Further physics - studying the universe Flashcards
How does the sun appear to move across the sky?
From east to west (I remember it by alphabetical order)
What is the time between the sun appearing at its highest point one day and the next called?
The solar day
How long is the solar day?
24hours
How long does it take for the moon to appear in the same place in the night sky from one day to the next?
24 hours + about 50 minutes, which is the time that the earth takes to rotate further to catch up with the moon as it orbits us.
How long does it take the stars to appear in the same place?
Slightly less than 24 hours.
What is a sidereal day?
The time taken for the earth to rotate 360 degrees
How long is the sidereal day?
23 hours 56 minutes
Why is the sidereal day shorter than the solar day?
We are orbiting the sun, so we need an extra 4 minutes to allow for the movement of the earth through space to put the sun where it was the previous day.
What does the moon orbit?
The earth
What causes the moon to be visible from earth?
It reflects some of the light from the sun.
How much of the moon is lit by the sun at any time?
Half.
Why does the moon appear to change shape from earth?
It is due to the change in the amount of the lit side of the moon that we can see.
What is the changing shape of the moon called?
The phases of the moon.
When the moon is almost directly between the earth and the sun, what shape moon do we see?
The crescent of the new moon
When the earth is almost directly between the sun and the moon what shape moon do we see?
The round full moon.
What is the time between one full moon and the next?
29.5 days.
What is the time between one full moon and the next called?
A lunar month.
What happens when the sun, moon and earth are directly in line?
It creates an eclipse
How does a solar eclipse happen?
The moon comes between the sun and earth, and the moons shadow falls on earth.
How can the tiny moon blot out the whole sun?
It is much closer to the earth, and they apparent size of the two objects from earth is almost the same.
Why can a solar eclipse only be seen from part of the earth?
The moon is a lot smaller than the earth, so the moons shadow covers a relatively small area.
How can a lunar eclipse happen?
When the moon passes into the earth’s shadow.
From where on earth is a lunar eclipse visible?
From any part where the moon is visible at that time.
Why don’t eclipses happen every time the moon orbits the earth?
The moon does not orbit the earth at the same angle as the orbit of the earth around the sun, the earth moon system is tilted slightly.
Which planets can be seen from earth with the naked eye?
Mercury, Venus, Mars. Jupiter and Saturn can also be seen, despite the distance, as they are very large objects.
What does the word planet actually mean?
It comes from the Greek for “wandering star”, as the planets move across the other stars.
As the planets orbit the sun, and we keep orbiting too, what happens to the distance between us and the planets?
It changes.
What happens to the apparent brightness of the planets as the distance between us changes?
It changes.
What happens to the stars as the earth rotates?
They, and the planets, appear to move east-west.
What happens to the position of the planets against the background of the stars?
It changes due to their orbits around the sun.
Sometimes the planets actually go backwards. What is this called?
Retrograde motion.
What happens to the speed of an object if the orbit is smaller?
It speeds up. (Think of a somersault, tight tuck = quicker movement)
What is the implication of the different speeds of the planets, as seen from earth?
Sometimes they will be moving towards us, at other times they will be moving away from us.
If Mars is on the opposite side of the sun from us, what will it’s speed seem to be, from earth?
Very fast, as we are moving in the opposite direction = we seem to see the sum of our speed, and the speed of Mars.
If we are on the same side of the sun, what speed is Mars doing (seen from Earth)
We would see the speed of Mars as the difference of the two speeds of Earth and Mars - but we are going more quickly, we are closer to the Sun, so it actually appears to go backwards for a bit.
What really happens to the speed of Mars as it orbits the sun?
It stays almost constant (remember that the orbits are not completely round, so the speed has to vary a little)
Where is the Pole star, relative to Earth?
Above the North Pole.
What is the Pole Star really called?
Polaris.
Why do the stars visible from earth change over the year?
We only see the stars at night, from the half that faces away from the sun, so we are facing a different part of the night sky as we orbit the earth. Polaris remains visible from the whole of the northern hemisphere of earth.
How would you describe the position of a star?
The angle of declination (distance above the equator) and the angle of right ascension (distance east-west).
What is the celestial sphere?
An imaginary globe around the earth that has the light of every celestial object projected on to it.
What is the angle of declination measured in?
Degrees, minutes, seconds.
What is the angle of right ascension measured in?
hours, minutes, seconds. 1 hour = 15 degrees.
When is the angle of right Ascension measured?
At the point where the sun crosses into the northern hemisphere, this happens at the spring equinox.
What is the spring equinox?
The day in spring when day and night are exactly the same length.
How does light travel?
As a wave.
How is light affected by the medium it is travelling through?
The speed varies.
What is the speed of light in a vacuum?
300,000,000 m/s = 3 x 10^8 m/s
If the speed of the light changes in different mediums, what happens to the frequency and the wavelength?
The frequency stays the same, so the wavelength has to be shorter if the speed is reduced.
When waves hit a boundary between two different mediums, where they will travel at different speeds, what will happen?
The light is refracted (bent).
Why is light refracted?
Think of a car going from the road to say, deep pebbles at an angle, the wheel that hits the pebbles first will slow down and cause the whole car to swing round it, changing direction.
How do we use refraction?
It is how a lens focuses light.
Draw the refraction diagram for a convex lens.
Make sure you use a ruler, and have sudden changes of direction at the edges of the lens, marks will not be awarded for bendy light waves!
What do convex lenses do to light?
Bring it together.
The power of a lens in measured in?
Dioptres
What is the formula for the power of a convex lens?
