The speed of a wave (eg light) is affected by the ❌ it passes through. When light passes from one substance into one with a different optical ❌, it will speed up or slow down. Because the ❌ (number of cycles in a given time period) of the light remains constant, the ❌ changes, and this causes a change in direction. This effect is called ❌.
Light rays slow down when passing from the air into the Perspex block, so they bend ❌ the normal (a line drawn at ❌o from the edge of the block of Perspex at the point where the light hits it). As they leave the block and pass into the ❌, they ❌ and bend ❌ from the normal.
If a wave is travelling into the more dense material along the normal, it will not be refracted.
Label the diagram of a converging (convex) lens.
The power of a lens can be calculated using the following formula:
power of lens (dioptres) = magnification/focal length of lens (metres)
When drawing ray diagrams of distant objects, we consider the rays to be parallel when they enter the lens as they are coming from so far away.
How many lenses are needed to make the image from a telescope the correct way up?
2
3
4
1
Diffraction is increased when waves pass through a ...
.... smaller gap
.... larger gap
What is a diffraction grating?
A set of narrow, evenly spaced parallel lines ruled onto a thin sheet of glass or a shiny surface, which allows different wavelengths of light to diffract different amounts when light shines on the grating, creating a spectrum.
A prism which light shines through, causing the different wavelengths to separate and form a spectrum.
A hole which a wave passes through, causing diffraction depending on its size and position, as well as the wavelength of the wave.
Professional ❌ are quite large. One reason for this is that they need to collect visible light and other ❌ radiation to detect faint objects.
Radiation is ❌ by the aperture of a telescope. The aperture is the hole through which the ❌ must pass. In order to produce ❌ images, the aperture must be very much ❌ than the wavelength of the radiation which is detected by the telescope.
Which of the following are advantages of using mirrors over lenses in telescopes?
They can be made very smooth - resulting in undistorted images.
Mirrors reflect rays of all colours in the same way.
Suitable reflectors can be used to focus all types of electromagnetic radiation.
Objective lenses can have a maximum diameter of approximately 1 metre before it begins to sag, whereas a mirror of several metres in diameter can be used.
They are quicker to make
They are cheap
They are not affected by light pollution
The angular magnification of a telescope can be calculated using the following formula:
angular magnification = focal length of objective lens/focal length of eyepiece lens
Major optical and infrared astronomical observatories on Earth are mostly located in Chile, Hawaii, Australia and the Canary Islands. Why are these locations favourable?
They are at high altitudes so there is less atmospheric gas above them for light to pass through.
They have frequent cloudless nights.
They have dry air with very little atmospheric pollution which would disturb light from space.
They are far from streetlights and other sources of light pollution.
It is cheaper to build them there.
They are in the southern hemisphere.
Operating a telescope remotely means that the astronomer is in the same place as the telescope. Operating it locally means that the astronomer could be as far away as a different country.
Telescopes are usually controlled by ❌. This has the following advantages: - astronomers can work ❌ - telescopes can be programmed to ❌ objects so that data can be collected over a ❌ period of time - the telescope can be ❌ positioned to find a distant object - the ❌ can be recorded and ❌ by computer
What are the disadvantages of space telescopes?
no atmosphere - therefore light from distant stars is not absorbed or refracted
they can use parts of the electromagnetic spectrum that are absorbed by the atmosphere
they are very expensive to manufacture and place in orbit around Earth
telescopes can be programmed to track objects so that data can be collected over a long period of time
maintaining and repairing a space telescope is very expensive
space programmes can be unpredictable and at risk from funding cuts and changes in government policies
The majority of recent developments in astronomical telescopes have been as a result of international co-operation. This has two main advantages: - the cost of the venture can be shared - expertise can be shared
Parallax makes stars that are nearer to ❌ appear to move over the course of a ❌, relative to stars that are ❌ away. In fact, it is caused by the ❌ of the Earth around the Sun. The parallax angle of a star is defined as half the angle that a ❌ moves in six months, against a ❌ of very distant stars. The effect of ❌ can be used to measure how far stars are from Earth. The ❌ the parallax angle, the further away the star is.
Finish labelling the diagram of parallax.
The nearer the star, the ..... the parallax angle.
larger
smaller
wider
A parsec (pc) is a unit of distance that astronomers use. A parsec is defined as being the distance to a star with a parallax angle of 1 second of an arc.
An arcsecond = 1/x degrees. What is x?
60
3600
distance (parsecs) = 1/parallax angle (seconds of arc)
The brightness of a star can also be used to estimate its ❌. However, this is not straightforward because a ❌ star that is very bright (luminous) might appear, to someone on Earth, as being very similar to a nearer star that is less ❌.
The ❌ of a star depends on two things: its size its temperature
Cepheid variable stars do not have a ❌ luminosity. They ❌ light and their luminosity depends on the ❌ of their pulses (how long each pulse lasts).This relationship allows astronomers to ❌ the distance to Cepheid variable stars - as long as they know how bright the star ❌ is and how bright the star appears to be.
If you know the period of a cepheid, you can work out its luminosity. Then, if you compare the luminosity to the apparent brightness of the star from Earth, you can work out how far away it is from Earth.
Supernovae can be used to calculate in space. A supernova is an explosion of a red /red supergiant at the of its life. All type 1a supernovae have the same peak , so we can compare the peak luminosity to the brightness to estimate the distance. However, supernovae are very rare, and it is necessary to follow supernovae from to end to ensure it is a type 1a.
In ❌, two leading astronomers - Heber Curtis and Harlow Shapley - disagreed on whether there was ❌ galaxy or many galaxies in the ❌. Observations with telescopes had identified that the Sun was one of many ❌ in our galaxy (the ❌ Way). Astronomers had also seen lots of fuzzy objects in the sky at night. Astronomers called these objects ❌.
Curtis argued that:
The Milky Way was not the only galaxy in the universe.
The Milky Way was the only galaxy in the universe.
These nebulae were distant galaxies.
The nebulae were gas clouds within the Milky Way.
Shapley argued that:
The debate wasn't solved until a few years later, when ❌ measured the distance to some ❌ Variables and found that they were much ❌ away than either Curtis or Shapley imagined the Milky Way galaxy to be. Hubble found that they were part of the ❌ spiral nebula, which was a galaxy in its own right (over ❌ light years away!).
Using redshift, Hubble worked out the recession speed (v) of some nearby galaxies which he knew the distance to.
Hubble found that the away a galaxy is, the faster it is moving away from us. The Hubble was originally 500km/s per Mpc, but it is currently stated as (70.6 +-3.1)km/s per Mpc. It has changed because our methods are improving and becoming accurate. Hubble's ideas and constant provide evidence for the . Hubble's law: the speed of of a galaxy is directly to its distance from the Earth - speed of recession = Hubble's constant x .
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