Telescopes Flashcards
What is a convex/converging lens?
Lens which focuses incident light.
What is a concave/diverging lens?
Lens which spreads out the incident light.
Define Principle focus/ Focal point.
In a convex lens: The point where incident beams parallel to the principal axis will converge.
In a concave lens: The point from which light rays appear to come from.
Define focal length.
The distance between the centre of a lens and the principal focus.
What does focal length tell you about the lens used?
Shorter focal length = stronger the lens.
Draw a ray diagran showing the action of a converging lens on a beam of (parallel) light.
FDFD
What kinds of image does a converging lens produce?
Produces both a real image and virtual image.
What is a real image?
Image formed when light rays cross after refraction (on the lens-air boundary).
What is a virtual image?
Image that appears to have come from behind the lens. Light rays do not cross.
What is one difference between real and virtual images?
Real images can be formed on a screen, a virtual image cannot.
When is a real image formed?
When an object is further away than the focal length a real image is formed, otherwisea virtual image is formed.
Draw a ray diagram showinf how a real and virtual image is formed when light is passed through a convex lens.
- Show three rays coming from object: one axial ray (ray parallel to the principal axis), one passing through the centre of the lens (this one does not get refracted at all), and a third one that travels to the end of the convex lens to be refracted.
- If object sits on principal axis, axial ray and ray through principles axis will be the same.
- Label ‘object’ and ‘convex lens’, the ‘focal length’ and ‘virtual or real image’ - depends which one you are drawing (DRAW BOTH).
When drawing the virtua image diagram you can draw just two of the rays - axial ray and ray that passes through principal axis.
What will happens to NON-AXIAL parallel light incident on a conve lens
They will be incident on the focal place. Check cgp book to see a picture.
You dont need to know this equation but it is helpful.
The distance between the object and the lens axis (u) and the distance between image and the lens (v) are related by the equation?
1/f = 1/u + 1/v
where:
f = focal length
u = distance between the object and the lens axis
v = distance between image and the lens
How do you know if the image is virtual or real from the equation: 1/f = 1/u + 1/v
If v is positive = real image. If v is negative = virtual image
What are the two main types of optical telescopes?
- Refracting telescopes
- Reflecting telescopes
What does a refracting telescope consist of?
It consists of two convex lenses that focus the incident light by refracting it.
What are the two convex lenses that it contains?
The objective lens which produces a real inverted image of the very distant object and the eyepiece lens magnifies and reinverts the image this image correcting it to produce a magnified inverted virtual image.
In a refracting telescope, how are the eyepiece and objective lens arranged?
The refracting telescope is put in normal adjustment.
What is a refracting telescope in normal adjustment?
A telescope in normal adjustment is set up so that the principal focus of the objective lens is in the same position as the principal focus of the eyepiece lens.
Why is the eyepiece lens adjusted so it hs the same principal focus as the objective lens? i.e. why is a refractive telescope put in normal adjustment?
This is so that the light rays from each point of the real image leave the eyepiece parallel to to one another (the reverse of what has happened with the objective lens). To the viewer these rays appear to come from a virtual image at infinity.
Draw a ray diagram showing light rays from a istant object passing through a refracting telescope in normal adjustment.
- Draw a straight non-axial ray that passes through the CENTRE of the object lens and ends at the eye lens axis.
- Draw two non-axial parallel rays parallel to the first one drawn on either side and end at objctive lens axis. Draw a straight line from where each of these rays meet the objective lens axis, so that all of the rays cross at the same point on the focal plane and reach the eye lens axis. A real imgage is formed where they cross.
- Draw a dotted line that passes through the point the rays cross and the centre of the eyepiece lens. Label construction line. Continue the three rays drawn so they are refracted at the eye lens axis and leave the lens parallel with the construction line. You can show the virtual image formed at infinity by extending these lines backwards using these dotted line. Label objective lens, eyepiece lens, objective lens focal length, eyepiece lens focal length
What is angular magnification? Equation?
Angular magnification is the magnifying power of a refracting telescope. It can be calculated in terms of angles.
M = θₑ/θₒ
M = angular magnification
θₑ angle subtended by image at eye.
θₒ = angle subtended by object at unaided eye.
Look at cgp book for a really good image on what these terms^^ mean.
*Units do not matter as long as they are the same for both angles.
If the refracting telescope is in normal adjustement you can also calculate the angular magnification in terms of? Equation?
If the refracting telescope is in normal adjustement you can also calculate the angular magnification in terms of focal lengths.
M = fₒ/fₑ
M = angular magnification fₒ = objective lens focal length fₑ = eyepiece lens focal length
*Units do not matter as long as they are the same for both lengths.
What are some of the fundamental flaws of refracting telescopes?
Refracting telescopes suffer from chromatic and spherical abberations.
What is chromatic abberation?
This is when refraction by the objective convex lens causes white light to seperate into its component colours (wavelengths) so that the different colours/wavelengths are focused by the objective lens over a range of focal lengths.
Why does chromatic abberation occur?
This is bcause the refractive index of the objective lens’ material is different for different wavelengths of light.
What kind of image does chromatic abberation produce?
This produces coloured edges/fringes around the image.
Draw chromatic abberation showing two different wavelengths of light e.g. red and violet
On the Collins Astrophysics book.
What is spherical abberation?
This is when light rays in a parallel beam are refracted and focused at slightly different positions. Light rays near the edge of the lens are deviated more than those near the principal axis.
What is spherical abberation caused by.
It is caused by the curvature of the lens being too spherical.
How can the effect of spherical abberation be reduced in a refracting telescope?
The effect can be minimised by making both surfaces of the lens contribte equally (but oppositely) to the ray deviation.
ASK SIR THIS QUESTION AND CHECK WITH HIM .
Which refracting telescopes is the effect of spherical abberation greater?
The greater the diameter of the lens the greater the effect of spherical abberation.
What type of image does spherical abberation produce?
Spherical abberation causes blurring and distortion of the image.