Physics Light and the EM Spectrum + Astronomy

Reflection/refraction, colour, lenses, EM spectrum, radiation & temperature, the solar system, mass/weight/gravity, orbits, stars, doppler effect & red-shift

Reflection

light ray bounces off a surface instead of passing through or being absorbed

refraction

light ray changes directions as it passes from one medium to another

normal

line at a right angle to the line separating the two mediums (dotted line in diagrams)

How to draw reflection on a curve

1. draw a tanglent (striaght line, in place of where a mirror would be)

2. draw the normal at 90’ to the tangent

3. use the normal to draw the diagram

what do waves do when they slow down & speed up

bend towards the normal when they go slower

bend away from the normal when they go faster

frequency

number of waves to pass a point per second, aka number of vibrations per second (f)

wavelength

distance from one point on one wave to the same point on the next wave (λ)

what happens to the frequency, wavelength & speed as a wave refracts (deep water → shallow water)

• wavelength decreases

• frequency stays the same

• therefore speed decreases (because of v = fλ)

• therefore wave bends towards the normal

what happens to the frequency, wavelength & speed as a wave refracts (shallow → deep water)

• wavelength increases

• frequency stays the same

• therefore speed increases (because of v = fλ)

• therefore wave bends away from the normal

critical angle

the angle of incidence at which the refracted ray travels along the edge of the glass, at 90’ to the normal

when angle of incidence (i) = critical angle
angle of refraction (r ) = 90 degrees

luminous object

something that emits its own light

what happens when light hits (non-luminous) objects

some light is reflected off, and some is absorbed into it

the light reflected is the colour the object appears, and all the other colours are absorbed

e.g. a blue object would reflect blue light and absorb all other light

black & white light

white light is made up of all the colours

white objects reflect all colours off it

back objects absorb all light

what happens if your shine coloured light on a different coloured surface

it will appear black, as the surface absorbs the light (as the light isn’t the colour of the surface), so there is no light to be reflected

example: If you shine red light onto a green surface it looks black. This is because the green surface will absorb the red light, and there is no green light to be reflected

2 types of reflection

specular reflection

• flat surface

• all light reflect evenly

• keeps a clear image

• e.g. a mirror

diffuse reflection

• rough surface (even just slightly)

• light reflects in different directions

• doesn’t give a clear image

filters

another way of producing coloured light

they transmit (let through) certain colours of light and absorb others

remember the actual filter itself does still reflect, as we can see

what happens if two different coloured filters are used together

no light gets through

filter 1 transmits only one colour, and filter 2 absorbs that colour

lenses definition

pieces of transparent material shaped to refract light in a particular way

they operate by refraction, bending light as it passes through a different matierial

focal point

the point where the rays of light from a lens converge

focal length

the distance from the centre of the lens to the focal point

power (of a lens)

how much the lens bends light

a more powerful lens has a shorter focal length & is more curved

converging lens

a type of lens that brings light rays together

diverging lens

a type of lens that spread light ways apart

real image

formed when light rays from a point are made to converge onto another point

can be captured & projected onto a screen

e.g. looking at a distant object using a converging lens

virtual image

light rays appear to have come from a place, and don’t converge

can’t be captured / projected

e.g. looking at a nearby object using a converging lens

lenses ray diagrams

they are a model of what happens, so we make the model simpler than the real situation

• concentrate on what happens with just a few rays of light

• show the direction changing in the middle of the lens

converging lens ray diagram (with a real image)

1. a ray parallel to the optical axis passes through the focal point on the other side of the lens

2. a ray that passes through the focal point emerges parallel to the optical axis on the other side (opposite of #1)

3. a ray that passes through the centre of the lens continues with no change in direction

the image is at the point at which these three lines meet, on the other side of the lens to the object

the image will be real as long as the object is in front of the focal point

converging lenses creating virtual images

if the object is in front of the focal point, it will form a virtual image, which will appear:

• on the same side of the lens as the actual object

• the same way up as the object

• magnified

diverging lens ray diagram

• a ray parallel to the optical axis emerges as through it was coming from the focal point

• a ray that passes through the centre of the lens continues with no change in direction

the virtual image is at the point at which these two lines meet, on the same side of the lens to the object

diverging lenses creating virtual images

diverging lenses always create virtual images, which will appear:

