Chapter 5• Basic properties of light and matter.• What can we learn by observing light from distant objects?• How do we collect light from distant objects?

Solar Spectrum

Light and Matter

Why do we need to study light?

Light is the messenger that carries information of distant astronomical objects to us.

We will first talk about what is light and matter, and how they interact. Then we can understand…

– How can we learn about the composition of cosmic objects by observing the light they emit.

– How can we study the motion of cosmic objects by observing the light they emit.

– How can we tell about the environment (temperature, density, etc.) of the cosmic objects by observing the light they emit.

What is Light?

• Light is a form of energy (radiative energy).

• White light is actually the mixture of equal intensity of light with different color (in the visible wavelength regime).

• Different color of light carry different amount of energy.– Different wavelength.

– Different frequency.

• Light has dual personality:

– Light as wave—electromagnetic waves.

– Light as particle—photons.

How do we generate light?Light can be generated by accelerating (NOT just moving with a

constant velocity) an electric charge…• For example, if we make an electric charge move in a circle with a

constant orbital speed, then this accelerating electric charge will emit light.

• To emit red light, this electric charge needs to complete 500 trillions orbits in one second (5 ×1014 circles/second, or a frequency of 5 ×1014 Hertz)

• Moving a electrical charge back and forth alone a straight line also work—antenna

• Light can also be generated by heating up an object—thermal radiation.

Conservation of Energy is at work here…It takes energy to keep accelerating the electric charges. These energy (for example, chemical potential energy stored in the battery) are converted into the radiation energy of light!

Electric and Gravitational Fields• Gravitational Field

Matters change the property of the space around it. The property of space associate with the presence of matter is call the gravitational field.

• Electric FieldElectric charges change the property of the space around it. The property of space associate with the presence of electric charges is call the electric field.

• The gravitational (electric) field interacts with a second matter (electric charge) by exerting a force on the second matter (electric charge).

• The formula for the electric force between two charges is identical to the Newton’s law of gravity in form. If we replace mass m1 and mass m2 with charge e1 and charge e2, and appropriate constant C, then we get the formula for electric force.

221

221

r

eeCF

r

mGmF

e

g

rm1 m2

e1 e2

Electric and Gravitational Fields

• One BIG difference between gravitational and electric force is that there is no negative mass. Gravitational force is always attractive. – Gravitational force is the dominant force acting

between astronomical objects. • The net electric charges on most of the objects

are all very close to zero. Therefore, we do not experience electric force very much most of the time.

• Electric force is important inside the atoms.

Light’s Dual PersonalityWe can think of light as wave:• When light travels through space, the electric field along the

path of the light would vary in the form of a wave. • If we place a row of electrons along the path, then the

electrons would move up and down as a line of corks in wavy water.

• The wavelength of the light wave determines the energy and color of the light.

• The speed of light is 300,000 km/sec in vacuum, It does not depend on its wavelength.

• The diffraction and interference phenomena of light is a characteristic property of wave.

We can also think of light as particle

• Some properties of light resemble that of particles. For example, we can ‘count’ the number of photons we receive.

• Photons can have different energy.

• Photons DO NOT have mass!

• The energy each photon carries is related to the color (wavelength, frequency) of the photon.

Click on image to start animation

The Electromagnetic SpectrumBecause of the close association between light and electrical charges and magnetism, light waves are also called electromagnetic wave.

Our eyes are sensitive to only a very small fraction of the EM spectrum, which happens to be the part of the spectrum in which the Sun emits most of its radiative energy.Radio waves, microwaves, infrared and ultraviolet light, X rays, and gamma rays are all light that our eyes cannot see.

• The speed of light is 300,000 km in vacuum. – It is not infinitely fast…– Nothing can be accelerated to speed higher than

the speed of light.

• Speed of light does not depends on color. It is the same for all the colors.

• The energy light carries depends on its color. Red is low energy. Blue is high energy.

Matter• Ancient Greeks considered that matters are composed by tiny, indivisible particles — called

atoms.

• In modern physics, we have identified more than 100 different chemical elements

– Each element is made of a different type of atom,

– Example of elements are: hydrogen, helium, carbon, oxygen, iron, gold, silver, etc.

• Atoms have a nucleus, made up of protons and neutrons, with positive charges, and a electron clouds surrounding the neucleus.– Protons: each proton carries one positive electrical charge.

– Neutrons: neutrons are electrically neutral. They don’t carry any electrical charge.

– Electrons: each electron carries one negative electrical charge.

– Proton and Neutron have about the same mass.

– Electrons are much lighter than protons and neutrons.

N

P+

e- P+ e-

Helium atom has two protons and two neutrons in its nucleus, surrounded by two electrons

Hydrogen atom has only one proton and one electron

Chemical Properties of Elements

• The chemical property of an element is determined by the total electrical charge (or the number of protons) of the nucleus.

• Elements with the same number of protons but different number of neutrons are called isotope.

Interaction Between Light and Matter

Matter can emit, absorb, transmit, or scatter/reflect light.• Emission:

– Black Body Emission: An object with a finite temperature will emit light with a spectrum described by a black body spectrum.

– Spectral Emission: The atoms of the object can absorb only light at certain frequency, and then re-emit light in these frequencies in all direction.

• Absorption: Matter can absorb light, result in the increase of its temperature (conversion of radiative energy into thermal energy).

• Transmission: Some matters (like glass) allow light to propagate through. The speed of light in these matters will be different from that in the vacuum. The direction of propagation will be changed also.

• Reflection/scattering: Photons may bounce off the surface of some matters, like mirror (a thin coating of aluminum on the surface of glass). – Reflection: When the incident light travels toward the matter in the same

direction are bounced toward a same general direction, or– Scattering: When the light that was bounced off the surface of an object is sent

into random direction.

Examples of Light/Matter Interaction• Something that is white (to human eyes) means it reflect all the visible

light…• Something that is black (to human eyes) means that it absorbs all the visible

light.