OCR 21st century science P7 flash cards
- Created by: isis
- Created on: 13-06-13 09:06
How often does the sun move across the sky?
The sun appears to travel from east to west once every 24 hours.
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How often does the moon move across the sky?
The moon moves slightly slower than the sun, reappearing every 24 hours and 49 minutes.
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What is a constellation?
A group of stars that form a pattern in the night sky.
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What is a solar day?
The time taken for the Earth to rotate so that it fully faces the sun again. It is exactly 24 hours.
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What is a sidereal ay?
The time taken for the Earth to rotate 360 degrees. It is 23 hours and 56 minutes.
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What is right ascension?
Right ascension measures the angle east from the vernal equinox point. The angle is commonly measured in hours, minutes and seconds.
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What is declination?
Declination measured the angle of the star above or below the celestial equator. The angle is usually measured in degrees and minutes.
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What planets can be seen by the naked eye?
Mercury, Venus, Mars, Jupiter and Saturn.
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What is retrograde motion?
An apparent reversal in a planet's usual direction of motion, as seen from the Earth against the background of fixed stars. This happens periodically with all planets beyond the Earth's orbit.
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What is a solar eclipse?
When the Moon comes between the Earth and the Sun, and totally or partially blocks the view of the Sun as seen from the Earth's surface.
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What is a lunar eclipse?
When the Earth comes between the Moon and the Sun, and totally or partially covers the Moon in the Earth's shadow as seen from the Earth's surface.
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Why does the moon precisely block the Sun?
It is because the have the same angular size (about 0.5 degrees). The Sun is 400 times the diameter of the Moon, and it is 400 times as far away.
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What is the effect of the sun being an extended source of light?
It means that the both the Earth and the moon do not have shadows with hard edges. Instead there is a region of total darkness ( the umbra) fringed by a region of partial darkness (the penumbra)
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How can the umbra be used to explain eclipses?
When the Moon's umbra touches the surface of the Earth, a solar eclipse is seen from inside the area of contact. When the Moon passes into the Earth's umbra, a lunar eclipse is seen.
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Why is it more likely to see a lunar eclipse than a solar eclipse?
The Earth's shadow is much bigger than the Moon's, so the moon is more likely to pass into it. Also solar eclipses are only seen in a very small area.
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Why are lunar and solar eclipses so rare?
It is because the Moon's orbit is tilted relative to the plane of the Earth's orbit by about 5 degrees. Usually the Earth, Sun and Moon are not in a line so no eclipse occurs.
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What is angular size?
It is the angle between two line drawn from and observer to each side of an object. It depends on the objects actual size and its distance away.
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What is an optical telescope?
They use visible light. Some are designed for people to look through, but most modern optical telescopes record the image using electronic detectors.
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What is an aperture?
The light-gathering area of the objective lens or mirror
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What does a bigger aperture mean?
It means that it collects more radiation, which allows fainter object to be studied.
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How have new types of telescopes changed people's ideas about stars and the Universe?
Since the mid-20th century, astronomers have developed telescopes to study radiation in all parts of the electromagnetic spectrum. These telescopes have produce results that came as a complete surprise. For example studying UV radiation.
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What is a real image?
An example of a real image is when light that is scattered, or given out, by an object passes through a pinhole and make an image on a screen. A real image can be recorded using a light-sensitive electronic detector or photographic film.
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What is a ray diagram?
A way of representing how a lens or telescope affects the light that it gathers, by drawing the rays (which can be thought oh as very narrow beams of light) as straight lines.
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What can a ray diagram for a pinhole camera help to predict and explain?
Why the image is upside down, what happens to the image if the screen is moved away from the pinhole and what happens if you enlarge the pinhole.
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What does a ray diagram for a pinhole camera look like?
Only rays travelling towards the pinhole get through and light up the screen. As the pinhole is very small, each point produces a tiny spot of light together these spots make up the image.
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What is a converging lens?
A lens that changes the direction of light striking it, bringing the light together at a point.
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What is the principle axis?
An imaginary line perpendicular to the centre of a lens or mirror surface. Rays of light parallel to the axis are refracted so that they meet at a point.
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What is the focus?
The point at which rays arriving parallel to its principle axis cross each other also called the 'focal point'.
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What is the focal lenght?
The distance from the centre of the lens to the focus. The longer the focal length of a lens, the larger (actual physical size) will be the real image that the lens produces of a distant object.
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How can you compare lenses by simply looking at them?
For lenses of the same material, a lens with a long focal length has surfaces that are not very strongly curved, and a lens with a short focal length has surfaces that are more strongly curved.
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How do you estimate the focal length of a converging lens?
Stand next to the wall on the opposite side of the room to the window. Hold up the lens and use it to focus an image of the window on the wall. Measure the distance from the lens to the wall- this will give a good estimate of the focal length.
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What sort of lenses have a greater power?
