The nature of light

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AtomsAtoms consist of a nucleus of protons (which have a positive electrical charge) and neutrons (no charge), surrounded by a cloud of electrons (negative charge)

Atoms

The type of element is determined the number of protons, also called the atomic number. Sometimes atoms of the same element can have different number of neurons; these are called isotopes of that element.

Atoms

The electrons can have different energy levels. What’s really interesting is that the electrons can’t have just any amount of energy, but can only have particular amounts of energy (i.e. the energy levels are discrete, or quantized. They are not continuous.)

Atoms

Atoms

Electrons aren’t like particles that orbit the nucleus in the way that planets orbit stars. Individual electrons really are “smeared out” into clouds. But we often visualize electron energy levels like this anyway:

(the ground state)

Atoms

n=5

n=2

n=4

n=3

n=1

If an electron gains or loses the exactright amount of energy, it can transitionbetween different energy levels.

Electrons aren’t like particles that orbit the nucleus in the way that planets orbit stars. Individual electrons really are “smeared out” into clouds. But we often visualize electron energy levels like this anyway:

Energy level diagrams: different elements have differentelectron energy levels

Atoms

Atoms

ionization — this is when an electron gets enoughenergy to escape the atom

Energy level diagrams: different elements have differentelectron energy levels

The nature of light

There are some key experiments that reveal the nature of light

• Diffracting light through a prism — reveals that white light is made of many colors.

• The double-slit experiment — shining (monochromatic) light on a screen with two slits produces strange patterns on the other side. Reveals the wave nature of light.

• The photoelectric effect — shining light on a surface can free electrons, but only if the light has the right color. Reveals the particle nature of light.

The nature of light

There are some key experiments that reveal the nature of light

• Diffracting light through a prism — reveals that white light is made of many colors.

• The double-slit experiment — shining (monochromatic) light on a screen with two slits produces strange patterns on the other side. Reveals the wave nature of light.

• The photoelectric effect — shining light on a surface can free electrons, but only if the light has the right color. Reveals the particle nature of light.

The diffraction of light

But Newton showed that if you shine light of only a single color it through a prism, it does not diffract. This shows that the different colors were present in the light originally, and that they are not created inside of the prism

The diffraction of light

The diffraction of light

You can make a plot of brightness versus color. This is called a spectrum.

Blackbody Radiation 57

Part I: Spectral Curves V

White light is made up of all colors of light. We can see the individual colorswhenwhite lightis passed through a prism or when we look at a rainbow. Light’can come,in: an array of typesor forms, which we call a spectrum. A spectral curve (like the one:shown:.below) isa graphthat displays the amount of energy given off by an object each second’versuSthe differentwavelengths (or colors) of light. For a specific color of light on the horizontal axis, the heightof the curve will indicate how much energy is being given off at’that particular wavelength..Figure 1 shows the spectral curve for an object emitting more red and orange !ight thanindigo and violet. Notice that the red end of the curve is higher than the, yioletend so theobject will appear slightly reddish in color.

1) Which color of light has the greatestenergy output in Figure 1?

2) Imagine that the blue light and___—

orange light from the source wereblocked. What color(s) would now bepresent in the spectrum of light observed?

I I I I I I IV I B G Y 0 R

3) Which of the following is the most Violet Indigo Blue Green Yellow Orange Redaccurate spectral curve for thespectrum described in Question 2? Figure 1

a) b) c)

0 0 0C) C) C)

VIBGYOR VIBGYOR VIBGYOR

4) What colors of light are present in 3b above?

5 What colors are resent in 3c above? Would this obect appear reddish or bluish?

©r2QO8iearson Education, Inc., LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY

PubIishingas Pearson Addison-Wesley. SECOND EDITION

The diffraction of light

You can make a plot of brightness versus color. This is called a spectrum.

Blackbody Radiation 57

Part I: Spectral Curves V

White light is made up of all colors of light. We can see the individual colorswhenwhite lightis passed through a prism or when we look at a rainbow. Light’can come,in: an array of typesor forms, which we call a spectrum. A spectral curve (like the one:shown:.below) isa graphthat displays the amount of energy given off by an object each second’versuSthe differentwavelengths (or colors) of light. For a specific color of light on the horizontal axis, the heightof the curve will indicate how much energy is being given off at’that particular wavelength..Figure 1 shows the spectral curve for an object emitting more red and orange !ight thanindigo and violet. Notice that the red end of the curve is higher than the, yioletend so theobject will appear slightly reddish in color.

1) Which color of light has the greatestenergy output in Figure 1?

2) Imagine that the blue light and___—

orange light from the source wereblocked. What color(s) would now bepresent in the spectrum of light observed?

