7.2 The Nature of Light

Question 1

What is the chief characteristic that light has in common with electrons?

Answer 1

The wave-particle duality.

Question 2

What is electromagnetic radiation?

Answer 2

A type of energy embodied in oscillating electric and magnetic fields.

Question 3

What is an electric field?

Answer 3

A region of space where an electrically charged particle experiences a force.

Question 4

What is a magnetic field?

Answer 4

A region of space where a magnetic particle experiences a force. (ex. The area around a magnet.)

Question 5

At what speed do electromagnetic waves travel inside a vacuum?

Answer 5

At a constant speed of 3.00 X 10^8 m/s (186,000 mi/s)

Question 6

How long does it take for an electromatic wave to circle Earth?

Answer 6

One-seventh of a second.

Question 7

What are the two main characteristics of an electromagnetic wave?

Answer 7

It’s amplitude and wavelength.

Question 8

What is amplitude?

Answer 8

The amplitude of a wave is the vertical height of a crest (or depth of a trough).

Question 9

What does amplitude determine about light?

Answer 9

It is related to the intensity or brightness of the light— the greater the amplitude, the greater the intensity.

Question 10

What is wavelength of a wave?

Answer 10

It is the distance in space between adjacent crests (or any two analogous points) and is measured in units of distance such as the meter, micrometer, or nanometer.

Question 11

At what angle are the electric field and magnetic field of a wave of light?

Answer 11

The two components are perpendicular.

Question 12

Are wavelength and amplitude independent properties?

Answer 12

Yes.

Question 13

What does wavelength determine about light?

Answer 13

It determines the color of the light.

Question 14

Wavelength and amplitude are both related to the amount of what within a wave?

Answer 14

Amount of energy.

Question 15

What kind of amplitude and wavelength do the most energetic waves have?

Answer 15

Large amplitudes and short wave lengths are the most energetic.

Question 16

What is the frequency of a wave?

Answer 16

The number of cycles (or wave crests) that pass through a stationary point in a given period of time.

Question 17

What are the units used to express frequency?

Answer 17

Cycles per second (cycle/s) or simply s^-1, and hertz (Hz), defined as 1 cycle/s.

Question 18

What is directly proportional to the frequency of a wave?

Answer 18

The speed at which the wave is traveling—the faster the wave, the more crests will pass a fixed location per unit time.

Question 19

What is inversely proportional to the wavelength of a wave?

Answer 19

The frequency of a wave—the farther apart the crests, the fewer that pass a fixed location per unit time.

Question 20

What is the formula for the relationship between the speed of light, c, wavelength, λ, and frequency, ν?

Answer 20

ν = c/λ

Question 21

What is the name of the symbol that represents wavelength?

Answer 21

It is λ (The symbol is a Greek letter, ‘lambda’.).

Question 22

What is the name of the symbol that represents frequency of light waves?

Answer 22

It is usually denoted by a Latin letter f or by the Greek letter (nu).

Question 23

What does frequency and wavelength have in common?

Answer 23

They represent different ways of displaying the same information.

Question 24

What is visible light?

Answer 24

Light that can be seen by the human eye.

Question 25

What is seen when white light passes through a prism?

Answer 25

We see the same colors as in a rainbow, ROYGBIV; red, orange, yellow, green, blue, indigo, and violet.

Question 26

What is the color of light that has the longest wavelength of visible light, and at about what in nanometers?

Answer 26

Red light has the longest wavelength at about 750 nanometers that is still visible.

Question 27

What is the color of light that has the shortest wavelength of visible light?

Answer 27

Violet light has the shortest wavelength at 400 nanemeters that is still visible.

Question 28

The presence of what within white light is responsible for the colors that we perceive?

Answer 28

A variety of wavelengths.

Question 29

Why does a red shirt appear red?

Answer 29

It reflects predominantly red light while absorbing most other colors, and our eyes see only the relfected light.

Question 30

Ex 7.1 Wavelength and Frequency
Calculate the wavelength (in nm) of the red light emitted by a barcode scanner that has a frequency off 4.62 X 10^14 s^-1.

Answer 30

You are given the frequency of the light and asked to find its wavelength. Use v = c/λ, which relates frequency to wavelength. You can convert the wavelength from meters to nanometers by using the conversion factor between the two (1 nm = 10^(-9) m).
v = c/λ, λ = c/v = (3.00 X 10^8 m/s)/(4.62 X 10^14 1/s) = 6.49 X 10^-7 m X (1 nm/ 10^-9 m) = 649 nm

Question 31

What are the characteristics of frequency and wavelength in regard to red light?

