Telescopes and Diffraction Flashcards
How are simple refacting optical telescopes made up?
From two convex lenses with different powers: An objective lens and a more powerful eye lens / eyepiece.
How does the objective lens in a telescope work?
It collects the light from the object being observed and forms an image of it
How does the eyepiece in a telescope work?
It magnifies the image so we can view it.
How are the two lenses positioned?
So that they have the same principal axis and their focal points are in the same place
Which of the two lenses is more powerful?
The eyepiece
What images do the 2 lenses make?
The objective lens makes a real image
The eyepiece lens makes a virtual image
What is the formula for angular magnification if you’re given the focal lengths?
Focal length of objective lens
(Divided by)
Focal length of eye lens
What is the formula for angular magnification if you’re given the powers?
Power of the eyepiece
(Divided by)
Power of the objective lens
What do astronomical telescopes use?
A concave mirror
What is a vertex?
The centre of the mirror’s surface
Where is the focal point on a concave mirror?
Halfway between the centre of curvature and the vertex. These points all lie on the axis
When do waves spread out / ‘diffract’?
When they pass through a gap or past an object
What does the amount of diffraction depend on?
The size of the gap relative to the wavelength of the wave: the narrower the gap (or the longer the wavelegth) then the more the wave spreads out
What is aperture?
The diameter of the objective lens
How does the aperture effect the image formed from a telescope?
The bigger the aperture, the more radiation can get into the telescope and the better the image formed
What is a reason for why many telescopes use concave mirrors instead of lenses?
Because making large lenses is difficult and expensive, whereas big mirrors are much easier to make accurately
Why must the aperture size be larger than the wavelength?
Because when the light passes through the gap it diffracts and spreads out, causing the image to blur. Having a wider aperture means that the radiation passing through won’t diffract much and it’ll produce a sharp image
What happens when white light passes through the gaps in a diffraction grating?
The different wavelengths of coloured light are all diffracted but by different amounts, creating a spectrum of coloured light
How do astronomers use the spectrums produced by a diffraction gratings?
To analyse the light coming from stars
