telescopes Flashcards
Why were telescopes invented?
The invention of refracting and reflecting telescopes has enabled scientists to study the solar system, galaxy and universe in more detail than they were able to do with the naked eye.
What is refraction?
The speed of a wave (eg light) is affected by the medium it passes through. When light passes from one substance into one with a different optical density, it will speed up or slow down.
Because the frequency (number of cycles in a given time period) of the light remains constant, the wavelength changes, and this causes a change in direction. This effect is called refraction.
what happens with refraction of a light ray passing into a Perspex block from air:
Light rays slow down when passing from the air into the Perspex block, so they bend towards the normal (a line drawn at 90o from the edge of the block of Perspex at the point where the light hits it). As they leave the block and pass into the air, they speed up and bend away from the normal.
How do converging lenses work?
A converging lens uses refraction to bend the light rays coming out of it inwards. This has the effect of focusing the light to a point called the focal point.
does a convex Lens have more power?
Lens power
A convex lens with a greater degree of curvature, but made from the same material, will have a shorter focal length and therefore be a more powerful lens
How is the power of a lens calculated ?
The power of a lens can be calculated using the following formula:
power of lens (dioptres) = 1/focal length of lens (metres)
For example, a lens which has a focal length of 0.1 metres has a power of 10 dioptres
why do rays of light enter Telescopes as parallel?
Telescopes
Astronomical objects such as stars are so far away that the light rays that reach us from them are effectively parallel. We can therefore draw the rays of light entering a telescope as parallel rays.
Because the light from distant stars will be very faint, a large telescope will be needed to collect enough light to make a visible image.
Why are large telescopes needed?
Because the light from distant stars will be very faint, a large telescope will be needed to collect enough light to make a visible image.
Describe a simple optical telescopes
A simple optical telescope will have two converging lenses of different powers. The more powerful lens will be the eyepiece lens.
Describe the objective lens and the eyepiece lens.
The objective lens focuses the light to the focal point of the lens. This point is also the focal point of the more powerful eyepiece lens. The eyepiece lens produces a magnified image of the image from the objective lens which the viewer can see.
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How do you calculate the distance between the lenses?
The distance between the lenses will be the sum of the two focal lengths of the lenses
How does an Astronomical (reflecting) telescope work?
In a reflecting telescope, instead of a convex objective lens, a concave mirror is used to collect parallel rays from the object and form an image at the focal point. Then the convex eyepiece lens is used to magnify this image for the viewer
How do concave mirrors work?
Concave mirrors focus light at a focal point using reflection, not refraction.
Why are mirrors are used instead of lenses?
Mirrors are used instead of lenses in large, professional telescopes. This is because an objective lens can have a maximum diameter of approximately 1 metre before it begins to sag, whereas a mirror of several metres in diameter can be used.
what advantages do mirrors have (rather than lenses)?
Mirrors (rather than lenses) have other advantages:
They can be made very smooth - resulting in undistorted images.
Suitable reflectors can be used to focus all types of electromagnetic radiation.
Mirrors reflect rays of all colours in the same way.
