lecture 2

Question 1

Why do we need to understand Light?

Answer 1

In astronomy, LIGHT is the only way we get information about what’s happening in the Universe.
Astronomers are passive observers of what is going on. Light is the messenger. We need to understand it.

light is a wave

Question 2

When describing waves, we often speak of their:

Answer 2

wavelength : the distance between two adjacent identical points (crest-to-crest distance)

and

Frequency : the number of crests passing a given point every second

Question 3

frequency and wavelength has what kind of relationship ?

Answer 3

Inverse Relationship:

Higher Frequency –> Shorter Wavelength

Lower Frequency –> Longer Wavelength

Question 4

does waves has speed ?

Answer 4

Yes,
Waves have speed and they can transmit energy
The SPEED with which a wave moves depends on both its: FREQUENCY and WAVELENGTH

Question 5

how to det. the speed of a wave ?

Answer 5

c=f*lambda

c= speed in m/s 
f = frequency in Hertz (cycles/sec) 
lambda = wavelength in meters

Question 6

Calculate the speed of a wave that has a frequency of 3.5 x 1014 Hz and a wavelength of 857 nm

Answer 6

Solution:

857 x 10^-9 m).
(n = nano = 10^-9)

c = f lambda
= (3.5x 1014 Hz) x (857 x 10^-9 m)
= 3 x 10^8 m/s

Question 7

What is the speed of a wave that has a wavelength of 4.0 x 10-8 m and a frequency of 7.5 x 1015 Hz?

Answer 7

Ans: 3.0 x 108 m/s

Question 8

A wave has a wavelength of 450 nm and its speed is 3 x 108 m/s. What is its frequency?

Answer 8

Ans: 6.7 x 1014 Hz

Question 9

What is the wavelength of a wave which has a frequency of 1.8 x 1013 Hz and a speed of 3.0 x 108 m/s?

Answer 9

Ans: 1.7 x 10-5 m

Question 10

what is light ?

Answer 10

Light is a wave! A particle of light (photon)

vibrates and moves (transfers information) at the same time.

Question 11

Why isn’t it more obvious that light is a wave?

Answer 11

Because the wavelength of light is tiny.

Depending on the vibration (frequency and wavelength), light comes in many forms.

Humans can only see ONE form of light called the ‘visible’ form

Question 12

The term “light” or “radiation”

Answer 12

refers to any electromagnetic (EM) wave listed below:

Question 13

The colour of light depends on

Answer 13

on its wavelength and/or frequency:

The shorter the wavelength, the higher the frequency.

Question 14

colour corresponds to

Answer 14

wavelength
What we see as different colours are in fact different wavelengths of light! Visible light is really a combination of 7 different colours (wavelengths) of light.

Question 15

Visible Light

Answer 15

The range of colours the human eye is sensitive to is from violet to red, or, in wavelengths, from 400 to 700 nm

Question 16

why is sky blue ?

Answer 16

Sky is blue because sunlight gets scattered to your eye from all directions

Blue light is scattered more easily than red light

Question 17

why is the sun red ?

Answer 17

Sun looks red at sunrise/set because blue light gets scattered away

Question 18

Does all type of Em travels at exactly the same speed ?

Answer 18

Yes

the speed is 30. x 10^8 m/s

Question 19

what is the relationship between frequency and energy ?

Answer 19

The higher the frequency of light, the more energy it has!

Question 20

Does the color of the metal object depends on the temperature ?

Answer 20

Yes. 
Red = hot 
reddish-orange = hotter
yellowish-white = even 
hotter 
As an object is heated, it emits light at different wavelengths (depending on the temperature)

Question 21

does any object emits light ?

Answer 21

It turns out that any object emits light (radiation)

The type of light (radiation) depends on the temperature of the object

The hotter the object, the shorter the wavelength of light (radiation) emitted.

Question 22

so, does human emit light ?

Answer 22

YES! But not visible light – infrared light.

A special camera is needed for our eyes to detect infrared light

Question 23

blackbody :

Answer 23

A blackbody re-emits the same amount of radiation (light) as it absorbs

Question 24

spectrum ?

Answer 24

A graph of the amount of light given off at different wavelengths

If you measured the spectrum for a glowing iron bar at different temperatures you would get results very much like what you obtained for a star.

Question 25

a blackbody spectrum ?

Answer 25

A Blackbody Spectrum is a graph showing the: amount of light emitted by an object at each wavelength. Note: Virtually any hot object emits electromagnetic radiation in the form of a blackbody spectrum

Question 26

As a object get hotter, it changes colour from :

Answer 26

red to orange to yellow to blue and beyond

Question 27

Does hotter object have shorter or longer wavelength ?

Answer 27

it has shorter wavelenghts | towards the blue end of the visible spectrum

Question 28

Wiens Law

Answer 28

An iron bar and a steel bar in fire will produce the same blackbody curve. So will a rock, lump of clay, a piece of glass, etc hence, Blackbody radiation is totally independent of the blackbody’s composition.

Question 29

relationship between peak wavelength and temperature

Answer 29

Peak wavelength is inversely related to temperature Higher Temperature -->Smaller Wavelength or Cooler Temperature -->Larger Wavelength Wavelength is the only factor in determining the temperature of a star

Question 30

What Color is Our 5800oK Sun?

Answer 30

The Sun emits all colors (wavelengths) Green is most intense Sun appears WHITE to the human eye because it is giving off nearly equal amounts of all the visible colors of light white = all colours combined

Question 31

Atoms

Answer 31

An atom consists of a small, dense nucleus surrounded by Electrons (− charge) The nucleus contains: Protons(+ charge) Neutrons (no charge)

Question 32

electrons

Answer 32

Electrons may have only certain, discrete energies (Levels are designated by “n”) We can think of those energies as only certain allowed distances from the nucleus

Question 33

Light Creation

Answer 33

A particle of light (called a photon) is created when electrons jump down from a higher shell to a lower shell This is called Emission

Question 34

Light Destruction

Answer 34

Light particles (photons) are destroyed when electrons jump up from a lower shell to a higher shell This is called Absorption

Question 35

How does electrons jump to a higher orbit ?

Answer 35

Electron needs energy to jump to a higher shell (orbit) like walking up a flight of stairs (a) This energy comes from absorbing a photon (b) As electron jumps to a lower shell (orbit) it does not need this extra energy and so releases it as light (emission)