Module P7 L5

Module P7 L5

Chromatic Aberration

One problem with very powerful telescopes that rely on refraction is chromatic aberration.

This is when the lens focuses different colours in different positions.

This leads to a blurred image.

The effect is worse with more powerful lenses.

Experiment

How do convex and concave mirrors affect light?

Mirror, MirrorA curved mirror can be used to focus light rays instead of a lens.

It will not suffer from chromatic aberration because it relies on reflection and not refraction. (All colours are reflected [but not refracted] equally.)

Newton’s Telescope

In 1670, Sir Isaac Newton designed a telescope that used a curved mirror to replace the objective lens.

Ray Paths in Reflecting Telescopes

You would expect that this mirror would block out part of the image: but surprisingly it does not.

Reflecting Telescopes

Now all powerful telescopes rely on large mirrors to focus light.

This is the mirror of the Hubble Space Telescope. It is quite small because it had to fit inside a satellite.

The largest telescope in the world has computer-controlled segmented mirrors which are

10 m across.

What is Diffraction?

wavelength

Diffraction is the spreading out of waves as they pass through a gap.

The spreading out is much more noticeable when the gap is small compared to the wavelength.

Large gap = little diffraction; Small gap = large amount of diffraction

WIDE GAP NARROW GAP

Diffraction and Telescopes

Picture two stars next to each other in the night sky.

Their light will be diffracted (spread out) as it passes through a narrow aperture.

It will be harder to tell them apart or resolve them.

A telescope should have a wide aperture for it to have the best resolving power.

Diffraction and TelescopesVisible light has a typical wavelength of 10-7 m.

m 000 000 10

1m

10

1m 10

77

0.10 m

Question 1: Is diffraction going to be a problem with the optical telescope shown below?

Answer: Diffraction is not going to be a problem with this telescope. The aperture is much larger than the wavelength it is meant to capture so there will be no observable diffraction.

Microwaves have wavelengths of a few cm. Radio waves have wavelengths of 10s or 100s of metres. Question: Which of the radio telescopes shown below should be used to

observe

a) 15 m radio waves; b) 15 cm Microwaves

Give a reason to support each answer.

Answer a: This is suitable for observing 15 m radio waves as its aperture is large enough to prevent noticeable diffraction

2 m76 m

Answer b: This is suitable for observing 15 cm micro waves as its aperture is large enough to prevent noticeable diffraction. However, it would diffract 15 m micro waves.

Observing Long Wavelength Radio Waves

305 m

The Arecibo radio telescope has an aperture of 305 m.

Two or more radio telescopes can work as one larger telescope. The VLBA (Very Long Baseline Array) can act as a radio telescope with an aperture of 36 km (the telescopes can be moved along railway tracks).