7.2 - Light, telescopes and images Flashcards
What is speed of a wave affected by?
The medium it is travelling through.
What does a change of speed cause? Why?
A change in wavelength because the FREQUENCY OF WAVES CANNOT CHANGE.
As a result of a change in wavelength, what does this lead to?
A change in direction.
What is refraction?
When waves hit a boundary between two materials at an angle they change direction as they pass from one medium to another.
As light enters a glass block it slows down and refraction causes it to what?
Deviate TOWARDS the normal and SLOWS DOWN.
When light leaves the glass block, what happens?
Light bends AWAY from the normal and SPEEDS UP.
What is the ‘normal’?
A construction line at right angles to the surface of the glass.
Generally, what is refraction used by?
A lens to bring light into focus.
What is a Convex Lens?
A lens that is shaped in such a way so that it is thicker in the middle than at the edges.
When light enters a convex lens, what happens?
Light will be turned INWARDS as it deviates TOWARDS the normal.
On leaving the lens, what happens?
The light deviates AWAY from the normal but the shape of the lens means that this also turns the light INWARDS.
When light rays all CONVERGE on a single point, what is the scientific name for this?
Focal point
What do Convex Lens do?
Brings rays of light together. Parallel rays entering the lens are brought together at the focal point.
How much the rays are turned inwards depends on….?
The material of the lens and how curved it is.
The more curved the surface of the lens is, the….?
The further the rays are turned inwards and the shorter the focal length.
What is Power measured in?
Dioptres
The more powerful a lens, the….?
…The shorter the focal length.
What is the formula for Power?
Power (Dioptres) = 1 /Focal Length (metres)
What two features does a more curved lens possess?
- MORE powerful
2. SHORTER Focal Length
Why do we present light rays as parallel lines?
Astronomical objects are so distant that the light rays from them reaching Earth are effectively parallel sets of rays.
When a lens is used to focus the light from a distant star which is on the PRINCIPAL AXIS of the lens then….?
The image will be formed at the FOCAL POINT.
What would happen if the object is off the principal axis?
The rays will still be brought to a focus but the image will be formed off the principal axis.
When objects look larger than a ‘point’ (due to their distance), what is this called?
Extended objects.
How would you construct an image using a Ray Diagram?
- The Ray that passes through the centre of the lens continues in a straight line. Construct this.
- Make all the other rays refract (bend) towards the point of the straight line.
- The image is formed.
What does a simple telescope contain?
Two converging (convex) lens: an EYEPIECE lens and an OBJECTIVE lens.
What is the Objective Lens?
A LOW power lens with a LONG focal length. It has a large diameter to collect more light and make a brighter image.
What is the Eyepiece Lens?
A smaller but a HIGH power lens with a SHORT focal length. It acts like a magnifying glass to magnify the image formed by the objective lens.
What is the function of a telescope?
To make objects appear larger.
How does a telescope work?
It works by making the angle between the light rays and the axis of the telescope BIGGER.
What does the objective lens form when the parallel rays are converged?
It forms a REAL image between the two lens.
What does the eyepiece lens form when the light enters it?
A MAGNIFIED, VIRTUAL image.
What is the formula for (angular) magnification?
Magnification = Focal Length of Objective Lens / Focal Length of Eyepiece Lens
What do most astronomical telescope use?
Concave Mirror
What is the brief description of a Concave Mirror?
They’re shiny on the inside of the curve. Parallel rays of light shining on a concave mirror reflect and converge.
What is the centre of the mirror’s surface called?
Vertex
Where is the focal point in a Concave Mirror? Where do all the points lie on?
Halfway between the centre of the curvature and the vertex. These points all lie on the axis.
How is the image received by the user?
When the light converges from the Concave Mirror, the light reflects on a Secondary Mirror which reflects on the Eyepiece. This performs the final magnification.
What are the 4 main advantages that Concave Mirrors have over Convex Lenses?
- It is easier to manufacture a large mirror than a large lens.
- A larger mirror can be much thinner than a large lens so is lighter.
- A large mirror can be supported all along its rear side but a lens can only be supported at the edge, otherwise the light passing through the lens is blocked.
- It does not cause CHROMATIC ABERRATION.
What is Chromatic Aberration?
When light passes through a lens it is refracted towards the normal, however different colours of light are refracted.
In terms of telescopes, what effect does Chromatic Aberration have on them?
The light is separated into different colours and the image will appear coloured where it should not be.
How did Isaac Newton solve the issue of Chromatic Aberration when building the Newtonian Reflecting Telescope in 1668?
Using a mirror in place of the objective lens.
What is diffraction?
When waves pass through a gap/obstacle and spread out.
How intense will diffraction be if a narrow gap was placed?
MORE diffraction.
The ______ the wavelength the _____the amount of diffraction.
The LONGER the wavelength the GREATER the amount of diffraction.
Other than light telescopes, what types of telescopes are there?
Radio, X-Ray, etc.
What happens when radiation enters a telescope?
It is diffracted by the APERTURE of the telescope.
What is the Aperture of a telescope?
The size of the objective lens/objective mirror.
Because the aperture is circular, what would happen to a diffracted image of a star?
It will appear blurry with a bright central disc surrounded by circular rings instead of a sharp point.
How can we produce sharp images?
The aperture must be very much larger than the wavelength. The longer the wavelength of the radiation being detected, the larger the aperture needs to be.
In terms of radio telescopes, what kind of aperture is needed?
Radio waves have a LONG wavelength so radio telescopes are very large and are often joined together in an array to create a telescope with an aperture HUNDREDS OF METRES across.
When studying the spectrum of radiation emitted by a star, what information can that give us?
The star’s COMPOSITION and TEMPERATURE.
How can we separate light into the spectrum of colours?
By using a prism.
What is white light?
A mixture of ALL the colours in the spectrum.
The ______ the wavelength of light, the _____ it is refracted.
The SHORTER the wavelength the MORE it is refracted.
How else can we produce a spectrum?
A diffraction grating.
What is a diffraction grating?
Either a series of thousands of finely spaced gaps that light shines through OR a series of finely spaced lines drawn on a reflective surface that light reflects off.
When astronomers discovered quasars, what did they notice?
That their spectrum was highly shifted to the RED end of the spectrum.
What does this very large redshift mean?
That the Quasar must be moving away from us at a very high speed (and must be very far away).
