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 = angle subtended by eye β/ angle subtended by image α
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
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
