Astrophysics Flashcards
What is the definition of a ray?
A representation of a light path
What is the definition of the normal line on a ray diagram?
The line perpendicular to the surface
What is the definition of the plane on a converging and diverging ray diagram?
The line drawn that is in the centre of your lens
What is the definition of the principal axis on a converging and diverging ray diagram?
The line that passes through the centre of the lens, perpendicular to its surface
What is the definition of the focal point on a converging and diverging ray diagram?
This is the point (drawn where your principal axis and plane intersect) where incoming rays travelling parallel to the principal axis will be refracted and converge/ are directed away from one another
What is the definition of divergence?
Light rays that spread apart over a distance/ spreads out incident light
What is the definition of convergence?
Light rays that come together over a distance/ focuses incident light
What is the definition of a real image?
A projected image formed from light focussing/crossing each other after refraction
What is the definition of a virtual image?
An apparent image formed on the same side of the lens - since the light rays are not focussed/ do not cross the image cannot be projected onto a screen
Which type of image can be formed on a screen?
A real image
In which type of lens are real images usually formed?
Converging lenses
In which type of lens are virtual images usually formed?
Diverging lenses - however, can be produced by converging lenses
When will a converging lens never form an image?
This is when your object is placed at your focal length - your refracted rays will be parallel to on another
When will a converging lens form a virtual image?
This is when your object is placed at a distance that is less than your focal length - your refracted rays will diverge hence you draw a dashed line backwards from them to find where your virtual image would form
What is the definition of focal length?
The distance between the centre of your lens and the principal focus
What are the 3 rays that you have to draw a converging and diverging ray diagram?
- Centre ray
- Parallel ray
- Focus ray
What is the definition of your centre ray on a converging and diverging ray diagram?
The ray that runs through the centre of your lens/ mirror curvature and does not deviate/ get refracted
What is the definition of your parallel ray on a converging and diverging ray diagram?
The ray that is drawn parallel to your principal axis that always goes through your principal focus, f:
- If your rays converge then your parallel ray will go through the principal focus on the opposite side of your object
- If the rays diverge then your parallel ray will go through the focus on the same side as your object (this focus is also focal length distance away from the centre of the lens)
What is the definition of your focus ray on a converging lens ray diagram?
The ray that runs through your focal point on the same side as your object and is then refracted parallel at the plane of your lens
What does a converging lens look like?
Has 2 convex lines facing outwards
How do you show where an image will be formed on a converging and diverging ray diagram?
Draw a line down to the principal axis from where the lines converge (real or virtual)
Where are your foci on a ray diagram?
Your foci are drawn on either side of your lens and are equidistant from one another and from the centre of the lens
What is the definition of your focus ray on diverging lens ray diagram?
The ray that runs through the principal focus virtually as it gets refracted parallel at the plane of the lens before it reaches this focus
What does a diverging lens look like?
Has 2 concave lines facing outwards
What does convex lenses normally do to rays?
Converge them - sometimes diverge when the object is placed at a shorter distance than the focal length from the lens
What do concave lenses do to rays?
Diverge them
What is the lens equation?
1/d(o) + 1/d(i) = 1/f
Where:
d(o) - is the distance from the centre of your lens to the object
d(i) - is the distance from the centre of your lens to the image you have formed
f - is the focal length of the lens (distance from the centre of your lens to the principal focus)
What is the definition of the focal plane on a converging and diverging ray diagram?
The line drawn straight down from your principal focus
What is an example of an infinitely far out object that we use lenses to view?
Stars
What are the sort of rays do we receive from infinitely far out objects?
parallel rays
Where is your f1 when you draw parallel rays coming from an infinitely far out object on a ray diagram?
Your f1 is where the first parallel ray passes the principal axis (this ray acts as your focal ray)
Where is your imaged formed when you have parallel light from an infinitely far away source coming into your converging lens?
At the principal focus
What do we use to view objects that are infinitely far away?
A telescope
What are the two types of telescopes?
- Refracting telescope
- Reflecting telescope
What are the two type of lenses used in a refracting telescope?
- Objective lens - the first lens
- Eyepiece lens - the second lens
What is the image formed from light passing through the objective lens of your refracting telescope?
- real
- inverted
- image formed at focal length (principal focus)
What is the image formed from light passing through the eyepiece lens of your refracting telescope?
- virtual
- inverted
- image formed at infinity
What is the main purpose of the objective lens?