I I I I I I IV I B G Y 0 R

3) Which of the following is the most Violet Indigo Blue Green Yellow Orange Redaccurate spectral curve for thespectrum described in Question 2? Figure 1

a) b) c)

0 0 0C) C) C)

VIBGYOR VIBGYOR VIBGYOR

4) What colors of light are present in 3b above?

5 What colors are resent in 3c above? Would this obect appear reddish or bluish?

©r2QO8iearson Education, Inc., LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY

PubIishingas Pearson Addison-Wesley. SECOND EDITION

Light can be made up of some combination of many different colors, or only one color.

The nature of light

There are some key experiments that reveal the nature of light

• Diffracting light through a prism — reveals that white light is made of many colors.

• The double-slit experiment — shining (monochromatic) light on a screen with two slits produces strange patterns on the other side. Reveals the wave nature of light.

• The photoelectric effect — shining light on a surface can free electrons, but only if the light has the right color. Reveals the particle nature of light.

The double-slit experiment

If you shine light of a single color through two slits, it makes a pattern on the other side

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

The double-slit experiment

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

The double-slit experiment

+

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

The double-slit experiment

+ =

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

The double-slit experiment

+

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

The double-slit experiment

+ =

The double-slit experimentWhat’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

The double-slit experimentWhat’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

WavesA wave is something that transmits energy without carrying material along with it

Waves

wavelength — the distance between adjacent peaks

frequency — the number of times each second that a point moves up and down

When wavelength goes up, frequency goes down:frequency = c/wavelength

A wave is something that transmits energy without carrying material along with it

WavesThe color of light is determined by it’s wavelength or frequency.

• Redder light has higher wavelength and lower frequency• Bluer light has lower wavelength and higher frequency

Waves

So a spectrum is really a plot of brightness versus wavelength

Waves

So a spectrum is really a plot of brightness versus wavelength

Waves

So a spectrum is really a plot of brightness versus wavelength

Waves

The human eye is only sensitive to a narrow range in wavelength or frequency. That’s why you can’t see light with shorter wavelengths (like x-rays) or longer wavelengths (like infrared and radio).

The thermal spectrum (aka “blackbody” spectrum)Dense objects emit light with a certain spectrum depending on their temperature. Objects with relatively cold temperatures emit most of their light at long wavelengths (like in the infrared). Objects with high enough temperatures can emit light in at visible wavelengths.

The thermal spectrum (aka “blackbody” spectrum)

• Midterm on Tuesday (scantron and fill-in-the-blank/short answer)

• No homework next week

ReviewAtoms consist of a nucleus of protons and neutrons surrounded by a cloud of electrons. The type of element is determined the number of protons, also called the atomic number. Sometimes atoms of an element can have different numbers of neutrons; these are called isotopes.

(the ground state)

Review

n=5

n=2

n=4

n=3

n=1

If an electron gains or loses the exactright amount of energy, it can transitionbetween different energy levels. But if it gets enough energy then it can escape the atom entirely.

Electrons aren’t like particles that orbit the nucleus in the way that planets orbit stars. Individual electrons really are “smeared out” into clouds. But we often visualize electron energy levels like this anyway:

Review

There are some key experiments that reveal the nature of light

• Diffracting light through a prism — reveals that white light is made of many colors.

• The double-slit experiment — shining (monochromatic) light on a screen with two slits produces strange patterns on the other side. Reveals the wave nature of light.

Review

Light can be made up of some combination of many different colors, or only one color. This is shown by looking at it’s spectrum.

Blackbody Radiation 57

Part I: Spectral Curves V

White light is made up of all colors of light. We can see the individual colorswhenwhite lightis passed through a prism or when we look at a rainbow. Light’can come,in: an array of typesor forms, which we call a spectrum. A spectral curve (like the one:shown:.below) isa graphthat displays the amount of energy given off by an object each second’versuSthe differentwavelengths (or colors) of light. For a specific color of light on the horizontal axis, the heightof the curve will indicate how much energy is being given off at’that particular wavelength..Figure 1 shows the spectral curve for an object emitting more red and orange !ight thanindigo and violet. Notice that the red end of the curve is higher than the, yioletend so theobject will appear slightly reddish in color.

1) Which color of light has the greatestenergy output in Figure 1?

2) Imagine that the blue light and___—

orange light from the source wereblocked. What color(s) would now bepresent in the spectrum of light observed?

I I I I I I IV I B G Y 0 R

3) Which of the following is the most Violet Indigo Blue Green Yellow Orange Redaccurate spectral curve for thespectrum described in Question 2? Figure 1

a) b) c)

0 0 0C) C) C)

VIBGYOR VIBGYOR VIBGYOR

4) What colors of light are present in 3b above?