• on the other side of the lens as the actual object

• smaller than the actual image

what type of waves are em waves

transverse waves - perpendicular oscillations (up & down)

what are em waves created by

the vibrations of charged particles

which two fields do em waves create oscillations in

electric and magnetic (hence electromagnetic)

how fast do em waves travel (in a vacuum)

they all travel at the speed of light

types of em waves (shortest → longest wavelength)

1. radio waves

2. microwaves

3. infrared

4. visible light (ROYGBIV backwards)

5. ultraviolet

6. x-rays

7. gamma rays

which types of em waves can our eyes sense & why

our eyes can only sense visible light

because the cells in the retina of the human eye are only sensitive to those wavelengths

uses of radio waves (3)

• broadcasting

• communications

• satellite transmissions

uses of microwaves (3)

• cooking

• communications

• satellite transmissions

uses of infrared radiation (6)

• cooking

• thermal imaging

• short-range communications

• optical fibres

• (tv) remote controls

• security systems

uses of visible light (3)

• vision

• photography

• illumination

uses of ultraviolet light (4)

• security marking

• fluorescent lamps

• detecting forged bank notes

• disinfecting water

uses of x-rays (3)

• observing the internal structure of objects

• airport security scanners

• medical x-rays

uses of gamma rays (2)

• sterilising food/medical equipment

• detection & treatment of cancer

dangers of microwaves

internal heating of body cellsd

dangers of infrared radiation

skin burns

dangers of ultraviolet light

damage to surface cells / the eyes, leading to skin cancer / eye conditions

dangers of x-rays

mutation / damage to cells

dangers of gamma rays

mutation / damage to cells

temperature

a measure of the amount of thermal energy in an object

the temp of an object affects the thermal energy it emits

what happens as the temp of something gets higher

• the intensity of emitted radiation gets higher

• the average wavelength of emitted radiation gets shorter

• the average frequency of emitted radiation gets higher

power (in temperature) & its unit

the amount of energy per time, measured in watts (w)

1 watt = 1 joule per second
1W = 1J/1s

what does it mean when radiation is emitted

energy is transferred away from an object

what does it mean when radiation is absorbed

energy is transferred towards an object

what needs to happen for an object to stay at the same temperature

it needs to emit the same amount of energy as it absorbs in a certain time

aka it needs to emit radiation at the same power as it absorbs

what happens if more radiation is emitted than absorbed

• the object loses more energy than it gains

• temperature decreases

• after a while it stars emitting less radiation and becomes balanced again

  • temperature stops decreasing and stays the same

what happens if less radiation is emitted than absorbed

• the object gains more energy than it loses

• temperature increases

• after a while it stars emitting more radiation again so it becomes balanced

• temperature stops decreasing

what do greenhouse gasses e.g. carbon dioxide do to earth’s temperature

they are gasses in the atmosphere that absorb energy radiated from the surface instead of letting it escape to space, creating with greenhouse effect and warming earth up

the sun

the star at the centre of the solar system that radiates energy

planets

celestial bodies moving in orbit around the sun at different distances & speeds

8 planets in solar system: 4 rocky/terrestrial & 4 gassy/giant

different sizes, but all large & rounded

natural satellites

celestial bodies that orbit other celestial bodies (planets or asteroids)

also known as moons

dwarf planets

small planetary-mass objects in orbit around the sun

like planets but smaller

asteroids

small rocky bodies orbiting the sun

there are load in the solar system (over 1 mil)

comets

icy objects that move in an elliptical orbit that can take a long time & take them very far away from the sun

lots in solar system but not as many as asteroids (4,000)

name the 8 planets, starting with the one closest to the sun

• mercury

• venus

• earth

• mars

• jupiter

• saturn

• uranus

• neptune

geocentric model

earliest model of the solar system

earth in the centre, everything else orbiting around it in circles

planets also going in small circles in their orbits

made by ptolemy

created just from observing the planets with eyes (no telescopes) and charting them

heliocentric model

second, more accurate (but still not completely accurate) model of the solar system

sun in the centre, other planets orbiting in circles around it. moon orbits the earth but no other planets have moons

created by copernicus

created with the use of telescopes, which ptolemy didn’t have for the geocentric model

our current model

the same as the heliocentric model, except for a few differences:

• planets orbit the sun in slightly elliptical orbits, instead of perfect circles

• other planets also have natural sattelites (moons) orbiting around them, not just us

• more planets - uranus and neptune, which aren’t on the heliocentric model

Mass

the amount of matter in an object measured in kilograms (kg)

