Light and the Electromagnetic Spectrum

Light Phenomenon • Isaac Newton (1642-1727) believed light consisted of particles • By 1900 most scientists believed that light behaved as a wave.

Light as a Wave • One way to think about light is as a traveling wave • A wave is just a disturbance in some medium (water, air, space) • A wave travels through a medium but does not transport material • A wave can carry both energy and information • Light is made up of small bundles of energy called photons. It travels in a straight line with a direction from its source.

Light as a Wave • However, light is composed of two transverse waves traveling simultaneously but at right angles to each other.

The Electromagnetic Spectrum The electromagnetic spectrum represents the range of energy from low energy, low frequency radio waves with long wavelengths up to high energy, high frequency gamma waves with small wavelengths.

Nature of Electromagnetic Waves • They are Transverse waves without a medium. (They can travel through empty space) • They travel as vibrations in electrical and magnetic fields. – They have some magnetic and some electrical properties to them.

• Speed of electromagnetic waves = 300,000,000 meters/second (Takes light 8 minutes to move from the sun to earth {150 million miles} at this speed.)

RADIO WAVES • A. Have the longest wavelengths and lowest frequencies of all the electromagnetic waves. • B. A radio picks up radio waves through an antenna and converts it to sound waves. • C. Each radio station in an area broadcasts at a different frequency. # on radio dial tells frequency. • D. MRI (MAGNETIC RESONACE IMAGING) – Uses Short wave radio waves with a magnet to create an image

MRI of the Brain

AM=Amplitude modulation—waves bounce off ionosphere can pick up stations from different cities. (535kHz-1605kHz= vibrate at 535 to 1605 thousand times/second) +

FM=Frequency modulation—waves travel in a straight line & through the ionosphere--lose reception when you travel out of range. (88MHz-108MHz = vibrate at 88million to 108million times/second)

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MICROWAVES • Microwaves—have the shortest wavelengths and the highest frequency of the radio waves. – Used in microwave ovens. • Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the form of heat to the food.

– Used by cell phones and pagers. – RADAR (Radio Detection and Ranging) • Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.

INFRARED RAYS • Infrared= below red • Shorter wavelength and higher frequency than microwaves. • You can feel the longest ones as warmth on your skin • Heat lamps give off infrared waves. • Warm objects give off more heat energy than cool objects. • Thermogram—a picture that shows regions of different temperatures in the body. Temperatures are calculated by the amount of infrared radiation given off. Therefore people give off infrared rays.

VISIBLE LIGHT

• Shorter wavelength and higher frequency than infrared rays. • Electromagnetic waves we can see. • Longest wavelength= red light • Shortest wavelength= violet (purple) light • When light enters a new medium it bends (refracts). Each wavelength bends a different amount allowing white light to separate into it’s various colors ROYGBIV.

Visible light is a small portion of this spectrum. This is the only part of this energy range that our eyes can detect. What we see is a rainbow of colors. RedOrangeYellowGreenBlueIndigoViolet ROY G BIV

No correction needed

a. Normal eye

b. Myopia (nearsightedness)

c. Hyperopia (farsightedness)

Corrected with concave lens

Corrected with convex lens

ULTRAVIOLET RAYS • Shorter wavelength and higher frequency than visible light • Carry more energy than visible light • Used to kill bacteria. (Sterilization of equipment) • Causes your skin to produce vitamin D (good for teeth and bones) • Used to treat jaundice ( in some new born babies. • Too much can cause skin cancer. • Use sun block to protect against (UV rays)

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X- RAYS

Shorter wavelength and higher frequency than UV-rays Carry a great amount of energy Can penetrate most matter. Bones and teeth absorb x-rays. (The light part of an x-ray image indicates a place where the x-ray was absorbed) • Too much exposure can cause cancer – (lead vest at dentist protects organs from unnecessary exposure) • Used by engineers to check for tiny cracks in structures. – The rays pass through the cracks and the cracks appear dark on film.

GAMMA RAYS • Shorter wavelength and higher frequency than Xrays • Carry the greatest amount of energy and penetrate the most. • Used in radiation treatment to kill cancer cells. • Can be very harmful if not used correctly.

• Brief SUMMARY • A. All electromagnetic waves travel at the same speed. (300,000,000 meters/second in a vacuum. • B. They all have different wavelength and different frequencies. – Long wavelength-lowest frequency – Short wavelength highest frequency – The higher the frequency the higher the energy.

Light behavior • reflection from an uneven surface - diffuse reflection • reflection from plane mirrors • Refraction(bending) of light = a change in the speed of light as it passes from one material to another.   The amount of bending depends on the speed of light in both materials. The greater the difference is, the more the light will bend. •  Index of refraction = a property of matter that indicates how much the speed of light is reduced/changed.

• Prisms - refract light twice; as it enters and exits the prisms The amount of bending of light depends on wavelength; longer wavelengths are refracted less than shorter wavelengths.