physics - light & the EM spectrum (5.1 - 5.24)

5.1 Ray diagrams

arrows show direction of light travel

normal: imaginary line perpendicular to surface

all angles measured from normal

angle of incidence: angle ray enters at

angle of reflection: angle ray exits at

5.1 reflection

angle of incidence = angle of reflection (law of reflection)

5.1 refraction

light ray → material where it travels at diff. speed: changes direction

light → medium where travels slower: bends towards normal

light → medium where travels faster: bends away from normal

(light meets interface at right angle - no change in direction)

5.1 total internal reflection

when angle of incidence > critical angle

light from water/glass → air with small angles of incidence - most light passes through interface, some reflected

angle of incidence increases - angle of refraction increases until refracted light passes along interface

angle of incidence increases more - light completely reflected inside glass

5.1 critical angle

angle of incidence TIR starts to occur at

5.2 specular vs diffuse reflection

specular: very smooth surfaces reflect light evenly

diffuse: rough surfaces scatter reflected light in all directions

5.3 colour of light - differential absorption at surfaces

white light: from sun/lamps; mix of diff. colours eyes see white

white light hits coloured surface - some colours that make it up absorbed & reflected

e.g. red object looks red - reflects red light, absorbs all other colours

e.g. white object looks white - reflects all colours

5.3 colour of light - transmission of light through filters

white light made into coloured light using filter

filter: transparent material, absorbs some colours in white light

e.g. blue filter - transmits blue light, absorbs all other colours

focal point

where parallel rays of light meet after passing through lens

focal length

distance between focal point & centre of lens

5.4 power of lens related to focal length & shape

more curved lens = shorter focal length = more powerful lens

5.5 similarities & differences in refraction of light by converging & diverging lenses (ray diagrams)

5.6 different types of lenses producing real images

can be projected onto screen - light rays converge

image on opposite side of lens to object

image inverted & smaller than object

produced by converging lenses

5.6 different types of lenses producing virtual images

cannot be projected onto screen - light rays don’t converge

image appears on same side of lens as object

image upright & bigger than object

produced by converging/diverging lenses

5.7 EM waves (type & speed)

transverse

travel at same speed in vacuum

5.8 EM waves transfer energy from source → observer - e.g.

e.g. light travels from source & reflected by object into eyes - light transfers energy from source → eyes (observer)

5.9 core practical: investigate refraction into rectangular glass blocks in terms of interaction of EM waves with matter

1. place piece of plain paper on desk; set up power supply, ray box & single slit so you can shine single light ray across paper

2. place rectangular glass block on paper & draw around it

3. shine ray of light into block, mark where light rays go with small crosses

4. take block off paper; use ruler to join crosses to show path of light & extend lines so they meet outline of block; join points where light entered & left block to show where it travelled inside block

5. measure angles of incidence & refraction where light entered & left block

6. repeat step 3-5 with ray entering block at diff. angles

7. move ray box to light ray reaches interface at right angles; note what happens to light as it enters & leaves block

5.10 EM spectrum order

radio waves

microwaves

infrared

visible

UV

x-rays

gamma rays

5.10 visible spectrum order

red

orange

yellow

green

blue

indigo

violet

5.11 EM spectrum (continuity, frequency & wavelength)

continuous

increasing frequency

decreasing wavelength

5.12 eyes detecting EM radiation

our eyes can only detect limited range of frequencies of EM radiation

5.13 what do diff. substances do?

absorb/transmit/refract/reflect EM waves in ways that vary with wavelength

5.14 effects of differences in velocities of EM waves in diff. substances

waves travel in straight lines unless reflected/refracted (bending of path of wave due to change in velocity)

EM waves travel at diff. speeds in diff. substances - causes reflection/refraction

5.15 bodies emitting radiation (intensity & wavelength - temp.)

all bodies emit radiation

object at higher temp. = greater intensity & shorter wavelength of radiation emitted

5.16 what is needed for body to be at constant temp.?

needs to radiate same average power it absorbs

power = amount of energy transferred in certain time

• 1W = 1J/s

5.17 what happens to body if average power it radiates is less/more than average power it absorbs?

radiates less power than absorbs = temp. increases

radiates more power than absorbs = temp. decreases

5.18 how is earth’s temp. affected by factors controlling balance between incoming emitted radiation?

earth’s surface absorbs about half of radiation from sun

re-radiates energy as infrared radiation (warms atmosphere)

greenhouse effect: some gases (greenhouse gases) in atmosphere naturally absorb some energy - keep earth at higher temp. than if no atmosphere

5.19 core practical: investigate how nature of surface affects amount of thermal energy radiated/absorbed

1. cover 4 boiling tubes in diff. coloured materials - shiny silver, dull grey, shiny black, dull black

2. pour same volume of hot water from kettle into each

3. inert bung with thermometer into each tube

4. measure temp. of water in each tube & start stop clock

5. record temp. of water in each tube every 2 mins for 20 mins

5.20 potential danger associated with EM wave

increases with increasing frequency

5.21 microwaves - harmful effects on people of excessive exposure

internal heating of body cells

5.21 infrared - harmful effects on people of excessive exposure

skin burns

5.21 UV - harmful effects on people of excessive exposure

damage to surface cells & eyes - leads to skin cancer & eye conditions

5.21 x-rays & gamma rays - harmful effects on people of excessive exposure

mutation/damage to body cells

5.22 radio waves - uses

broadcasting

communications

satellite transmissions

5.22 microwaves - uses

cooking - microwaves transfer energy → food, food heats up

communications

satellite transmissions

5.22 infrared - uses

cooking - grill/toaster transfers energy → food, food absorbs radiation & heats up

thermal imaging - shows amounts of infrared radiation emitted by diff. objects

short range communications

optical fibres

TV remote controls

security systems - detect infrared radiation emitted by intruders

5.22 visible light - uses

vision

photography - cameras detect visible light & record images

illumination

5.22 UV - uses

security marking - fluorescent materials only visible when UV light shines on them

fluorescent lamps - gas in lamps produces UV radiation when electric current passes through, coating inside glass absorbs UV & emits visible light

(fluorescence: some materials absorbs UV radiation & re-emit it as visible light)

detecting forged bank notes

disinfecting water - kills microorganisms

5.22 x-rays - uses

observing internal structure of objects - pass through materials visible light can’t

airport security scanners

medical x-rays - pass through muscles & fat, absorbed by bone

5.22 gamma rays - uses

sterilising food & medical equipment - transfer lots of energy & can kill cells (kill potentially harmful microorganisms)

detecting cancer - chemical that emits gamma rays injected into blood, collects in cancer cells, scanner outside body finds source of gamma rays & locates cancer

treating cancer - kills cells (radiotherapy)

oscillation definition

variation in current & voltage

5.23 radio waves & electrical circuits

radio waves produced by/themselves induce oscillations in electrical circuits

5.24 what can changes in atoms & nuclei do?

generate radiations over wide frequency range (e.g. EM radiation produced by changes in electrons/nuclei in atoms)

be caused by absorption of range of radiations (e.g. makes atoms lose electrons)