Refraction P 7.2 LIGHT TELESCOPES AND IMAGES

You should understand that the wave speed will change if a wave moves from one medium into another a change in the speed of a wave causes a change in wavelength this may cause a change in direction as the frequency cannot change

Waves through a lens

light wavefront

wavelength

light ray focal length focal point

Ray diagrams

light from a distant star How do you work out where the image will be for a converging lens? astronomical objects are so distant that light from them reaches the Earth as parallel sets of rays

F F principal axis Mark the principal axis and the principal focus on either side of the lens. Then draw a vertical axis through the lens – this is where some rays will change direction.

The first ray to draw is one passing through the centre. A ray passing through the centre does not change direction. Remember light travels in straight lines, so rays should always be drawn with a ruler. F F

All these rays – and any others through the centre – are drawn in this way. None of them changes direction. F F

The second ray to draw is one through the focal point. F F

What will happen to this ray? F F

F F A ray through the focal point will leave the lens parallel to the principal axis.

F F All rays from the focus end up travelling parallel to the principal axis…

F F …and the reverse is also true. All rays arriving parallel to the principal axis end up passing through the focus on the other side.

image of a point object such as a star F F Where these two rays cross is the position of the image. These two rays are chosen because you can predict where they go.

F F There are many more rays that could be drawn (but you do not know where to draw them until you know where the image is).

F F

F F

F F

F F If you draw all the rays, you will end up with a complete beam of light. The beam will be in three dimensions, even through this diagram is drawn in two dimensions.

F light from one side of the Moon light from the other side of the Moon image of the Moon A star is a point object. The Moon is not a point object. Objects like it are called ‘extended’ objects. Rays from all over the Moon pass through the lens to give the image. This diagram shows two rays from one side and two rays from the other side. F

Focal length and power of a lens

Parallel rays of light from a distant object will converge and form an image at a point called the focus. light from a distant star focus

It doesn’t matter which direction the rays come from, they will be parallel and converge at the focus. light from a distant star focus

The distance from the middle of the lens to the focus is called the focal length. focal length focus

power of lens (D) = The power of a lens is measured in dioptres (D) when the focal length is measured in metres. REMEMBER..... f in metres!!!! 1 focal length (m)

Worked example A lens has a focal length of 25 cm, what is its power? Write the focal length in metres. focal length = 0.25 m Use the equation. power of lens = power of lens = = 4 D 1 focal length m

Diffraction

recall that light can be diffracted, and that the effect is most noticeable when light travels through a very small gap, comparable to the wavelength of the wave

Radiation is diffracted by the aperture of a telescope, The aperture must be very much larger than the wavelength of the radiation detected by the telescope to produce sharp images Views of a region of the night sky, taken with telescopes with different resolving powers.

The Very Large Array in New Mexico, USA, is the world’s largest radio telescope. It has 27 dishes that can be moved along the 21-km- long arms of a Y- shaped track.

Telescopes

Looking through the eyepiece, you should see an inverted (upside-down) image of a distant object. recall that a simple optical telescope has two converging lenses of different powers, with the more powerful lens as the eyepiece

astronomical objects are so distant that light from them reaches the Earth as parallel sets of rays Telescopes have: an objective lens (or mirror) to collect light from the distant object and form an image of it an eyepiece which magnifies this first image

The angular magnification of a refracting telescope focal length of objective lens focal length of eyepiece lens magnification = Can you see that magnification has no units?

Here is one way to gather a lot of light – build four identical telescopes. This quadruplet telescope is part of the European Southern Observatory in Chile. Each has an aperture of 8 m European Southern Observatory so when combined they equal a single telescope with an aperture of 16 m. Light from distant stars spreads out in all directions and the light can be very faint by the time it reaches Earth. Large (aperture) telescopes are needed to collect this weak radiation from faint or very distant sources

Problems with lens (refracting) telescopes Large aperture objective lenses are difficult to manufacture and are very heavy Refracting telescopes produce images with coloured edges because, (a little bit like a prism), the glass refracts different wavelengths of light by different amounts. This effect is called chromatic aberration

This is why most astronomical telescopes have concave mirrors, not converging lenses, as their objectives. Reflecting surfaces do not cause this chromatic aberration and large diameter mirrors can be more easily manufactured and supported. Rays parallel to the axis of the reflector are reflected to the focus. Parallel rays from another direction are focused at a different point.

The Arecibo radio telescope Puerto Rico is built into a natural crater. It cannot be steered about.

A problem with a reflecting telescope is where to place the observer. In the top diagram, the observer must be inside the telescope. In a different design, a small plane mirror close to the focus of the objective reflects light out of the telescope to an external eyepiece

Stars and galaxies emit radiation at many wavelengths (radio waves to gamma rays). By splitting the radiation into its spectrum we can learn a lot about stars and galaxies. A spectrum of the visible radiation from a star can be produced by refraction in a prism The Pleiades is a group of bright stars. Here the light from each star is broken up into a spectrum. Can you spot the black lines in the spectrum?

Astronomers use diffraction gratings to produce a series of spectra. Different colours (wavelengths) are diffracted by different amounts