The shorter the focal length of the lens the greater the power, and the longer the focal length the lower the power. As one increases, the other decreases, they are inversely proportional.
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What is power measured in?
dioptres, D
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What is the formula for the power of ta lens?
power (D) = 1/focal length (m)
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What is light like when it reaches a lens from a very distant point?
All the line entering the lens are parallel. Parallel rays at an angle to the principal axis converge to one side of the principal focus.
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What is refraction?
Waves change their wavelength if they travel from one medium to another in which their speed is different. Light changes direction when it passes at an angle from one material to another.
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What is the eyepiece lens?
It is then lens next to your eye. This is the stronger of the two lenses.
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What is the objective lens?
It is the lens nearer to the object you are observing. This is the weaker lens.
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How does a ray diagram explain how a telescope of two converging lens works?
The objective lens collects light from a distant object. Parallel rays of light enter the objective lens from a point on the distant object. A real image is formed. The eyepiece lens is magnifying glass, which is used to look at the real image.
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What does the amount of radiation gathered by a telescope depend on?
The collecting area of its objective lens. The larger the area the more that will be able to be seen.
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What are telescopes mainly used for?
As a telescope doesn't make distant stars bigger, they are used to reveal detail in object that are much bigger than stars ( galaxies or clouds of glowing gas) or much closer (planets)
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What are the different ways of thinking about a telescope with magnification of 50?
The telescope makes the Moon seem 50 time bigger than its actual size. The telescope makes the Moon seem to be only 1/50th of its actual distance away. The telescope make the Moon's angular size look 50 times bigger.
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What is angular magnification?
The ratio of the angle subtended by an object when seen through the telescope to the angle subtended by the same object when seen with the naked eye.
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What is the formula for magnification?
magnification=(focal length of objective lens)/(focal length of eyepiece lens)
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What is an extended object?
An astronomical object made up of many points, for example the Moon, or a galaxy. By contrast a star is a single point.
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What is a specrometer?
An instrument that divides a beam of light into a spectrum and enables the relative brightness of each part of the spectrum to be measured
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Why is using a spectrometer useful for astronomers?
By analysing the make-up of radiation received, astronomers can find out more about its source. Radiation can provide important clues about the temperature of an object and its chemical composition.
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What is a spectrum?
One example of a spectrum is when a beam of white light passes through a prism, the merging ray is coloured,
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What is dispersion?
It is the splitting of white light into colours.
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What causes the different colours?
The colour of light depends on its frequency (it therefore also depends on it wavelength, as the two are link.0
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What does the visible light spectrum extend from?
The visible band extends from red through to violet. Red has the lowest frequency (longest wavelength) and violet has the highest frequency (shortest wavelength).
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What causes dispersion?
Dispersion happens because light of different frequencies travels through glass at different speeds. The differences are small but they are enough to split the light up so that different colours are refracted through different angle.
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Why can't a an objective lens be larger than 1 metre?
Any larger and the lens would sag and change shape under its own weigh, making it useless for focusing light.
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What is a problem with glass of a large diameter?
It is very difficult to ensure that the glass is uniform in composition all the way through.
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What is problem if the middle of a converging lens is quite fat?
Some light is absorbed on its way through, making fainter objects appear even fainter.
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Why are glass lenses only useful in looking at visible light?
Radiation in other parts of the electromagnetic spectrum is either completely absorbed (UV) or goes straight through (radio).
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What is the law of reflection?
angle of incidence=angle of reflection
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What are the advantage of using a parabolic mirror instead of a lens?
A mirror reflects rays of all colours in the same way. It is possible to support a mirror several metres in diameter so that it does not sag. Its weight can be supported from the back and sides.
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What are other advantages of using a mirror instead of lens?
A mirror can be made very smooth so the image is not distorted. By choosing suitable materials, reflectors can be made to focus most types of electromagnetic radiation.
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What is a problem with using a reflecting telescope and what are the solution?
Where to place the observer. In some the the observer must be inside the telescope, in others a small plane mirror close to the focus of the objective reflects light out of the telescope to an external eyepiece.
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What are the reasons for telescopes having to be so big?
In order to collect a lot of radiation in order to detect faint object and to make the image clearer so that more detail can be seen.
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What is meant by resolving power?
The ability of a telescope to measure the angular separation of different points in the object that is being viewed. resolving power is limited by diffraction of the electromagnetic waves being collected.
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What happens when waves hit a barrier?
They bend a little at the edge and travel into the shadow region behind the barrier. This effect is called diffraction. The longer the wavelength of the wave, the more it diffracts.
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What happens at a gap between two barriers?
Waves bend a little at both edges. If the width of the gap is similar to their wavelength, the waves beyond the gap are almost semicircles. If the gap is really tiny, much less than the wavelength of the waves, the waves do not go through at all.