To collect the light
What is the main purpose of the eyepiece lens?
To magnify the image
What is the same about the objective and eyepiece lens in your refracting telescope?
- Both convex lenses
- Have the principal focus at the same point (doesn’t mean they have the same focal length)
What happens when your refracting rays from your objective lens meet your eyepiece lens?
They refract to become parallel to one another
Describe the ray diagram of light coming into a refracting telescope from infinity
- Your rays come into the lens parallel to one another, the first ray to cross your principal axis acting as your focal ray and the ray that goes through the centre of your lens acting as your centre ray
- At the plane of your lens your rays will refract: the focal ray parallel and the centre ray will not deviate therefore you can find where they cross and draw all your other refracted rays to meet at this point.
NB: this point is your principal focus, therefore it should roughly be as far away from the centre of your lens as the focus on the other side. - Continue the paths of your rays until you reach the eyepiece lens (this should not be too far away as your want to maximise your magnification) This distance that your rays have just traveled is your focal length as the principal focus of your eyepiece lens in the same as your objective lens.
- At the plane of the eyepiece lens your rays will refract, becoming parallel to one another and going towards the principal axis. The parallel line that reaches the principal axis at the focal length is your focal point.
- You draw dashed lines going back from your refracted rays as we observe a virtual image at infinity
What is the equation for your telescope length in a refracting telescope?
f(o) + f(e) = length of telescope
What are two properties that an objective lens should have?
- Long focal length
- Large - in order to have a large collecting power
What is the magnification equation?
M = Eyepiece angle/Objective angle = beta/alpha = f(o)/f(e)
What is the definition of the eyepiece angle?
The angle subtended by the height of the real image from the principal axis, h, a distance, d, away (focal length)
What is the objective angle?
The angle subtended by the height of your object, h, and distance d away
How do you derive the equation for magnification, M?
For small angles tan(theta) = theta
Objective angle:
alpha = h/f(o)
Eyepiece angle:
beta = h/f(e)
beta/alpha = h/f(e) x f(o)/h
= f(o)/f(e)
How do you increase the magnification of a refracting telescope?
Increase the objective focal length and decrease the eyepiece focal length
What is the ultimate goal of a telescope?
To collect parallel rays of light and focus them onto a singular point
What is spherical aberration (description)?
Spherical aberration is when light is passed through a spherical lens and instead of the light being refracted to meet at a certain point, the light rays cross at different points due to the radial line being the normal
How do you draw spherical aberration on a diagram?
All your rays come in parallel into your spherical lens or mirror. Then your outer rays are refracted/reflected most and so cross each other first followed by the next two rays crossing further away etc.
What is the definition of spherical aberration?
The image blurring and distortion produced from a lens due to its curvature as the rays of light at the edge are focussed in a different position to the ones in the centre
In which types of telescopes will spherical aberration occur?
Refractive and reflective
How can you reduce the impact of spherical aberration in reflecting telescopes?
By using a parabolic mirror
What is the definition of chromatic aberration?
The image produced having coloured fringing due to the different focal lengths of the colours in white light as they are refracted by different amounts
In which types of telescopes will chromatic aberration occur?
Refractive
NB: it will occur a little bit in reflective telescopes but only in your eyepiece lens
What is the equation that demonstrates the order that the colours focus in, in chromatic aberration?
n = c/v = f(lambda)c/f(lambda)v = lambdac/lambdav
Therefore, the smaller the wavelength the more it is refracted, hence blue is refracted the most, then green and red is refracted the least.
Which colour is refracted the most in spherical aberration?
Blue -> Green -> Red
What are the two types of reflecting telescopes you need to know about?
- Cassegrain Telescope
- Newtonian Telescope
What type of primary mirror do both Cassegrain and Newtonian telescopes use?
Concave, parabolic with a long focal length
Which type of telescopes have longer focal length and, therefore, a greater magnification?
Reflective telescopes
What is the magnification equation for reflecting telescopes?
M = f(o)/f(e) (same equation for both types of telescopes)
Describe and draw parallel light rays entering a Newtonian telescope
- Your light rays enter in parallel to the rectangular telescope, one on either side of the plane mirror in the centre of your telescope/rectangle
- Your parallel light rays hit the concave, parabolic mirror at the back of the telescope and are reflected towards the central plane mirror. NB: they have not crossed at this point.
- Your plane mirror is angled in a 45 degree position facing downwards. After the two rays are reflected off of this plane mirror they cross before reaching the eyepiece lens. NB: continue your light rays as dashes through your plane mirror till they cross as this is your principal focus.