Power (dioptres) = 1/focal length
What is the focal length?
The distance from the lens that all the light will meet.
Light entering the eye from the objects around us is not parallel. Why do we say that the light from stars is?
The distances are so large that the light rays are effectively parallel.
If an object is on the principal axis of a lens, where will the image be?
At the focal point.
If an object is off the principal axis of a lens, what will happen to the image?
It will still be focused, but at one side of the focal point (opposite to the real object).
What are astronomical objects that are bigger than a point of light called?
Extended objects.
Name some extended objects?
Sun, moon, galaxies, planets and their moons
You will need to be able to draw ray diagrams - page 44 of the revision guide.
Ruler!!!
What does a simple refracting telescope contain?
Two convex lenses, the objective lens and the eyepiece lens.
What does the objective lens in a refracting telescope do?
It is a low power lens, with a long focal length. It has a large diameter to collect the most possible light.
What does the eyepiece lens in a refracting telescope do?
It is a small lens, higher power, short focal length. It acts as a magnifying glass to the image formed by the objective lens.
What is the function of the telescope?
To make objects appear larger.
How does the telescope make the image larger?
It increases the angle between the image and the axis of the telescope - so it is spreading the image out.
How do you measure magnification of a refracting telescope using the angles from the axis?
The angle from the axis of light leaving the telescope/ the angle from the axis of the light entering the telescope.
What does the magnification of a telescope actually depend on?
The ratio of the power of the lenses, and therefore the ratio of the focal lengths.
How would you measure magnification of a refracting telescope?
Magnification = focal length of objective lens / focal length of eyepiece length.
What is the magnification of a telescope always greater than?
1 - otherwise it is not helping
How does focal length relate to power?
f = 1 / power
Work out the magnification of a telescope where the eyepiece has a power of 100, and the objective has a power of 20.
f = 1 / power. fe = 1/ 100 = .01 fo = 1/20 = .05 magnification = focal length of obj. lens/ eye.lens magnification = 0.05 /0.01 = 5
How does a concave mirror reflect light?
As the mirror is curved, and the angle of reflection must equal the angle of incidence, it brings the rays together at a focal point.
How can a concave mirror be used in a telescope?
It is used instead of the objective lens, then a second mirror is used to reflect the captured image into the eyepiece
Why us a concave mirror in a telescope rather than a convex lens?
A mirror is lighter (as thinner), easier to support (as can be supported all the way behind, not just at the edges like a lens) so can be made much larger, easier to manufacture, no chromatic aberration.
What is chromatic aberration?
When light passes through a lens different colours (wavelengths) of light are diffracted by different amounts (like a prism) so you get colours on a telescope image where there should not be any (eg the moon has a coloured edge)
Who first built a telescope using a mirror?
Newton, in 1668. Mr Apple himself :)
What sort of lens is the objective lens of the telescope?
Convex
What sort of mirror in a telescope?
Concave.
What is diffraction?
The waves spread out after going through a small gap
What does the amount of diffraction depend on?
The wavelength of the wave, and the size of the gap in comparison to the wavelength.
What does the long wavelength of radio waves mean for the diffraction of radio waves?
They can be diffracted considerably, so,for instance, you can receive radio waves in a valley because the hills have diffracted the waves, even if you don’t have a direct line of sight to the transmitter.
What sort of telescope is Jodrell Bank?
Radio
What other sorts of telescopes are there?
eg X-ray, infrared, microwaves
What is the aperture of a telescope?
The size of the objective lens/mirror
What happens when the waves entering the telescope are diffracted by the small aperture?
You get blurry images.
What must you do to produce clear images?
Increase the size of the aperture to be longer than the wavelength.
What type of telescope will be most affected by diffraction?
Radio telescopes, as the wavelength of radio waves is so large.
What do we do to reduce diffraction in radio telescopes?
Join multiple dishes together in an array to create a telescope with an aperture hundreds of metres across.
What does a star’s spectrum show?
How much energy of each wavelength it emits.
What can studying the spectrum of a star tell us?
Information about composition and temperature.
When can you see the spectrum of visible light from the sun?
In a rainbow.
What could you use to create a spectrum?
A prism.
How does a prism split light?
The light is refracted as it enters the prism, different wavelengths are refracted by different amounts.
What are the colours of sunlight?
Red, Orange, Yellow, Green, Blue, Indigo, Violet (Richard Of York Gave Battle In Vain)
If the wavelength is shorter, what happens to the amount of refraction?
It increases
What colour light is refracted the most?
Violet.
What else can produce a spectrum?
A diffraction grating
What is a diffraction grating?
A series of very finely spaced gaps that light shines through; or a series of finely drawn lines on a surface that light reflects off (eg a cd!)
How many lines per cm on a diffraction grating?
Thousands.
As different colours are diffracted by different amounts, what would you see if you look at a diffraction grating, then move your head?
The colours will change
What did astronomers notice about quasars when they were first discovered?
The spectrum showed a very large redshift.
What does a large redshift mean?
The object must be moving away from us very quickly
Where would an object that is moving away so quickly be?
A long way away from us (expanding universe)
Why did some astronomers not believe that quasars were as far away as the red shift suggested?
Something that far away would not be visible.
Why are quasars only seen far away?
There are no quasars in the recent universe, they seem to have happened only at the beginning.
What are quasars?
No one really knows. They seem to be tied up with galaxy formation in the early universe, they only last a short time (maybe a billion years or so), none are being formed now, and they (probably) leave behind a black hole.
