{Year 2} Chapter 13 - Astrophysics (Missed out pgs 236 - 240 inclusive => After that up to top of 250)

Question 1

What is a converging lens?

Answer 1

A lens that is designed to focus light rays on a specific point

Question 2

What happens when a light ray passes through a lens at exactly its principal axis?

Answer 2

It is left undeviated

Question 3

What is the principal axis of a lens?

Answer 3

An imaginary line that passes through the centre of a lens and through the centres of curvature of the faces of the lens

Question 4

What is the focal point of a lens?

Answer 4

The point at which rays parallel to the principal axis of the lens are brought to a focus

Question 5

What is the focal length of a lens?

Answer 5

The distance between the centre of the lens and the point at which rays parallel to the principal axis are brought to focus

Question 6

What happens, in terms of rays, when an image is real?

Answer 6

They converge at a point. A real image can be focused on to a screen

Question 7

Which will produce a larger image of a distant object - a telescope with a larger or smaller focal length? (i.e is it better for the objective lens of a telescope to have a larger or smaller focal length)

Answer 7

A telescope with a larger focal length

Question 8

What is a magnifying glass?

Answer 8

A lens that causes rays to diverge, causing a large virtual image

Question 9

What is more powerful as a magnifying glass - one with a smaller or larger focal length?

Answer 9

Shorter focal length for magnifying glasses

Question 10

What is the equation to find the angular magnification of a telescope?

Answer 10

Angle subtended by image at eye / angle subtended by object at unaided eye, otherwise also expressed as focal length of objective / focal length of eyepiece

Question 11

What is spherical aberration?

Answer 11

As most lenses are made in a spherical shape, despite it not actually being the ideal shape for a lens. As a result, sometimes a lens will bring two rays into two different focal points, resulting in a blurring of the image we see

Question 12

What is one solution to spherical aberration and what are its drawbacks?

Answer 12

You can use a parabolic lens, which is more suited to refracting the rays to an exact focal point, however they are much more expensive and only give the advantage when the rays are exactly parallel to the principal axis

Question 13

What is chromatic aberration and how can you reduce it?

Answer 13

When a ray of light that consists of multiple wavelengths together, such as white light, refracts through the lens it can cause the different wavelengths to separate as they travel through the medium at different speeds. Chromatic aberration can be reduced by using a lens with two different types of glass

Question 14

What does it mean when it says the ‘angle subtended by an object’?

Answer 14

The angle between two light rays coming from its extremities - aka its very top and its very bottom

Question 15

What advantages does a reflecting telescope have over a refracting one?

Answer 15

Good astronomical telescopes are large, with a diameter of 15cm or more, for which it is hard to make a lens that size. However, it is much easier for a mirror of that size to be made // A reflecting mirror has no chromatic aberration // Spherical aberration can be reduced much more easily in a mirror by making it parabolic // It is possible to make reflecting telescopes with larger diameters that refracting telescopes

Question 16

What is the collecting power of a telescope?

Answer 16

A measure of the light intensity gathered by the telescope which is proportional to the square of the telescope’s diameter

Question 17

What are the advantages of a refracting telescope?

Answer 17

The lenses in a refractor are held in place by a metal tube, so little maintenance is required, whereas in a reflector the mirror is open to the air and might need recoating // The secondary mirror in a reflector has the disadvantage of blocking some light from entering the primary mirror // The secondary mirror and its supports will cause some diffraction and degrade the image // The mirrors in a small reflector can be knocked out of alignment, the strong construction of a refracting telescope makes such things unlikely

Question 18

In telescopes, what specific feature of an image does diffraction effect?

Answer 18

How well the telescope can resolve fine detail

Question 19

Who was Hipparchus and what did he do?

Answer 19

He was an ancient Greek who lived 2200 years ago, and was the first person to classify stars by their brightness

Question 20

What is the brightness of a star?

Answer 20

A measure of how much visible light from the star reaches our eyes

Question 21

What is the luminosity of a star?

Answer 21

The energy a star emits per second

Question 22

What is the steps between each magnitude of star?

Answer 22

approx. 2.5 times - e.g. a first magnitude star is 2.5 times brighter than a second magnitude star

Question 23

How do you work out the difference in apparent magnitudes between two stars?