5 What colors are resent in 3c above? Would this obect appear reddish or bluish?

©r2QO8iearson Education, Inc., LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY

PubIishingas Pearson Addison-Wesley. SECOND EDITION

Blackbody Radiation 57

Part I: Spectral Curves V

White light is made up of all colors of light. We can see the individual colorswhenwhite lightis passed through a prism or when we look at a rainbow. Light’can come,in: an array of typesor forms, which we call a spectrum. A spectral curve (like the one:shown:.below) isa graphthat displays the amount of energy given off by an object each second’versuSthe differentwavelengths (or colors) of light. For a specific color of light on the horizontal axis, the heightof the curve will indicate how much energy is being given off at’that particular wavelength..Figure 1 shows the spectral curve for an object emitting more red and orange !ight thanindigo and violet. Notice that the red end of the curve is higher than the, yioletend so theobject will appear slightly reddish in color.

1) Which color of light has the greatestenergy output in Figure 1?

2) Imagine that the blue light and___—

orange light from the source wereblocked. What color(s) would now bepresent in the spectrum of light observed?

I I I I I I IV I B G Y 0 R

3) Which of the following is the most Violet Indigo Blue Green Yellow Orange Redaccurate spectral curve for thespectrum described in Question 2? Figure 1

a) b) c)

0 0 0C) C) C)

VIBGYOR VIBGYOR VIBGYOR

4) What colors of light are present in 3b above?

5 What colors are resent in 3c above? Would this obect appear reddish or bluish?

©r2QO8iearson Education, Inc., LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY

PubIishingas Pearson Addison-Wesley. SECOND EDITION

Blackbody Radiation 57

Part I: Spectral Curves V

White light is made up of all colors of light. We can see the individual colorswhenwhite lightis passed through a prism or when we look at a rainbow. Light’can come,in: an array of typesor forms, which we call a spectrum. A spectral curve (like the one:shown:.below) isa graphthat displays the amount of energy given off by an object each second’versuSthe differentwavelengths (or colors) of light. For a specific color of light on the horizontal axis, the heightof the curve will indicate how much energy is being given off at’that particular wavelength..Figure 1 shows the spectral curve for an object emitting more red and orange !ight thanindigo and violet. Notice that the red end of the curve is higher than the, yioletend so theobject will appear slightly reddish in color.

1) Which color of light has the greatestenergy output in Figure 1?

2) Imagine that the blue light and___—

orange light from the source wereblocked. What color(s) would now bepresent in the spectrum of light observed?

I I I I I I IV I B G Y 0 R

3) Which of the following is the most Violet Indigo Blue Green Yellow Orange Redaccurate spectral curve for thespectrum described in Question 2? Figure 1

a) b) c)

0 0 0C) C) C)

VIBGYOR VIBGYOR VIBGYOR

4) What colors of light are present in 3b above?

5 What colors are resent in 3c above? Would this obect appear reddish or bluish?

©r2QO8iearson Education, Inc., LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY

PubIishingas Pearson Addison-Wesley. SECOND EDITION

ReviewIf you shine light of a single color through two slits, it makes a pattern on the other side. This is because light behaves as a wave, adding up constructively or destructively

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

Review

+ =

What’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

Review

+ =

ReviewWhat’s going on? Light behaves like a wave. And when you add two waves together, sometimes they add up constructively, and sometimes they add destructively

Review

wavelength — the distance between adjacent peaks

frequency — the number of times each second that a point moves up and down

When wavelength goes up, frequency goes down:frequency = c/wavelength

A wave is something that transmits energy without carrying material along with it

Review

The human eye is only sensitive to a narrow range in wavelength or frequency. That’s why you can’t see light with shorter wavelengths (like x-rays) or longer wavelengths (like infrared and radio).

ReviewDense objects emit a thermal (aka blackbody) spectrum based on their temperature. Hotter objects are brighter at all wavelengths, and peak and a shorter wavelength

The nature of light

There are some key experiments that reveal the nature of light

• Diffracting light through a prism — reveals that white light is made of many colors.

• The double-slit experiment — shining (monochromatic) light on a screen with two slits produces strange patterns on the other side. Reveals the wave nature of light.

• The photoelectric effect — shining light on a surface can free electrons, but only if the light has the right color. Reveals the particle nature of light.

The photoelectric effectFor centuries people argued over whether light is continuous or whether it consists of a lot of individual particles. Discovery of the wave nature of light suggested that light is continuous. But this idea was challenged by an experiment:

The photoelectric effect

• In the 19th century physicists knew that light was a wave, and that light carries energy.

• So they thought that shining light onto a material would give the electrons enough energy to become ionized if the light was shining for long enough, or if the light was bright enough.

