Telescopes & Light: Part 2All About Light

What is light?• In the 17th Century, Isaac Newton argued

that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves.

• In the 19th and 20th centuries Maxwell, Young, Einstein and others were able to show that Light behaves both like a particle and a wave depending on how you observe it.

What does light do?

• Light transfers energy from place to place.

• Light transfers information from place to place.

• Everything we know about astronomical objects, we have learned through the analysis of light.

Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light

must be a combination of electric and magnetic fields.

In 1905, Einstein calculated the energy of

a particle of light (photon) and proposed the photoelectric effect.

Ephoton = hc/

e-photon

But, where does light actually come from?

Light comes from the acceleration of charged particles (such as electrons

and protons)

Waves• Wave - (general definition) a pattern that repeats itself cyclically

in both time and space• Electromagnetic radiation travels through space in the form of

waves - a wave is a way in which energy is transferred from place to place without physical movement of material from one location to another.

• A wave is not a physical object.• Wave period - number of seconds needed for the wave to

repeat itself at some point in space (time from crest to crest).• Wavelength - the number of meters needed for the wave to

repeat itself at a given moment in time (length from crest to crest).

• Amplitude - maximum departure of the wave from the undisturbed state (maximum height).

• Frequency - number of crests passing any given point per unit time (= 1 / period).

Wave Motion

• Period is given in seconds, frequency is 1/seconds or Hz (Hertz).

• Wave velocity - a wave moves a distance equal to one wavelength in one period.

• Velocity = wavelength x frequency• Speed of light in a vacuum is constant and is called

“c” - it equals 3.0 x 108 m/s.

Lecture Tutorial: EM Spectrum (p. 45)

• Work with a partner!• Read the instructions and questions

carefully.• Discuss the concepts and your answers

with one another. Take time to understand it now!!!!

• Come to a consensus answer you both agree on.

• If you get stuck or are not sure of your answer, ask me or another group.

Diffraction and Interference

• Diffraction - bending of a wave around a barrier (light shining around a corner, etc.).

• Interference - when two or more waves interact. Interference can be constructive (the waves add) or destructive (the waves subtract, or even cancel each other out completely).

So how do we actually get light?

An atom consists of a small, dense

nucleus (containing protons and neutrons)

surrounded by electrons

- Model Proposed by Niels Bohr

1913

The electron should be

thought of as a distribution or

cloud of probability around the

nucleus that, on average,

behaves like a point particle on a fixed circular

path.

Interactions Between Charged Particles

• Electrons - negatively charged.• Protons - positively charged.• Like charges repel, unlike charges attract.• Electric field - extends outward from a

charged particle.• The electric field extends outward in a wave if

the particle is moving - we can learn about the particle from afar from studying this wave.

Electromagnetic Waves

• Electromagnetic waves are caused by changing electric and magnetic fields (magnetic fields accompany magnetized objects, just like electric fields accompany charged objects).

• We learn about distant stars from their electromagnetic waves (radiation).

• Since the speed of light is finite and constant, when we say an object is 3 LY away (where 1 LY is the distance light travels in a year), we are looking at light that left the star 3 years ago.

Photons (light-waves) are emitted from an atom when an electron

moves from a higher energy level to a lower energy level.

Nucleus

Photons can also be absorbed by an atom when an electron moves

from a lower energy level to a higher energy level.

Nucleus

Each chemical element produces its own unique set of spectral lines when it

is excited

Tutorial: Light and Atoms – p. 63

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask me

or another group.

If an electron in an atom moves from an orbit with an energy of 5 to an orbit with an energy of 10,

A. a photon of energy 5 is emittedB. a photon of energy 15 is emitted.C. a photon of energy 5 is absorbed.D. a photon of energy 15 is absorbed.E. None of the above

Which of these shows the atom emitting the greatest amount of light?

e-

Ae- e-

B

e-

C e-

D e-

E

There are three types of spectra.

Continuous Spectrum

Hot/Dense Energy Source prism

Emission Line Spectrum

prismHot low density cloud of Gas

Absorption Line SpectrumCooler low density cloud of Gas

Hot/Dense Energy Source prism

Tutorial: Types of Spectra – p. 61

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask me

or another group.

All stars produce dark

line absorption spectra.

So what do we learn from light?

The Blackbody Spectrum

• Intensity - amount of strength of radiation at any point in space.• Blackbody - an object that absorbs all radiation falling upon it.• Blackbody curve - describes the distribution of re-emitted

radiation.• Blackbody objects absorb radiation and then re-emit that

radiation if it is too remain in a steady state (does not increase or decrease in temperature).

• Wien’s Law - wavelength of peak emission 1 / temperature• Stefan’s Law - total energy radiated per second temperature4

Temperature

• We can use blackbody curves as thermometers to determine the temperatures of distant objects (thanks to Wien’s law - which relates the wavelength of peak emission to the temperature of the object).

Spectroscopy

• Spectral lines are indicators of chemical composition. As such, they can be used to identify the chemical composition of any body that emits (or absorbs) light.

• This is how we know the composition of the sun and other stars.

The Doppler Effect

• (Apparent wavelength / True wavelength) = (True frequency / Apparent frequency) = 1 + (Recession velocity / Wave speed)

• Objects moving toward us are blueshifted (higher frequency) and objects moving away from us are redshifted (lower frequency).

• By determining the amount of shift in spectral lines, we can determine an object’s recessional velocity (in general how fast it’s moving away from us, or sometimes how fast it’s moving towards us).

• The recessional velocity (this applies only to objects moving away from us) relates directly to the object’s distance from us, giving us a new way to measure distance.

Real Life Examples of Doppler Effect

• Doppler Radar (for weather)

• Airplane radar system

• Submarine radar system– Ok, anything with radar

• Radar gun, used by Law Enforcement Officers…

Doppler Effect• When something which is giving off light

moves towards or away from you, the wavelength of the emitted light is changed or shifted.

V=0

Doppler Effect• “Along the line of sight” means the Doppler

Effect happens only if the object which is emitting light is moving towards you or away from you.– An object moving “side to side” or

perpendicular, relative to your line of sight, will not experience a Doppler Effect.

Astronomy Application

V=0

Lecture Tutorial: Doppler Shift (p. 73)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on and

write complete thoughts into your LT.• If you get stuck or are not sure of your answer, ask me

or another group.

The Doppler Effect causes light from a source moving

away to:A. be shifted to shorter wavelengths.

B. be shifted to longer wavelengths.

C. change in velocity.

D. Both a and c above

E. Both b and c above

You observe two spectra (shown below) that are redshifted relative to that of a stationary source of light. Which of the following statements best describes how the sources of light that produced the two spectra were moving?

BLUE RED

Spectrum A

Spectrum B

A. Source A is moving faster than source B.

B. Source B is moving faster than source A.

C. Both sources are moving with the same speed.

D. It is impossible to tell from looking at these spectra.

Spectral Line Analysis• The composition of an object is determined by matching its

spectral lines with the laboratory spectra of known atoms and molecules.

• The temperature of an object emitting a continuous spectrum can be measured by matching the overall distribution of radiation with a blackbody curve.

• The (line-of-sight) velocity of an object is measured by determining the Doppler shift of its spectral lines.

• An object’s rotation rate can be determined by measuring the broadening (smearing out) of its spectral lines.

• The pressure of the gas in the emitting region of an object can be measured by its tendency to broaden spectral lines.

• The magnetic field of an object can be inferred from a characteristic splitting it produces in many spectral lines when a single line divides into two (known as the Zeeman effect).