Topic 5- Light and the EM Spectrum

reflection

a wave hits a boundary between two media and does not pass through, but stays in the original medium

where is the angle of a wave measured?

between the wave and the normal

angle of incidence (i) / angle of reflection (r)

angle of a wave approaching/leaving the boundary

law of reflection

angle of incidence = angle of reflection

refraction

the bending of light as it passes from one medium to another, caused by a change in its speed

when light rays pass from less dense to more dense media, the light bends…

towards the normal (slows down)

when light passes from a more dense medium to a less dense medium, it bends…

away from the normal (speeds up)

which wave properties change during refraction?

speed and wavelength

NOT frequency (frequency determines colour)

Critical Angle

the minimum angle of incidence for TIR to occur at a boundary

Total Internal Reflection (TIR)

ray is reflected back into the medium instead of being refracted at the boundary. occurs when angle of incidence is greater than the critical angle and the incident medium is denser than the second.

specular reflection

reflection from a smooth surface in a single direction, so i=r (reflection can be seen clearly)

Diffuse reflection

reflection from a rough surface that causes scattering (light leaves surface in all directions) (gives objects dull appearance)

absorption

energy is transferred from the wave into the particles of a substance

light will be absorbed (and later reemitted as heat) if…

the frequency of light matches the energy levels of the electrons in the substance

how do colour filters work?

they absorb certain frequencies of light and transmit other frequencies

lens power definition

how strongly it focuses the light

lens power (equation + units)

power = 1 / focal length

measured in dioptres

How does shape & focal length affect lens power?

the more curved the lens, the shorter the focal length

The shorter the focal length, the greater the power

convex lens (other name + what it does)

converging lens

rays of light brought to a focal point/principal focus

concave lens (other name + what it does)

diverging lens

rays of light diverge from a point

focal point is the point from which rays appear to diverge from (focal length is negative)

real image

formed when the light rays from an object converge and meet each other and can be projected onto a screen

always inverted

virtual image

image formed when the light rays from an object do not meet but appear to meet behind the lens

cannot be projected onto a screen

always upright

3 things to consider when describing an image

real or virtual

magnified or diminished

upright or inverted

if an object is placed further than the focal length from converging lens, the image will be…

real

inverted

magnified

if an object is placed closer than the focal length from a converging lens, the image will be…

Virtual

upright

magnified

if an object is placed further than the focal length from a diverging lens, the image will be…

virtual

diminished

upright

electromagnetic waves are…

transverse waves that transfer energy from the source of the waves to an absorber

that can travel through a vacuum, all at the same speed

examples of energy transfer by EM waves

microwaves in a microwaves oven transfer energy from the electronics to heat the food inside

all hot objects emit infrared radiation, which can be absorbed by other objects, heating them

the sun emits visible light (so we can see), infrared radiation (heat the earth) and ultraviolet radiation (provides plants with energy)

type of electromagnetic waves (longest wavelength to shortest)

radio waves

microwaves

infrared

visible light

ultraviolet

x-rays

gamma rays

patterns in EM spectrum from radio to gamma waves

Increasing

wave energy, frequency

Decreasing

wavelength

in the visible light spectrum, ____ has the longest wavelength (and lowest frequency), and ___ has the shortest wavelength (and highest frequency)

Red

violet

wavelength and frequency are…

inversely proportional

dispersion

different wavelengths of light change speed (and therefore are refracted) by different amounts.

In visible light, white is split up into all its colours.

EM waves travel ___ in denser substances

slower

when EM waves pass from one medium to another, they can be:

transmitted

absorbed

reflected

refracted

what affects how an EM wave will interact with different substances?

the wavelength of the waves and the substance itself

as frequency of waves increases, energy…

also increases

which types of EM waves are ionising?

higher frequency than visible light (UV, X-rays, Gamma rays)

ionisation

removing electrons from an atom leaving a charged ion

what affects the danger of EM waves?

frequency (higher frequency = highly ionising = high danger)

intensity of radiation (how ‘concentrated’)

potential dangers of excessive exposure to EM radiation

microwaves: internal heat damage to body cells

infrared: skin burns

UV: eye damage, sunburn, skin cancer

X-rays/Gamma rays: damage/kills cells, leading to mutations and cancer

uses of radio waves

broadcasting/communications (radio + tv)

uses of microwaves

heating food

communication (wifi, mobile phones, satellites)

uses of infrared

heating and cooking

thermal imaging

remote controls

optical fibres

security systems

uses of visible light

vision (duh!)

photography

illumination

uses of UV

security marking (on bank notes)

fluorescent lamps

disinfecting water

sunbeds

uses of x-rays

x-ray imaging for medicine, airport security and industry

uses of gamma rays

sterilising food + medical instruments

detecting + treating cancer

interaction of a EM wave with an atom

when it collides, it can be absorbed by an e-

this e- gains energy and jumps up an energy level

it then moves back down to a lower energy level and emits this energy as another EM wave

EM waves can be absorbed and emitted by atoms over a wide range of frequencies, but the energies that tend to be emitted by electron transition are:

visible and UV (sometimes X-rays)

higher energy waves can only be emitted when an EM wave interacts with…

the nucleus

how are radio waves produced + received

by connecting antenna to a high frequency a.c. power source

oscillation of charge produces radio waves of same frequency

radio waves absorbed by an aerial at a distance,

inducing an alternating current with the same frequency as the transmitted wave

all bodies emit…

(a spectrum of) thermal radiation (in the form of EM waves) e.g. infrared/visible

temperature of the body affects the _______ of any emitted waves

intensity and wavelength distribution

the hotter an object, the more ____ it radiates in a given time

infrared radiation

as the energy and therefore heat of an object increases, the wavelength…

decreases

Thermal equilibrium

when an object reaches a point of constant temperature when it absorbs radiation at the same rate it emits radiation.

if a body absorbs radiation at a higher rate than it emits, it…

heats up (and vice versa)

Greenhouse Effect and its effect on Earth’s teperature

• majority of Earth’s heat received from thermal radiation from Sun

• Earth emits its own thermal radiation (of a longer wavelength, so the Earth is significantly cooler than the Sun)

• Greenhouse gases in the atmosphere absorb and emit back some of this radiation in all directions, trapping some in the atmosphere

• some emitted heat passes through atmosphere into space

• this process makes the Earth warmer

the temp of the Earth depends on a number of factors, e.g….

the rate that light and infrared radiation from the sun are:

• reflected back into space

• absorbed by Earth’s atmosphere/surface

• emitted from earth’s atmosphere/surface into space