- The eyepiece lens is drawn just below your plane mirror and comes out of your rectangle slightly. When the light rays go through the lens they are refracted to become parallel to one another and then go into your eye.
Describe and draw parallel light rays entering a Cassegrain telescope
- Your light rays enter in parallel to each other and the rectangular telescope, one on either side of your convex secondary mirror that is placed in the centre of your telescope/rectangle.
- Your parallel rays hit the concave parabolic mirror at the back of the telescope and are reflected towards the central convex secondary mirror. NB: The rays have not crossed at this point and remember your concave primary mirror has a gap in the middle.
- Your convex secondary mirror faces your concave primary mirror. After the two rays are reflected off of the convex secondary mirror they cross before reaching the eyepiece lens. NB: continue your light rays as dashes through your convex secondary mirror till they cross as this is your principal focus.
- The eyepiece lens is drawn just behind your concave primary mirror and comes out of the rectangle slightly. When the light rays go through the lens they are refracted to become parallel to one another and then go into your eye.
Which reflecting telescope has a greater magnification? Why?
The Cassegrain telescope has a greater magnification. This is because its effective focal length of the objective is made greater by using a convex secondary mirror.
Which reflecting telescope is easier to manoeuvre? Why?
The Cassegrain telescope is easier to manoeuvre. This is because it is shorter than a similarly powered Newtonian telescope.
State the differences, advantages/disadvantages between refracting and reflecting telescopes
REFLECTING TELESCOPES
- Can be made much larger/have wider objectives than refracting telescopes (therefore greater collecting power and so can view fainter objects) as mirrors can be supported from behind, whereas a lens can only be supported from its edges
- Suffer from much less chromatic aberration
- Suffer from spherical aberration, however this can be eliminated by using a parabolic mirror
- Large magnifying power with small diameters
REFRACTING TELESCOPES
- A large diameter lens cannot be used as it can only be supported by its edges and may break under its own weight
- Suffer from both chromatic and spherical aberration
- Large magnifications require large diameters
Which types of telescopes are preferred to use in the modern day?
Reflecting telescopes
What causes a slight reduction in the amount of light viewed from a reflective telescope?
The secondary plane/convex mirror
What are the two regions of the EM spectrum that are least effected by the earths atmosphere?
Radio waves and Visible light (although still preferred to be placed in space)
Why do most telescopes have to be placed in orbit?
- Due to light pollution and other interference at ground level
- Due to absorption of the EM waves by the atmosphere
- As the wavelength of light that they are receiving would be distorted when entering the atmosphere (many different refractive indexes so the ray gets bent many times)
What is the definition of collecting power?
A measure of the ability of a lens or mirror to collect incident EM radiation
What are the 2 advantages of larger diameter telescopes?
- Greater collecting power so images are brighter
- Greater resolving power so images are clearer
What is the definition of resolving power?
A measure of the ability of a telescope to produce separate images of close together objects
What is the relationship between collecting power and the area of the objective lens?
Collecting power is directly proportional to the area of the objective lens
Since area is given by pi(d)^2/4 it is generally said that the collecting power is directly proportional to the (diameter)^2
What is required for an image of two objects to be resolved?
It is required that the angle between the 2 straight lines from earth to each object must be at least the minimum angular resolution (theta)
What is the equation for the minimum angular resolution?
theta = lambda/D
lambda - wavelength of EM radiation
D - Diameter of the objective lens/objective mirror
What is the equation of Rayleighs Criterion?
theta = lambda/D
lambda - wavelength of EM radiation
D - Diameter of the objective lens/objective mirror
What is the equation of resolving power?
theta = lambda/D
lambda - wavelength of EM radiation
D - Diameter of the objective lens/objective mirror
When analysing the resolving power of different telescopes, is it better to have a smaller or larger resolving power?
Smaller - as this means the telescope is able to resolve objects that are separated by a smaller angle
What are two ways, other than increasing diameter, that you can increase the collecting power of an aperture?
- Increase exposure times
- Use very sensitive detectors e.g CCD’s
What is the Rayleigh Criterion used to determine?
The smallest angular separation between two point objects which can be resolved by a telescope
What is meant by the Rayleigh Criterion in terms of diffraction patterns?
Two sources will (just) be resolved if the central maximum of the diffraction pattern of one source coincides with the first minimum of the other