Astrophysics Telescopes Flashcards
How do you know if an image is virtual
When an object is on the same side of the focal point from the lens (this means the image is not inverted)
How do you know if an image is real
When the object is opposite to the focal point (this means the image is inverted)
What does a convex light do?
Focuses the incident light
What does a concave light do?
Spreads out the incident light
What is the principal axis
The straight line passing through the centre of the lens 90 degrees to its focus
What is the principal focus on convex and concave lens
Convex lens: the point where the incident light rays converge on the principal axis
Concave: the point where light rays should have appeared from behind the lens on the principal axis
What is the focal length?
The horizontal distance between the centre of the lens and the principal focus
What are real images
When light rays cross after refraction, real images can be formed on screen
What are virtual images
Formed on the same side of the lens, the light rays do not cross so a virtual image appears on the screen
What is the power of a lens>
A measure of how closely a lens can focus a beam that is parallel to the principal axis
- converging lens: positive
- diverging lens: negative
What is a refracting telescope? 1 mark
A telescope with two convex lenses
- objective lens and eyepiece lens
What is the role of the objective lens in a refracting telescope?
To collect light and create a real image of distant object.
It has a long focal length and is large to collect as much light as possible
What does the eye piece lens of a refracting telescope do?
Magnifies the image of the objective lens
lens produces a virtual image at infinity
reduces eye strain
What is normal adjustment for a refracting telescope
When the distance between the objective lens and eye piece lens is the sum of their focal length (fo + fe)
What is the formula for angle of magnification in a refracting telescope?
M = alpha / beta = fo/fe
Alpha is always bigger than beta,
What is the cassegrain reflecting telescope?
A telescope comprising of a small convex mirror and large concave mirror and an eye piece lens
Why are the mirrors in cassegrain reflecting scopes very thin and coated by silver atoms?
Allows mirrors to be as smooth as possible and minimise distortions in the image
What is chromatic aberration?
The focal length is of red light is greater than blue light which means they focus at different points (blue is refracted more than red)
- this causes a white object to produce an image with coloured fringing
- chromatic aberration is caused by refraction
What is spherical aberrration?
The curvature of the lens or mirror can cause light rays at the edge to be focused at different position, this leads to image blurring
- occurs mostly in lenses with large diameters which can be avoided using parabolic objective mirrors.
What is Achromatic doublet
A method to reduce chromatic and spherical aberration, this is done by cementing together a convex lens and concave lens to allow all rays of light to focus in the same position
What are the disadvantages of refracting telescopes?
- glass must be pure from defects
- large lenses can bend under their own weight
- chromatic and spherical aberrations
-refracting telescopes are heavy - large magnifications require large diameters
- lenses must be support only from the edges
Advantages of reflecting telescopes
- mirrors are few nanometers thick and give excellent image quality
- unaffected by by chromatic aberrations
- mirrors not as heavy
- easier to support from behind
What is collecting power
A measure of the ability of a lens or mirror to collect incident EM radiation
What is collecting power proportional to?
Pc = pi/4 * d^2