Weight

the force due to gravity experienced by an object in newtons (N)

Equation linking weight, mass & gravity

w = mg

w = weight in N

m = mass in kg

g = gravitational field strength in N/kg

2 factors that effect gravity & how they effect it

mass of planet

the more mass a planet has, the larger the gravity will be on the surface

how far away the object is from the planets centre - so radius of the planet

the smaller the radius, the larger the gravity on the surface

how does mass effect gravity

a larger mass causes gravity’s pull to be stronger

How does radius of a planet effect its gravity

because objects will be closer to the core (the centre of mass)

orbits of planets (what shape, around what)

slightly elliptical, around the sun

orbits of moons (what shape, around what)

orbit around planets or asteroids, orbit varies - it can be circular or elliptical

orbits of comets (what shape, around what)

very elliptical orbit, around the sun

low earth orbit of an artificial satellite

• close to the earth’s surface

• takes short time to complete an orbit - around 2hr

• used for imaging and mapping

polar orbit of an artificial satellite

• like a low earth orbit but around the north & south poles

• eventually will pass over all parts of the earth (because of the earth’s spin)

• used for monitoring the poles and weather forecasting

geostationary orbit of an artificial satellite

• high orbit

• always over the same place on the earth

• takes 24h to complete 1 orbit

• used for broadcasting and transmissions

how does height effect orbits of an artificial satellite

the higher the satellite is above the surface, the slower it orbits

centripetal force

• the force that keeps satellites in their orbits

• points towards the centre of the orbit - the centre of the earth

• constantly changes the velocity of a satellite

what happens if a satellite fires its rockets to speed up

it will speed up and enter a higher orbit, then slow back down again

what are the colour of stars determined by

the surface temp of the star

cooler stars are yellow, hotter ones are red, the hottest are white

life cycle of stars

1. nebula

2. protostar

3. main sequence star

4. red giant or red supergiant

5. shell of gas or supernova

6. white dwarf or either neutron star or black hole

nebula

a cloud of dust and gas

protostar

the gravity within the nebula pulls particles together, forming a hot ball of gas

main sequence star

• the protostar becomes stable once it’s hot enough (inward force due to gravity = outward force from expanding gasses)

• the star is born

• this is what our sun is

red giant

• star starts to run out of fuel and die

• the core shrinks and heats up

• the outer part of the star expands

• red because of the outer surface cooling

red supergiant

• star starts to run out of fuel and die

• core shrinks and heats up

• outer part of the star expands

• red because of the outer surface cooling

• bigger than red giant

shell of gas

• comes from a red giant

• star runs out of fuel

• core collapses and gets denser

• outer part of the shell gets thrown off

• also known as a planetary nebula

supernova

• comes from a red supergiant once reactions inside it ends

• core suddenly collapses then rebounds, causing a giant explosion

• outer remnants of the star are ejected into space

white dwarf

• comes from a shell of gas (red giant)

• core of the star keeps collapsing until all the energy is concentrated into a very small space

• very hot

neutron star

• comes from a supernova (red supergiant)

• created if the mass after the explosion isn’t more than 4x that of the sun

• dense body that is formed in the centre of the supernova

• ball of neutrons squished together after everything collapses

black hole

• comes from a supernova (red supergiant)

• created if the mass after the explosion is more than 4x the sun’s mass

• star keeps collapsing, forming an extremely dense point that not even light can escape from

doppler effect

if the sound source moves towards the observer, the observed frequency is higher, and the observed wavelength is shorter

if the sound source moves away from the observer, the observed frequency is lower, and the observed wavelength is longer

red-shift

as a galaxy gets further away from earth, the wavelength of its light becomes longer, and objects appears red (as red has the longest wavelength)

blue-shift

as a galaxy gets closer to earth, the wavelength of its light becomes shorter, and objects appear blue (as blue has the shortest wavelength)

spectral lines in red-shift & what it means

the spectral lines on the emission spectrum shift towards the red side

this means that the further away a galaxy is, the more red-shifted it is

so by seeing how spectral lines for light given out by galaxies have shifted, you can work out how far away they are

it also shows that the universe is expanding

big bang theory

• all matter in the universe was formed at once

• matter in the universe gets more spread out over time

• universe began about 13.8 billion years ago

• universe is expanding

steady state theory

• matter is continuously being created

• universe has always existed - has no start

• universe is expanding