Answer 31

Red light has longer λ (wavelength), therefore lower v (frequency).

Question 32

What are the characteristics of frequency and wavelength in regard to blue light?

Answer 32

Blue light has a higher v (frequency), therefore a shorter λ (wavelength).

Question 33

Which wavelengths of electromagnetic radiation are included in the electromagnetic spectrum?

Answer 33

It includes all known wavelengths of electromagnetic radiation.

Question 34

What is the range of the main regions of the electromagnetic spectrum?

Answer 34

Beginning in wavelength from 10^-15 m (gamma rays) to 10^ m (radio waves).

Question 35

Does long-wavelength light inherently have greater energy than short-wavelength light?

Answer 35

No, the shorter the wavelength the more the inherent energy.

Question 36

What form of electromagnetic radiation has the shortest wavelengths?

Answer 36

Gamma rays have the shortest wavelength, and are produced by stars, and certain unstable atomic nuclei on earth.

Question 37

Why is human exposure to gamma rays dangerous?

Answer 37

Because the high energy of gamma rays can damage biological molecules.

Question 38

Why do we use X-rays for medical use?

Answer 38

X-rays pass through many substances that block visible light and are therefore used to image bones and internal organs.

Question 39

What is the first type of light that is above our ability to see with a human eye?

Answer 39

Ultra-Violet Radiation

Question 40

What kind of energy does the right side of the electromagnetic spectrum consist of?

Answer 40

High energy, high frequency, and short-wavelength radiation.

Question 41

What kind of energy does the left side of the electromagnetic spectrum consist of?

Answer 41

Low-energy, low frequency, and long-wavelength radiation.

Question 42

What portion of the electromagnetic spectrum does visible light constitute?

Answer 42

A small portion in the middle of the spectrum.

Question 43

Visible light consists of a mixture between what?

Answer 43

All visible colors.

Question 44

What can result from excessive exposure to ultraviolet light?

Answer 44

Increase the risk of skin cancer and cataracts and cause premature wrinkling of the skin.

Question 45

What spectrum ranges between violet light and red light?

Answer 45

The visible light spectrum.

Question 46

Visible light does not carry enough energy to damage biological molecules under constitution?

Answer 46

As long as the intensity is not too high.

Question 47

What light lies beyond visible light that you can feel?

Answer 47

Infrared radiation is the heat you feel when you place your hand on a hot object.

Question 48

What light is utilized within night vision goggles?

Answer 48

Infrared light, because it is invisible to our eyes but can be detected with technology. All warm objects emit infrared light, therefore they are detected.

Question 49

Why do microwaves heat objects so effectively?

Answer 49

Microwaves have long wavelengths and low energy, but it is efficiently absorbed by water and can therefore heat substances containing water.

Question 50

What light waves have the lowest longest wavelengths?

Answer 50

Radio waves, which are used for communication.

Question 51

What is the characteristic way in which waves, including electromagnetic waves, interact with each other?

Answer 51

Interference.

Question 52

If waves of equal amplitude from two sources align with overlapping crests, what is this called, and what is the resulting effect to amplitude.

Answer 52

This is called constructive interference, and the resulting amplitude will be twice as much.

Question 53

What does it mean for two waves to be “in phase”?

Answer 53

It means the two waves align with overlapping crests.

Question 54

What does it mean for two waves to be “out of phase”?

Answer 54

They align so that the one crest from one source overlaps the trough from the other source.

Question 55

If two waves align so that the crest from one source overlaps the trough from the other, what is the resulting effect and what is it called?

Answer 55

The waves would cancel by means of destructive interference.

Question 56

What is diffraction?

Answer 56

When a wave encounters an obstacle or a slit that is comparable in size to its wavelength, it bends around it.

Question 57

What does the diffraction of light through two slits separated by a distance comparable to the wavelength of the light result in?

Answer 57

An interference pattern.

Question 58

What was the chief discovery that brought the classical view of electromagnetic theory into question?

Answer 58

The photoelectric effect.

Question 59

What is the photoelectric effect?

Answer 59

The observation that many metals eject electrons when light shines upon them.

Question 60

What is classical electromagnetic theory?

Answer 60

It was thought that light was purely a wave phenomenon largely due to the discovery of the diffraction of light.

Question 61

What is a trough?

Answer 61

The opposite of a crest in amplitude; the farthest point down on the negative dip of a wave in a wave’s cycle.