Physics notes

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  • Created on: 23-06-13 12:23
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Bidur Bhattarai
Solar day is the time from noon on one day to the noon on the other day. I.e. 24 hours
Sidereal day is the times it takes for Earth to rotate on its axis.
A solar day is longer than a sidereal day because as earth rotates 360 degrees on its axis, it also
orbits the sun. The sun will no longer be directly overhead; the earth has to rotate more than 360 in
order for the sun to be directly overhead.
Earth rotates from West ­East, which makes the stars appear to move East- West. We know that
the Earth is rotating because the rotation makes the star move from east ­west.
It takes a little more than 24 hours to see the moon in the same spot every day because as the Earth
has rotated, the moon has also moved on so it takes us more than 24 hours to see the moon.
Phases of the moon are the different appearance of moon due to its relative position to Earth.
We only see the part of the moon that it lit by the sun, as it orbits the Earth, we see different
proportion of it lit up.
Lunar month is the time between two successive full moons.
Solar eclipse I s when moon sometimes passes between the sun and the moon casting a shadow on
earth and obscuring the sun.
Lunar Eclipse is when moon travels in Earth shadow and can no longer reflect suns light.
Lunar eclipse is longer than solar eclipse because the shadow casted by Earth on moon is larger than
the shadow casted by moon on earth. The shadow is bigger due to the earth's size.
We do not see eclipses every month because the moons orbit is tilted by about 5° relative to the
plane of the Earth's orbit so the sun and the moon do not line exactly often. It doesn't orbit the Earth
in the same plane as the Earth orbits the sun.
During a lunar eclipse moon can be seen, it looks red because red light from the sun is refracted by
Earth's atmosphere, so it can still reach the moon's surface.
Ecliptic shows the path of the sun during the year.
Eclipses can only occur when the moon passes through the ecliptic. This is more likely to occur when
the moon is to side of the Earth rather than between the Earth and the Sun.
Retrograde motion is backward motion of planet over a number of days as seen against the
backward motion of stars.

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When Earth catches up with Mars and goes past it which makes it seem like Mars is going backward.
Earth takes over Mars because it has a smaller orbit.
The amount of light a planet reflects to Earth depends on:
How close the planet is to Earth
How close the planet is to Sun
Its natural surface
The size of the planet
Earth's moon; the earth; the sun; the solar system; the galaxy; the universe.…read more

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Refraction is a change in direction of wave as it travels from one medium to the other, due to
change in wave speed.
Speed of light varies in different mediums.
When refraction occurs, the frequency remains the same but the wavelength changes which causes
change in wave speed which causes waves to change direction.
Diffraction is a spreading out of waves through a gap or an obstacle.
There are two types of lenses:
Convex lenses are thicker at the middle.…read more

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If a lens has long focal length, it will have low power. A stronger lens has short focal length and
refracts light more.
Lenses are placed in a telescope so that the length of the telescope is the sum of their focal length.
The objective lens is bigger than the eyepiece because it needs more area to collect light in, to make
a brighter image.
The eye piece is smaller than the objective but has a greater power. It magnifies the image.…read more

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Stars emit all types of electromagnetic radiation not just visible light.
Radio telescopes collect radio waves coming from stars using huge parabolic reflecting dish.
There are also telescopes in space, star emits X- rays and gamma rays, telescopes in space can form
images from these radiations. These radiations cannot be observed from Earth as the Earth's
atmosphere absorbs it.
Aperture is the opening in the telescope through which light can enter.…read more

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Telescopes on space
High cost of maintaining
High cost of setting up
There are also size limitations.
Resolving power of a telescope measures how well it can distinguish stars that appear distinct.
A spectrum of light can be seen by passing light through a glass prism.
Shorter wavelengths are refracted more than the longer wavelengths.
The colours that can be seen on the spectrum depend on the temperature and chemical composition
of a light source.…read more

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Even though, some stars are really bright, when viewed from Earth, they look faint. This is because
the light spreads out further as it travels through space. The radiation given out spreads equally in all
The Rutherford-Geiger-Marsden showed the evidence for nuclear atom.
Variable stars are stars which brightness increases over a period of time.…read more

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Hubble found out that the further the galaxy, the faster it is moving away from us.
Hubble constant is the ratio of recession of galaxy from Earth.
Speed of recession (km/s) = Hubble constant * distance Mpc
Hubble constant is 75
The fact that all galaxies are moving away from us suggests that we are in the centre of the universe,
and they are being repelled on us.…read more

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Nebulas are enormous clouds of hot gas and dust.
Gravity is constantly acting to pull the molecules and dust together. As the cloud collapses, the GP
energy gets turned to kinetic energy. The particles move faster and this lead to change in
temperature, which increases pressure. Eventually the pressure of the gas pushing outward is
balanced by the gravity pushing outwards and the star is in equilibrium
Proto star is a star in its early formation, before nuclear fusion has happened.…read more

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Once the core stops releasing energy, the balance of forces keeping the star in equilibrium is lost and
the gravity will start crushing the star.
Hertzsprung-Russell diagram is a chart that plots the luminosity of a star in the vertical axis and the
surface temperature in the horizontal axis which is measured in Kelvin.
Giant stars are star which is 10-100 times larger than the sun
Super giant are star that release enormous power and can fuse heavier elements.…read more


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