Answer 23

Subtract the magnitudes from one another and then put 2.51 to the power of this number. The result of this is the brightness of the second star (the subtractor) over the brightness of the first star

Question 24

What is an astronomical unit?

Answer 24

The average distance between the Earth and the Sun, or 1.5 x 10^11m to 2 sig. fig.

Question 25

What is a light year?

Answer 25

The distance travelled by light in 1 year, or 9.46 x 10^15m

Question 26

Why and how can parallax be used to measure our distance from stars or astronomical objects?

Answer 26

As further objects appear to move less over large periods of time than closer objects do, and thus we can determine that they are further away

Question 27

What is absolute magnitude?

Answer 27

The apparent magnitude (measure of brightness as it appears in the sky) a star or object would have if it was 10 parsecs away

Question 28

What is the equation linking apparent magnitude, absolute magnitude and distance from us?

Answer 28

Apparent Magnitude (m) - Absolute Magnitude (M) = 5log(distance from Earth/10)

Question 29

What is black body radiation?

Answer 29

Radiation that is emitted from a black or non-reflective body, which is held at a constant uniform temperature

Question 30

What are the two important trends shown by the wavelength/power from black body graph?

Answer 30

As a body gets hotter, more radiation is emitted. The total power emitted is proportional to the area under the graph // The intensity of the radiation peaks at a shorter wavelength at higher temperatures

Question 31

What is Stefan’s Law?

Answer 31

Power = Stefan’s Constant x Area x Surface Temperature^4

Question 32

What is Wien’s Law?

Answer 32

Peak Wavelength x Absolute Temperature = 2.9 x 10^-3 meter-Kelvin

Question 33

What are Fraunhofer Lines and what causes them?

Answer 33

Dark lines on colour spectra from the Sun which have been caused by certain photons having exactly enough energy to excite electrons up to a higher energy level in the cooler outer gases of the Sun, resulting in the colour corresponding to the photon not being present in the spectrum

Question 34

What is useful about absorption spectra in learning about astronomical objects?

Answer 34

We can use them to find which elements are present, since each element has a corresponding set of absorption spectra caused by the electrons within it being excited, and thus when we map these against the spectra we get from stars or other objects, we can identify which elements are present

Question 35

What is a spectral class?

Answer 35

A way of measuring a star’s temperature by seeing which elements are present in them by seeing which absorption lines are present on their spectra

Question 36

How do scientists, still using the stellar spectra, differentiate between two hydrogen stars?

Answer 36

As, for example, in a cold star the electron in a hydrogen atom might be in its ground state of n=1, nearest the nucleus, but in a hotter star the electron might be in the n=2 state. The UV light missing from the spectrum is more visible in the hotter stars than in the colder ones

Question 37

What temperatures can the hottest of the blue stars reach in K?

Answer 37

25,000 to 50,000

Question 38

What colour are the coldest stars and what temperatures are they normally around?

Answer 38

Red, <3500K

Question 39

In the coolest stars, what causes prominent absorption lines?

Answer 39

Titanium oxide

Question 40

What causes the most prominent absorption lines in the hottest stars?

Answer 40

He and He+

Question 41

How was our sun originally formed?

Answer 41

From a large cloud of gas that gradually compressed under gravity and heated up as a result - since gravitational potential turns to kinetic and kinetic turns to heat as the atoms crash into each other. The temperature in the centre of the ball of gas becomes astronomical, and fusion is kickstarted as hydrogen atoms start to fuse to make helium. The energy produced as a result of the fusion is released as electromagnetic radiation.

Question 42

What size can a star be in its conception?

Answer 42

Stars formed can range in size from 0.1 solar masses to 100 solar masses, however stars below or above these limits are too small to start fusion or too unstable to remain as a whole respectively.

Question 43

What is the typical lifecycle of a star like the Sun?

Answer 43

A bunch of gases that collapse under their gravity // heat up and start fusion of hydrogens // outer fusion radiation balanced by gravitational field pull // core runs out of hydrogen - collapses slightly but this heats it up further so that it fuses helium - it is at this point where the star becomes a red giant as the grav. forces become too little for the even greater amount of electromagnetic radiation // eventually the star runs out of helium and shrinks to a white dwarf, powered by the energy of the star slowly contracting // the white dwarf runs out of energy after a very long time and becomes a black dwarf