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What happens when a beam of radiation from a distant object goes through an aperture?
Diffraction occurs and the waves spread out to give a blurred image. Even a very small amount of diffraction causes blurring,.
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To reduce diffraction what are astronomers doing?
They are designing telescopes with apertures much larger than the wavelength of the radiation they want to gather. For optical astronomers, diffraction is not too big a prob
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Why is diffraction not much of a problem for optical astronomers?
The wavelength of light is so small. More blurring is usually caused by 'twinkling' as light passes through the atmospher.
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Why is diffraction a problem for radio astronomers?
The wavelength they use are normally a few centimeters. Even a radio telescope a mile across does not have a very good resolving power.
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What is being done to get a big aperture for radio telescopes?
Radio telescopes often have an array of several small dishes linked together. This arrangement can have the same resolving power as a single telescope with an aperture equal to the distance across the array.
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What radiation is allowed through and which is absorbed by the atmosphere?
The atmosphere transmits visible light, microwaves, radio waves and some infrared, It absorbs X-rays, gamma rays and much infrared.
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Why do stars twinkle?
The scintillation is caused when starlight passes through the atmosphere.
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Why can the atmosphere be described as not uniform?
There are areas of different densities. The atmosphere is in constant motion, so areas of different density move around. This causes a ray to be refracted in different directions, and is the cause of scintillation.
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Why is being able to record images from astronomical telescopes electronically useful?
Computer software allows astronomers to reduce or remove completely the effects of scintillation from the images they produce.
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What is light pollution?
Light created by humans, for example, street lighting, that prevents city dwellers from seeing more than a few bright stars. It also causes problems for astronomers.
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What other problems are there for astronomers?
Electrical equipment can produce weak radio waves, particularly when being switched on and off. Radio waves used for broadcasting and mobile phone can interfere withe the work of radio telescopes.
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What is a light-year and how far is it in kilometers?
It is the distance electromagnetic radiation travels through a vacuum in a year. One light-year = 9.5 x 10'12km
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What is one method of measuring the distance of stars?
To use the idea of parallax.
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What is parallax?
The apparent shift of an object against a more distant background, as the position of the observer changes. The further away an object is, the less it appears to shift. This can be used to measure how far away an object is.
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What is the parallax angle?
When observed at an interval of six months, a star will appear to move against the background of much more distant stars. Half of ots apparent angular motion is called its parallax angle.
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What can be said about the distance of stars and the parallax angle?
The closer the star, the greater its parallax angle.
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How is a parallax angle measured?
Parallax angles are usually measured in fractions of a second of arc. There are 360 degrees in a full circle, 60' (minutes) of arc in 1 degree, and 60" (seconds) of arc in 1'. A second of arc is 1/3600 of a degree.
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What is a parsec (pc)?
A unit of astronomical distance, defined as the distance of a star that has a parallax angle of one arcsecond, A parsec is about 3.1 x 10'13km.
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What is the formula for distance in parsecs?
Distance= 1/(parallax angle (sec))
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What is the observed brightness?
A measure of the light reaching a telescope from a star.
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What is luminosity?
The amount of energy radiated into space every second, This cane be measured in watts, but astronomers usually compare a star's luminosity to the Sun's luminosity.
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What does a star's luminosity depend in?
It's temperature- a hotter star radiated more energy per second from each square metre of it's surface.
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What does a stars observed brightness depend on?
It depends on the star's luminosity and its distance from Earth.
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What is a variable star?
It is a star where the luminosity varies.
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What did John Goodricke discover?
A new type of variable star. He noticed that a star called delta Cephei went from dim to bright and back again with a time period of about a week and that this varibale was very regular.
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What causes the Cepheid variable to change in luminosity?
It is thought that the star is contracting and expanding so that its temperature and luminosity vary.
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What did Henrietta Leavitt notice?
She noticed that the brightest Cepheids varied with the longest periods, and drew a graph to represent this. She realised that the stars that appeared brightest were also the ones with the greatest luminosity - they were all roughly the same distance
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What is the first part of measuring galaxies using cepheid variables?
Find some nearby Cepheids whose distances have been measured using other methods. Measure their brightness and work out their luminosities. Plot a graph of luminosity against period.
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What is the second part of measuring galaxies using Cepheid variables?
Look for a Cepheid in a star cluster or galaxy of interest. Measure its observed brightness and period of variation. From the period, read its luminosity off the graph. Use the luminosity and observed brightness to work out distance.
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How far has the Cepheid method been used to measure galaxies?
Used to measure galaxies up to a few megaparsecs away.
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Other cards in this set
Card 2
Front
How often does the moon move across the sky?
Back
The moon moves slightly slower than the sun, reappearing every 24 hours and 49 minutes.
Card 3
Front
What is a constellation?
Back
Card 4
Front
What is a solar day?
Back
Card 5
Front
What is a sidereal ay?
Back
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