Lecture 15

Question 1

Properties of light

Answer 1

-White light is made from the whole visible spectrum
-Light is a form of energy
-Light travels at a finite speed (3x10^8m/s)

Question 2

How does light interact with matter?

Answer 2

-Emission
-Absorption
-Transmission: transparent objects allow light to pass and opaque objects absorb light
-Reflection: changes in the light direction (but not energy)

Question 3

Properties of waves

Answer 3

-Wavelength: distance between two peaks
-Frequency: number of times a wave vibrates up and down
-Wave speed= frequency x wavelength

Question 4

What are photons?

Answer 4

-Particles of light
-Photons have a frequency and a wavelength
-Photon energy depends on frequency (E=hf)

Question 5

What is spectroscopy?

Answer 5

Viewing the amount of light from each wavelength that is emitted by an object

Question 6

What is the continuous spectrum?

Answer 6

Light produced over the range of the visible spectrum without interruption.
-Incandescent light globes produce a continuous spectrum

Question 7

What is an emission line spectrum?

Answer 7

The different wavelengths of light emitted by unique elements.
-Low-density gas clouds emit light of specific wavelengths characteristic of the composition and temperature

Question 8

What is an absorption specturm?

Answer 8

The opposite of an emission spectrum. Light absorbed by unique elements.

Question 9

What is a chemical “fingerprint”?

Answer 9

The wavelengths emitted by atoms due to the unique set of energy levels of the electron shells. Each transition corresponds to a unique photon energy.
-Transitions of electrons back to their ground state produce an emission spectrum
-Transitions of electrons to higher energy levels produce an absorption spectrum

Question 10

How do the energy levels of molecules differ from individual atoms?

Answer 10

Molecules have additional energy due to their bonds which can rotate and vibrate.

Question 11

What does thermal radiation depend on?

Answer 11

Temperature

Question 12

Properties of thermal radiation

Answer 12

1. Hotter objects emit more light at all frequencies
2. Hotter objects emit more photons with a higher average energy
3. Thermal energy=blackbody radiation

Question 13

What does Wien’s Law predict?

Answer 13

The peak in thermal radiation

Question 14

How does light tell us the speed of a distant object?

Answer 14

The Doppler effect

Question 15

How does the Doppler Effect describe the motion of objects?

Answer 15

Objects coming towards us will experience shortening of their wavelengths producing blueshift. Conversely, objects moving away from us will experience longer wavelengths producing redshift

Question 16

How is the Doppler Effect measured?

Answer 16

It is measured from shifts in the wavelengths of the spectral lines

Question 17

How do spectral lines change in a rotating object?

Answer 17

Faster rotating objects produce wider spectral lines

Question 18

What does redshift tell us about the universe?

Answer 18

That it is expanding.