Astrophysics: Lenses and Telescopes Flashcards
Do mirrors have a principle axis
Yes
Perpendicular to its surface and passes through its centre
What do lenses do
Change the direction of light rays
By refraction since light travels slower in glass than in air
Converging lenses
Convex
Thicker across the middle than the edges
So refract light rays towards each other
Principle axis
Horizontal axis through the centre of a lens
Lens axis
Vertical axis though the middle of a lens
What are axial rays
Rays parallel to the principle axis of a lens
Converge at the principle focus
What are non-axial rays
Rays that aren’t parallel to the principle axis
Converge somewhere along the focal plane but not the principle focus
Focal plane
The plane perpendicular to the principle axis that contains the principle focus
Focal length
Perpendicular distance between the lens axis and the focal plane
f
What do ray diagrams show
How light rays passing through a lens travels and where and what type of image is formed when they converge
How do you draw a ray diagram
One ray parallel to the principle axis straight through the top of the lens
That gets refracted through the principle focus
Another ray passing straight though the centre of the lens
That doesn’t get refracted at all
What 7 things can a ray diagram tell you
Real or virtual
Magnified or diminished or same size
Inverted or not
If an object hasn’t been magnified or diminished where will it be
The same distance from the lens axis as the object is from the lens axis
If an object has been magnified then where will it be
Image is further from the lens axis than the object is from the lens axis
If an object has been diminished then where will it be
Image is closer to the lens axis than the object is to the lens axis
What is a real image
Formed when light rays from one object are made to pass through another point in space
The light rays are actually there
Can be captured on a screen
What is a virtual image
Formed when light rays from a point on an object seem to have come from another point in space
Light rays aren’t where the image appears to be
So can’t be captured on a screen
Image is located at infinity
What does how big an object appears depend on
The angle which it subtends at the eye
Equation for how powerful a telescope is
Angle subtended by image at eye / angle subtended by object at unaided eye
Equation for angular magnification
Angle subtended by image at eye / angle subtended by object at unaided eye
How do reflecting telescopes work
Use a mirror to collect light and form an image
How do refracting telescopes work
Use two converging lenses
Function of objective lens in astronomy/normal adjustment
The object/planet is effectively at infinity
A large lens known as the objective lens forms a real, inverted and diminished image at its focal point
Two lenses in astronomy
Objective lens
Eye piece lens
Function of eye piece lens in astronomy/normal adjustment
Produce a final image of the first image at infinity
Has principle focus coinciding with the focal point of the objective lens
Final image is inverted
Assumption for ray diagrams
The bottom of the object lies on the principle axis
Angular magnification equation in normal adjustment
angle at objective/angle at eye piece
Expression for the length of a telescope in normal adjustment diagrams
fo+fe
Essentially the length from the centre of objective to centre of eye piece
Equation linking angular magnification to fo and fe
a/b=fo/fe
Obtained using trigonometry and the small angle approximation
Disadvantages of refracting telescopes
DDDMM
Using glass of sufficient clarity and purity free from defects to make a large diameter telescope is extremely difficult
Large diameter lenses are heavy and tend to distort under their own weight
Suffer from distorting factors (chromatic and spherical aberration)
Heavy and difficult to manoeuvre quickly
Large magnifications require large objective lenses and long focal lengths
Advantages of reflecting telescopes
Large single mirrors can be made which are light and easily supportable from behind
Mirror surfaces can be made a few nanometres thick so give excellent image properties
No chromatic or spherical aberration when using parabolic mirrors
Relatively light mirrors allow rapid responses to astronomical events
Two types of aberration
Chromatic
Spherical
Explain chromatic aberration
Different wavelengths of light are refracted by different amounts
Issue with lens telescopes since they use refraction
Effect is more pronounced at the edges
Which wavelengths of light are refracted most in chromatic aberration
Blue since shorter wavelength
Explain spherical aberration
Blurry images
Refraction increases at lens edges
