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.

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.

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

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.

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.

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 ❌.

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!).