical density is measured using a scale called the index of refraction (n) Air has an index of refraction of 1.00 Values for index of refraction are found in the reference tables The higher the index of refraction, the more optically dense the medium and therefore the harder it is for light to get through

Keep Writing!!! Waves that go from a material with a low index of refraction to a high index of refraction will: --slow down --decrease their wavelength. Waves that go from a material with a high index of refraction to a low index of refraction will: --speed up --increase their wavelength.

Next… The amount the light speeds up or slows down can be calculated using this formula: v= c v=speed in new medium n c=speed of light n=index of refraction

Sample What will the speed of light in crown glass be?

Second Part II. Properties of light waves Light in general has all the same properties of other waves. It has speed, wavelength and frequency It also has the properties of reflection, diffraction, refraction, polarization and dispersion.

Monochromatic Light Monochromatic light consists of light of a single color, that is light of a single wavelength or frequency. If all the colors of visible lights are mixed together; The result is white light. Black is the complete absence of visible light.

Monochromatic Light Monochromatic Light generated so that all of the waves have a constant phase relationship is said to be coherent. Lasers produce intense beams of coherent light.

Diffraction Diffraction occurs when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings.

Interference Demonstrated by passing light through a single or double-slit arrangement. Interference also occurs with the reflected light from thin films and is responsible for the colors seen on soap bubbles and oil slicks.

More Notes… Dispersion A property that is unique to light. Light can come in two forms: monochromatic or polychromatic Monochromatic means the light has one color Polychromatic refers to a multicolored (white) beam Dispersion takes place when a polychromatic beam is separated into a spectrum by a substance

Rainbow Colors Therefore, dispersion is a property of polychromatic light only Sunlight Light is separated into its spectra Prism

Polarization of Light Polarization is the separation of a beam of light so that the vibrations are in one plane. It is an exclusive property of transverse waves. When a light wave is produced, it vibrates in many directions.

Plane Polarization If, however, a beam of light passes through a polarizing filter, the beam that emerges will vibrate in one plane only and is said to be plane polarized. When light is reflected from a nonmetallic surface, it is polarized. This is why polarized sunglasses can be used to eliminate glare

Polar polar….. Polarization A property of light and all other transverse waves Polarization is defined as the ability to organize waves so the ones chosen are all moving in the same direction Normally, a typical light beam consists of a number of waves traveling in different directions in different planes

Give me some more…. Normally, the slits in a polarizer are very tiny (1 X 10-5m), so they don’t bother your eyesight. The fact that light can be polarized proves that it is a transverse wave

Reflection Reflection is the change in direction of a wave. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors and flat surfaces exhibit specular reflection.

Mirror Mirror…. Reflection 1) Regular Reflection Light waves reflect off surfaces just as other waves do The angle of incidence of the light wave equals the angle of reflection It is helpful to draw diagrams when asked to solve reflection problems with light

Sample Problems!!! A light ray hits a mirror with an angle of incidence of 45°. What is the angle of reflection? 45

Another one!!!!!!!! A light ray makes an angle of 60° with a glass window when it strikes it. What is the angle of reflection?

Diffuse Reflection Occurs when light rays reflect off an irregular surface. Since the surface is not smooth, the angle of incidence may not equal the angle of reflection

                                           Moving on Refraction a property of waves caused as the wave changes media in the case of electromagnetic waves, they will bend and change speed as they enter the new medium the frequency of a wave DOES NOT change as it enters the new medium the ray must hit the medium at an angle in order for refraction to take place

Examples of Refraction 1.) A light ray enters water from air What we learn from this diagram: Water is more optically dense than air (it has a higher index of refraction) Because of this, the light ray slows down as it enters the water, the light bends towards the normal øi air/water interface ør

Finishing example “1” Øi = the angle of incidence Ør = the angle of refraction In this case, the angle of incidence is greater than the angle of refraction

Example “2” 2.) A light ray leaves quartz and enters the air What we learn from this diagram: Quartz is more optically dense than air (it has a higher index of refraction) Because of this, the light ray will speed up as it enters the air, the light ray will bend away from the normal øi ør

Here’s Snell’s Law Snell’s Law A Dutch mathematician, Willeboard Snell, described the mathematical relationship between the angle of incidence and the angle of refraction. Snell saw a relationship between the angle of incidence and the angle of refraction.

Graph…. This is shown in the following graph: i r

The law allows you to compare variables as light rays cross between various substances. Mathematically: n1=index of refraction of 1st medium i= angle of incidence n1 sin i = n2 sin r n2= Index of refraction of 2nd medium r = angle of refraction

Sample Problems 1.) A ray of light crosses from crown glass to diamond at an angle of incidence of 51°. What is the angle of refraction?

Another Sample!!! 2.) Given the diagram below, calculate the index of refraction of the 2nd medium 38° water 51°

Notes Notes Notes Remember also that the index of refraction can be used to determine the speed of light in a medium, using n=c/v

More notes Snell’s law can also be used to explain “everyday” refraction The “puddle effect” which you see as you drive along a dry highway is caused by differences in air temp from the road towards the sky Warm air near the road has a different index of refraction than cool air away from the road. This causes the light to bend and refract, making the puddles.

Summary Light is a transverse, periodic wave with constant speed. Light is used in most of our technology. Many people have searched for the speed of light. The actual speed of light is 2.9979x108 m/s. By diffraction and interference, wavelengths are determined. In monochromatic light, all of the waves have a constant phase relationship. Polarization filters such as sunglasses make light vibrate in north and south orientation thus reducing glare depending on the angle. Light ranges from red to violet.

Summary cont. Monochromatic light is a single color. Reflected light from a surface, the angle that the incident ray equals the angle that the reflected ray makes with the normal to the surface. Polished surfaces produce regular reflection. An irregular surface produces diffuse reflection.