P7 Quiz - Lenses and Telescopes, Mapping the Universe, The Astronomical Community

The speed of a wave (eg light) is affected by the [blank_start]medium[blank_end] it passes through. When light passes from one substance into one with a different optical [blank_start]density[blank_end], it will speed up or slow down. Because the [blank_start]frequency[blank_end] (number of cycles in a given time period) of the light remains constant, the [blank_start]wavelength[blank_end] changes, and this causes a change in direction. This effect is called [blank_start]refraction[blank_end]. Light rays slow down when passing from the air into the Perspex block, so they bend [blank_start]towards[blank_end] the normal (a line drawn at [blank_start]90[blank_end]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 [blank_start]air[blank_end], they [blank_start]speed up[blank_end] and bend [blank_start]away[blank_end] 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?

Diffraction is increased when waves pass through a ...

What is a diffraction grating?

Professional [blank_start]telescopes[blank_end] are quite large. One reason for this is that they need to collect visible light and other [blank_start]electromagnetic[blank_end] radiation to detect faint objects. Radiation is [blank_start]diffracted[blank_end] by the aperture of a telescope. The aperture is the hole through which the [blank_start]light[blank_end] must pass. In order to produce [blank_start]sharp[blank_end] images, the aperture must be very much [blank_start]larger[blank_end] 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?

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?

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 [blank_start]computer[blank_end]. This has the following advantages: - astronomers can work [blank_start]remotely[blank_end] - telescopes can be programmed to [blank_start]track[blank_end] objects so that data can be collected over a [blank_start]long[blank_end] period of time - the telescope can be [blank_start]precisely[blank_end] positioned to find a distant object - the [blank_start]data[blank_end] can be recorded and [blank_start]processed[blank_end] by computer

What are the disadvantages of space telescopes?

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 [blank_start]Earth[blank_end] appear to move over the course of a [blank_start]year[blank_end], relative to stars that are [blank_start]further[blank_end] away. In fact, it is caused by the [blank_start]movement[blank_end] of the Earth around the Sun. The parallax angle of a star is defined as half the angle that a [blank_start]star[blank_end] moves in six months, against a [blank_start]background[blank_end] of very distant stars. The effect of [blank_start]parallax[blank_end] can be used to measure how far stars are from Earth. The [blank_start]smaller[blank_end] the parallax angle, the further away the star is.

Finish labelling the diagram of parallax.

The nearer the star, the ..... the parallax angle.

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?

distance (parsecs) = 1/parallax angle (seconds of arc)

The brightness of a star can also be used to estimate its [blank_start]distance[blank_end]. However, this is not straightforward because a [blank_start]distant[blank_end] star that is very bright (luminous) might appear, to someone on Earth, as being very similar to a nearer star that is less [blank_start]bright[blank_end]. The [blank_start]luminosity[blank_end] of a star depends on two things: its size its temperature Cepheid variable stars do not have a [blank_start]constant[blank_end] luminosity. They [blank_start]pulse[blank_end] light and their luminosity depends on the [blank_start]period[blank_end] of their pulses (how long each pulse lasts).This relationship allows astronomers to [blank_start]estimate[blank_end] the distance to Cepheid variable stars - as long as they know how bright the star [blank_start]actually[blank_end] 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 [blank_start]distances[blank_end] in space. A supernova is an explosion of a red [blank_start]giant[blank_end]/red supergiant at the [blank_start]end[blank_end] of its life. All type 1a supernovae have the same peak [blank_start]luminosity[blank_end], so we can compare the peak luminosity to the [blank_start]apparent[blank_end] brightness to estimate the distance. However, supernovae are very rare, and it is necessary to follow supernovae from [blank_start]beginning[blank_end] to end to ensure it is a type 1a.

In [blank_start]1920[blank_end], two leading astronomers - Heber Curtis and Harlow Shapley - disagreed on whether there was [blank_start]one[blank_end] galaxy or many galaxies in the [blank_start]universe[blank_end]. Observations with telescopes had identified that the Sun was one of many [blank_start]stars[blank_end] in our galaxy (the [blank_start]Milky[blank_end] Way). Astronomers had also seen lots of fuzzy objects in the sky at night. Astronomers called these objects [blank_start]nebulae[blank_end].

Curtis argued that:

Shapley argued that:

The debate wasn't solved until a few years later, when [blank_start]Edwin Hubble[blank_end] measured the distance to some [blank_start]Cepheid[blank_end] Variables and found that they were much [blank_start]further[blank_end] away than either Curtis or Shapley imagined the Milky Way galaxy to be. Hubble found that they were part of the [blank_start]Andromeda[blank_end] spiral nebula, which was a galaxy in its own right (over [blank_start]2.5 million[blank_end] 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 [blank_start]further[blank_end] away a galaxy is, the faster it is moving away from us. The Hubble [blank_start]Constant[blank_end] 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 [blank_start]more[blank_end] accurate. Hubble's ideas and constant provide evidence for the [blank_start]Big Bang[blank_end]. Hubble's law: the speed of [blank_start]recession[blank_end] of a galaxy is directly [blank_start]proportional[blank_end] to its distance from the Earth - speed of recession = Hubble's constant x [blank_start]distance[blank_end].