3.9.1 Telescopes Flashcards
What is a lens?
A piece of equipment which forms an image by refracting light
What are the 2 types of lenses?
Convex - converging lens
Concave - diverging lens
What is the focal point?
The point on a converging lens where lays of light meet
What is the focal length?
The distance between the centre of the lens to the focal point
What does the focal length depend on?
How curved/thick the lens is - more curved = shorter focal length.
The shorter the focal length the more powerful the lens is.
What is the difference between real and virtual images?
Real - light converges, image is inverted, can be projected into a screen, image formed in front of the lens.
Virtual - Light diverges from the focal point, image is upright, can be projected on a screen, image formed behind the lens.
What are the 3 rays needed to construct a ray diagram?
Ray passing through the principle axis and the lens (diagonal)
Ray that is parallel to principal axis and passes through principle focus
Ray that passes through the principle focus and emerges parallel to principle axis
What type of image is formed by a converging lens at varying distances?
d>2f:
image between f and 2f, real, inverted, diminished
d=2f:
image at 2f, real, inverted, same size
f<d<2f:
image beyond 2f, real, inverted, magnified
d<f:
image at 2f, virtual, upright, magnified
What is the lens equation?
1/f = 1/u + 1/v
- if the image is virtual, the value of v is negative
v = distance of image from lens
u = distance of object from lens
What is the formula for magmification?
M=h_1/h_0 = v/u
What is normal adjustment for a refractor?
The adjustment which enables the final image to be at infinity
conditions:
both lenses must be arranged so their focal points meet in the same place
Focal length of the objective lens must be longer than the focal length of the eyepiece lens
What is the formula for angular magnification?
θ= h/d = beta/alpha = angle subtended by the image/angle subtended by the object
What is the derivation for the formula of magnification in terms of angles?
tan α = h/f_o, using small angle,
α = h/f_o
hence tan β = h/f_e => β=h/f_e
M= β/α = h/f_e / h/f_o ……rearrange….
M=f_o/f_e
What is required to achieve greater magnification for refractors?
Objective focal length needs to be longer and eyepiece focal length needs to be shorter (for refractors).
They need to be very long
l = f_o+f_e
What are the properties of mirrors used by the Cassegrain telescope?
2 mirrors used:
primary mirror: large and concave
secondary mirror: small and convex
What are the features of the ray diagram for a Cassegrain telescope?
Rays enter telescope parallel to principal axis
Curvature of mirrors does not need to be the same
Rays do not cross before the secondary mirror
What is the Rayleigh Criterion?
States that two sources will be resolved if the central maximum of one diffraction pattern overlaps with the first minimum of the other
How can resolving power of a telescope be increased?
Increase diameter of aperture
Operate at a shorter wavelength of light
What is the formula for the Rayleigh Criterion?
θ =λ/D
λ = separation between 2 objects?? (wavelength)
D = horizontal distance to eye
What is the formula for minimum angular resolution?
θ ≈ λ/D
θ >λ/D - sources are resolvable
θ = λ/D - sources are just resolvable
θ <λ/D - sources are not resolvable
What is the collecting power of a telescope?
A measure of the amount of light energy it collects per second.
It is proportional to diameter^2
What is the advantage of having a higher collecting power?
Images are brighter and clearer
What is the definition of quantum efficiency?
The percentage of incident photons which cause an electron to be released in CCDs
What is the formula for quantum efficiency?
QE = no electrons produced per sec/ no photons absorbed per sec
*100
Why are CCDs better than the human eye?
Higher quantum efficiency, can observe a broader range of wavelengths, images can be stored and analysed digitally, better resolution
Describe the operation of CCDs
CCDs are divided into pixels. When a photon is incident on a pixel, it causes electrons to be released via the photoelectric effect. The electrons are confined to the pixel so it accumulates charge. The position and magnitude of the charge can create an image. The number of electrons released is proportional to the intensity of incident light.
