G Notes - 5-2 EM Waves Sub-Unit.pdf

Waves Unit / Electromagnetic Waves Sub-Unit

Regents Physics

• Charged objects generate electric fields.

o When they move, the field changes

• Changing electric fields produce ________________________________ • Changing magnetic fields produce _______________________________ • The E-fields and B-fields are perpendicular to each other.

• Waving a charge back and forth generates an ______________________

The Speed of Light (well, EM Waves really)

• EM Waves travel at _______________________________ (in a vacuum) • They are slower in denser mediums.

• Speed of a medium depends on its ______________________________ • The closer the index is to 1.0 the faster the medium.

Example

What is the speed of light in a medium with an index of refraction of 2.0?

EM Wave Types

An EM Wave’s type depends on its frequency/wavelength.

All objects emit EM radiation

Wein’s Law – the electromagnetic radiation produced by an object depends on its temperature.

Equation

Mechanical waves require a medium.

EM waves DO NOT require a medium.

10-5 10-2

103 10-7 10-8 10-10 10-12

Gamma Ray X-ray Ultraviolet Visible Infrared Microwave Radio

Building Human Honey Bee Pinpoint Protozoan Molecules Atoms Atomic Nuclei

104 108 1012 1015 1016 1018 1020

1 K 100 K 10000 K 10 million K Temperature of

bodies emitting the wavelength

Freq (Hz) Wavelength (m)

PRACTICE

1. The constellation of Orion is a highly recognizable arrangement of stars. There are two very distinct stars in this constellation, Betelgeuse which is a red star and Rigel which is a bluish star. The color red has a wavelength of about 750 nm and the color blue has a wavelength of about 400 nm.

a. Which color has the higher frequency?

b. Which of the two stars is hotter? Betelgeuse or Rigel?

2. An EM wave is traveling through outer space with a frequency of 8.0 x 1013 hertz. a. What is the wavelength of this wave?

b. What type of wave is this?

3. Listed below are the properties of several EM Waves. State whether the property given is a wavelength or frequency, then state which type of EM Wave it is.

a. 500000 hertz __________________ ____________________ b. 5.0 x 10-9 meter _________________ ____________________ c. 3.0 x 1013 hertz _________________ ____________________ d. 4.5 x 1014 hertz _________________ ____________________ e. 200 meters _________________ ____________________

Review Questions

5.2.1 – Determine the part of the EM Spectrum that a particular wave belongs to. Explain the origin of electromagnetic waves and contrast this with the production of mechanical waves. Use equation to determine speed in a medium or index of refraction for a medium.

- What type of electromagnetic radiation is a wave with a __________?

o wavelength of... (a) 3.5 x 103 meters (b) 4.5 x 10-8 meters (c) 2.0 x 10-12 meters (d) 6.0 meters

o frequency of… (a) 3.0 x 106 hertz (b) 2.2 x 1015 hertz (c) 1.5 x 1010 hertz (d) 5.5 x 1014 hertz

[radio; UV; x-ray/gamma; radio] [radio; UV; microwave; visible]

- Electromagnetic waves can be produced by which of the following? Which would produce a mechanical wave?

(1) An electron moving at a constant velocity (2) A plastic slinky vibrating back and forth (3) An electron oscillating back and forth (4) A neutron oscillating back and forth

(5) A string with a single disturbance moving through it (6) A positively charged sphere vibrating back and forth

[EM: 3, 6; Mech: 2]

- Determine the speed of a ray of light with a frequency of 5.09 x 1014 hertz as it passes through water (index of refraction = 1.33).

[2.3E8m/s]

- Determine the absolute index of refraction for a medium in which microwaves travel at a speed of 1.8 x 108 meters per second.

Waves Unit / Electromagnetic Waves Sub-Unit

Regents Physics

Reflection

• Waves that change mediums or hit boundaries obey the Law of Reflection. • Angles are always measured from the normal!

Refraction

• Waves that change mediums obey Snell’s Law.

o Fast  Slow = bend toward normal

o Slow  Fast = bend away from normal

• Wavelengths change with corresponding differences in speed.

