Unit 5 – Light

ASTR 101Prof. Dave Hanes

The Importance of Light Light is radiant energy. Its propagation is the

transport of energy from one place (or one body) to another

An energetic (i.e. hot) body can simply lose energy (light) to its surroundings: e.g. a star shining into the void

But if the light is received and absorbed, there is an exchange of energy, with warming effects (possibly even life-sustaining, as here on Earth)

Finally, the light can be interpreted: there is information to be gleaned from its analysis. Astronomers do that!

Three Ways Light Can Reach Us

By direct emission: All bodies emit light, with very hot (‘incandescent’ ) objects emitting visible light. Astronomers study all wavelengths of light, not just the visible.

visible light infrared (heat) radiation

After transmission through a medium (like the Earth’s atmosphere, or clouds of gas in space). The medium may partly absorb or scatter the light in ways that allow us to interpret its properties.

After reflection by an object, giving us superficial (surface) information. We use this daily, as we look at everyday objects under natural or artificial illumination.

Its Propagation Through Space

Light provides our most important and essential ‘window on the Universe’

It travels through the nearly perfect vacuum of space, at the highest possible speed (symbolized “c”)

Measuring That Very High Speed!

Galileo: placed two people on separate mountain tops, each with a shielded lantern. One opens the shutter; the other responds in kind. What is the elapsed round-trip time?

Nowadays electronic devices make this feasible!

How Astronomers Study LightWe use telescopes and instruments of various kinds for two

reasons: to collect lots of light (to study faint, remote objects) to magnify images (to see details). But light provides much more than just “pretty pictures.”

The most important tool in astrophysics is spectroscopy, ourability to spread the light emitted by an object out into itsconstituent colours: a spectrum.

Light Tells Us

The chemical composition of the stars Their rotation speeds Their temperatures Their velocities The strengths of their magnetic fields Their pulsation and variability … and more -- all from looking at a ‘dot’ of light!

Moreover, Light is a Time Machine

Light gives us a way of looking into the past. We see remote objects as they used to be.

Such effects do not matter unless the distances are very large.

In ASTR 101, the most remote objects we consider (nearby stars) are a few light-years away, but the stars last millions or billions of years, so this is inconsequential. It matters in ASTR 102!

Consider the Speed

“c” = celerity (swiftness) = 300,000 km/sec across the room in less than a microsecond Kingston to Montreal in a millisecond

1.3 light-seconds away 8 light-minutes away

Two million light-years away

Billions of light-years away

What Is Light?

It transports energy, but in what form? There are two possibilities:

1. It is a lump (like a bullet): a photon

2. It is a disturbance(a wave) that propagates through some medium

Amazingly…

Light is both these things, and we will need to use both concepts at different times as we progress.

It acts like a photon (a discrete lump with a fixed energy) when interacting with atoms, yielding information that tells us the composition of the stars. We will return to this later.

In other contexts, it acts like a wave, with its energy distributed as a broad disturbance.

Think About Waves FirstThe notion of waves conjures up an image of

inherent fuzziness – energy spread over a region of space rather than localized into a little lump.

http://www.youtube.com/watch?v=QPxLs0Cv4zY/

Properties of Waves

There are two kinds: longitudinal (‘along’)transverse (‘across’)

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/TwoWaves.mp4

Note that the medium through which a wave passes moves back and forth, but there is no net displacement of the material.

Sound Waves in Air

Transverse Waves

Two Obvious Questions

1. How can we demonstrate that light is wave-like?

2. Given that, what sort of a wave is it? Longitudinal or transverse?

1. How Is Light Wave-Like?

Q: What do waves do that is special and different from ‘bullets’?

Watch

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/WavePart.mp4

(excerpted from https://www.youtube.com/watch?v=DfPeprQ7oGc)

Evidence for Waves: Diffraction. Light Does This!

Multiple Holes: Interference Patterns

Light Does This Too!

Newton’s Rings

PolaroidFilters

2. What Sort of Wave Is It?

Two possibilities:

http://www.youtube.com/watch?v=Rbuhdo0AZDU

Longitudinal The back-and-forth disturbance is along the direction the wave moves.

Transverse – like water waves, or the up-and-down undulations in a shaken rope. The back-and-forth disturbance is across (transverse to) the direction the wave moves.

Transverse Electromagnetic

Analogy to Wave in Water

Different Wavelengths (distances

fromcrest to crest)

One Puzzle with Light

But light travels through a perfect vacuum – no medium at all! How can it??

Before 1900, physicists assumed the

existence of the so-called ether, a mysterious medium that permeated everything.

Following Einstein, the need for the ether was abandoned. Light indeed travels through the vacuum of space: it needs no ‘medium.’

Interrelated Properties

Wavelength Frequency Speed of the Wave

A Simple Rule

Since speed (velocity) = frequency x wavelength, then:

high (large) frequency small (short) wavelength

Conversely,

low frequency large (long) wavelengths.

In Water

The wavelengths might be some metres

And the waves pass by moderately slowly

So the frequency can be very low (perhaps one ‘up and down’ motion every few seconds, as you float in place)

Staggering Numbers!

For yellow light, the wavelength is a bit more than 500 billionths of a meter.

But c = 300 000 km/sec.

So the associated frequency is almost 600 trillion ‘up-and-down’ motions per second.

The Critical PointDifferent frequencies (or wavelengths)

correspond to different colours (and also to different amounts of energy being carried along).

Red = longer wavelength, lower frequency, less energy

Yellow = intermediate

Blue = shorter wavelength, higher frequency, more energy

Even More Than Meets the Eye

ROYGBIV ?

Consider a PianoThe different frequencies of the vibrating

strings give varied sensations of pitch (high vs low)

Now imagine an infinite keyboard, with extremes way beyond our audible range.

The Full Electromagnetic Spectrum

Light also has this property – its spectrum is essentially boundless.

There exist wavelengths (frequencies) which are invisible to the eye, but which are nonetheless also light -- electromagnetic (EM) radiation.

We study all of them in astronomy, not just the visible light.

Our Eyes See OnlyOne ‘Octave’ of Light

Blue light has about twice the frequency of red light, so is (in musical terms) an ‘octave higher.’

A Reminder: Radio Astronomers are Not

‘Listening’ No Sound Reaches Us from the Stars!