• But this turned out not to be the case! Experiments showed that electrons are released only if the light has high enough frequency. If the frequency is too low, it does not matter how bright the light is nor how long it shines on the material.

The photoelectric effect

So what does this mean? Light is emitted or absorbed in discrete packets. In other words, light behaves like a particle. We call these particles photons.

energy of a photon: E=hf(where h is a constantand f is the frequency)

The photoelectric effect

So an individual photon can free an individual electron only if it has enough energy (i.e. a high enough frequency). Otherwise, no matter how many photons you shine on a material, the electrons will never absorb the photons.

Q: The higher energy of a photon, A. the longer it’s wavelengthB. the shorter it’s wavelengthC. energy is independent of wavelength

Q: The higher energy of a photon, A. the longer it’s wavelengthB. the shorter it’s wavelengthC. energy is independent of wavelength

The nature of light

We’ve established three very important things about light:• Light can be described by it’s spectrum (brightness

vs. wavelength)• Light behaves like a wave; it has a wavelength and a

frequency (l=c/f)• Light also behaves like a particle; it is absorbed and

emitted in individual photons (also sometimes called wavepackets) with energy E=hf

How does light interact with matter?

There are three ways:• emission• absorption• transmission (i.e. light passes through the object)• reflection

How does light interact with matter?

Q: You see a rose in a garden. It is red. Which process describes the light from the rose?

A. the rose emits red lightB. the rose absorbs red lightC. the rose transmits red lightD. the rose reflects red lightE. none of the above

Q: You see a rose in a garden. It is red. Which process describes the light from the rose?

A. the rose emits red lightB. the rose absorbs red lightC. the rose transmits red lightD. the rose reflects red lightE. none of the above

A red object absorbs the light at most wavelengths but it reflect the red light

Three different types of spectra

Emission line spectrum:

Continuous spectrum:

Absorption line spectrum:

Three different types of spectra

Three different types of spectra

A dense object will emit a continuous spectrum. This will depend on it’s temperature, which is why it’s also called a thermal spectrum (also a blackbody spectrum)

Three different types of spectra

A hot cloud of gas will emit an emission line spectrum. Because of the thermal energy in the gas, some of the electrons will get bumped up to higher energy levels. When they move back down to lower energy levels, they will emit photons corresponding to the difference in energy.

Three different types of spectra

If an electron is bumped up to a higher energy level, it will move back down very quickly, releasing a photon

If an electron is bumped up to a higher energy level, it will move back down very quickly, releasing a photon

Three different types of spectra

Three different types of spectra

A hot dense object illuminating a cool cloud of gas will produce an absorption line spectrum. Most of the electrons in the gas will be in the ground state. But photons from the source that have exactly the right amount of energy will be absorbed by the electrons, moving them into a higher energy state.

Chemical fingerprintsDifferent elements have different electron energy levels. So by studying looking at the wavelengths of the emission/absorption lines, you can figure out what element you’re looking at!

Chemical fingerprintsDifferent elements have different electron energy levels. So by studying looking at the wavelengths of the emission/absorption lines, you can figure out what element you’re looking at!

• 05_MysteryGasComposition.htm

Example: the solar spectrumThe sun emits an absorption line spectrum, because it has a hot core (thermal spectrum) with cooler gas on the outside that produces absorption lines. These lines tell us that the sun is mostly made of hydrogen and helium with smaller amounts of heavier elements

Doppler shift

The Doppler shift refers to the change in wavelength and frequency of a wave if the emitting object is moving towards or away from you.

Doppler shift

The Doppler shift refers to the change in wavelength and frequency of a wave if the emitting object is moving towards or away from you.

The waves get bunched up if the object is moving towards you, or stretched out if it is moving away

Doppler shift

The Doppler shift refers to the change in wavelength and frequency of a wave if the emitting object is moving towards or away from you.

Doppler shift

This happens for any type of wave, including sound waves and light waves.

• For an object moving toward you, the wavelength goes down and the frequency goes up. In the case of light, we call this a blueshift.

• For an object moving away from you, the wavelength goes up and the frequency goes down. We call this a redshift.

Doppler shift

Stationary

Moving away

Away faster

Moving toward

Toward faster

We generally measure the Doppler shift from shifts in the wavelengths of spectral lines

Doppler shift

Stationary

Moving away

Away faster

Moving toward

Toward faster

We generally measure the Doppler shift from shifts in the wavelengths of spectral lines

Redshifted

Blueshifted

Doppler shift• By measuring the shift in the spectrum, we can estimate

very accurately the speed with which an object is moving towards or away from us.

• But this only gives us the speed along the line-of-sight

Large redshift

No shift at all, since it is moving perpendicular to the line-of-sight

Small redshift, since it is moving away from us but not very quickly