• Special cases in which no bending occurs…

o Two mediums with the same index of refraction.

o A ray entering another medium at an angle of 0°

Example

- A ray of light with a frequency of 5.09 x 1014 hertz moves from air into flint glass. The angle with which the ray strikes the surface of the flint glass is 40° away from the normal. Which is the faster medium? Determine the angle of refraction with which the ray would move into the flint glass. Draw the refracted ray.

air diamond

Equation Equation

Equation corn oil

glycerol

air flint glass

PRACTICE

1. A ray of light strikes a mirror at an angle of 20° when measured from the normal. What will the angle between the incident and reflected rays be when the ray bounces off the mirror?

2. A ray of light (f = 5.09 x 1014 Hz) moves from air into water. The ray strikes the interface between the air and water at an angle of 60° from the normal.

a. Determine the ray’s wavelength in air.

b. Determine the ray’s wavelength in water.

c. Determine the ray’s angle of refraction as it enters the water.

3. Use a protractor and ruler to measure and sketch the reflected rays in each of the following pictures.

mirror

incident ray angle between incident and normal = 20°

angle between reflected and normal = _______°

angle between incident and reflected = _______°

Waves Unit / Electromagnetic Waves Sub-Unit

Regents Physics

into a new medium where its speed is 2.5 x 108 meters per second. The angle with which the light ray enters the second medium is 30° from a line that is perpendicular to the interface between the two mediums.

a. What is the index of refraction for the first medium?

b. Was the ray’s angle of incidence greater or less than 30°?

5. Measure the angle of incidence, draw the reflected ray, calculate the angle of refraction and draw the refracted ray. Label each ray.

air diamond

fused quartz zircon flint glass water

The Wave Nature of Light

• Consider each of the 8 phenomenon. Which of them can be demonstrated with light? Which of them are PROOF that light has a wave nature? Which can also occur with particles?

o Reflection

o Interference

o Standing Waves

o Resonance

o Doppler Effect:

o Diffraction

o Refraction

Waves Unit / Electromagnetic Waves Sub-Unit

Regents Physics

medium. Determine the index of refraction of the unknown medium.

[1.64]

- The diagram below shows a set of polarizers acting on a wave. What is the end product of having two polarizers set at 90° relative to one another?

[no waves pass through]

- Which types of waves can be polarized? (1)mechanical, transverse waves (2)mechanical, longitudinal waves (3)electromagnetic, transverse waves

[1 and 3] 45°

35° Water

Unknown Medium

Review Questions

5.2.2 – Explain the Law of Reflection; identify the two types of reflection; and identify ray diagrams in which reflection occurs. Explain Snell’s Law and identify ray diagrams in which refraction occurs. Use Snell’s Law to determine angle of incidence; angle of refraction; or index of refraction . Explain the phenomenon of polarization and recognize diagrams showing that polarization is occurring. Explain how Young’s Double Slit Experiment demonstrates that light has a wave nature.

- The Law of Reflection states that the angle at which anything is reflected is equal to the _____________________________ with which it hits a surface. - Which of the following pictures shows light hitting a highly reflective surface

(like a mirror)? Which type occurs on a surface that is not highly reflective (like the page of a book)?

[angle; mirror=specular; page=diffuse]

- A ray of light moving from air (n = 1) into Lucite (n = 1.5) enters the Lucite at an angle of 35° relative to a line perpendicular to the Lucite surface. What angle will the light be bent at as it moves through the Lucite?

[22°] Diffuse Reflection Specular/Regular Reflection

- The diagram below shows light passing through two slits. What will occur at the points at which the arrows in the diagram hit the screen? (Note the type of interference occurring at the points where the wave-fronts overlap along these lines!)

[bright spots (constructive interference]

- Which of these phenomena show that light has a wave nature? (There may be more than one!)

(1) Reflection (4) Refraction (2) Interference (5) Polarization (3) Doppler Effect (6) Diffraction

[2, 3, 5, 6] screen