Light and the em spectrum

reflection

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

Law of reflection

angle of incidence = angle of reflection

normal line

Imaginary line perpendicular to the surface.

What to remember when doing ray diagrams

arrows

lines with a ruler

refraction

The bending of a wave as it passes at an angle from one medium to another. This is due to a change in speed upon entering a medium of different density and therefore a change in direction

Acronym for refraction

Faster Away

Slower Towards

When will light bend towards the normal and why?

Upon entering a medium of higher density as speed decreases

When will light bend away from the normal and why?

Upon entering a medium of lower density as speed increases

When will light not bend from the normal?

When light is passing through the normal i.e. light passing perpendicular to the boundary

What factors of light waves are constant during refraction and which factors change?

- frequency stays the same as light doesn't change colour during refraction(different frequencies correspond to different colours of light)

- Wave speed and wavelength change during refraction.

What is the critical angle?

The angle of incidence needed to produce an angle of refraction of 90* (along the surface of the boundary)

What is total internal reflection?

Total internal reflection is where all the light is reflected inside a material.

Conditions for total internal reflection?

1. The angle of incidence > the critical angle (i > θc)

2. The incident material is denser than the second material

What is specular reflection?

Reflection from a smooth surface in a single direction

Example of specular reflection

Light reflecting off mirrors as mirrors are smooth - this is what gives them their shiny appearance

What is diffuse reflection?

Where incoming light rays are reflected off of a rough surface and become scattered

absorption

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

What is white light a mixture of?

A mixture of different colours of light

Why a red object looks red

It reflects red light and absorbs all other colours of light

(adjust this to blue object, green object, etc. black=absorbs all, white=reflects all)

How a green filter makes a white light become a green light

It transmits green light and absorbs all other colours of light

How a red object under blue light appears black

The red object absorbs the blue light and doesn't reflect any other colour of light (as there is no red light), making it appear black as black is the absence of light)

What is the power of a lens?

A more powerful lens bends light through a greater angle

two types of lenses

converging, diverging

What happens in a converging lens?

parallel rays of light are brought to a focus - this is called the principal focus

alternate name for converging lens

convex lens

focal length

the distance from a lens to its principal focus

What happens in a diverging lens?

parallel rays of light are made to diverge (spread out) from a point (principal focus)

alternate name for diverging lens

concave lens

Difference between principal focus for converging and diverging lens

converging: the PF is where the rays meet

diverging: the PF is the point from where the rays appear to diverge from

How focal length and shape of a lens affects its power

A more powerful lens either:

• has a shorter focal length

• has a more curved lens

As the lens becomes more curved what happens to the focal length

Gets shorter

lens power equation

power = 1/focal length

(The focal length can be negative also)

real image

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

qualities of real image

- produced by the convergence of light towards a focus

- always inverted

- can be projected onto pieces of paper or screens i.e. cinemas

virtual image

An image that is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen

qualities of virtual image

- formed by the divergence of light away from a point

- always upright i.e. mirror

- cannot be projected onto a piece of paper or a screen

What an image looks like if the object is twice the focal length away from the converging lens

• Real

• Inverted

• Diminished

What an image looks like if the object is past the focal length away , but not twice the focal length, from the converging lens

• Real

• Inverted

• Magnified

What an image looks like if the object is less than the focal length away from the converging lens

• Virtual

• Upright

• Magnified

What an image looks like formed by a diverging lens

• Virtual

• Upright

• Diminished

How to draw Converging Lens?

1Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line

2Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly through the principal focus f

3The image is the line drawn from the axis to the point where the above two rays meet.

4. If the rays diverge then it is a virtual image. Draw dashed lines continuing the ray upwards and the virtual image produced if where the dashed lines meet.

How to draw Diverging Lens?

1.Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line

2.Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly upwards away from the axis

3.Draw a dashed line continuing this ray downwards to the focal point, f

4.The image is the line drawn from the axis to the point where the above two rays meet

List the electromagnetic spectrum, longest to shortest wavelength/lowest to highest frequency

• Radio

• Microwaves

• Infrared

• Visible light

• Ultraviolet

• X-rays

• Gamma rays

What happens to the energy of radiation as frequency increases

The higher the frequency, the higher the energy of the radiation

Features of electromagnetic waves

• do not need matter to travel through(can travel through a vacuum)

• speed of light (3x10^8m/s)

• transverse

• transfer energy as radiation from the source to the absorber

EM wave emission from Sun and uses of it

Visible light - lets use see

Infrared - heat up earth

UV - provide plants with energy for photosynthesis

Visible light colours longest to shortest wavelength

Red orange yellow blue green indigo violet

What kind of radiation do hot objects emit?

infrared

Effects of high frequency radiation

Radiation with higher energy is:

1.Highly ionising

2.Harmful to cells and tissues causing cancer (e.g. UV, X-rays, Gamma rays)

What types of radiation are ionising?

• UV radiation

• X-rays

• Gamma rays

If an EM wave has a shorter wavelength, is it refracted more or less when the medium changes?

more

relationship between frequency and wavelength

Inversely proportional (the higher the frequency, the shorter the wavelength)

How are different parts of the EM spectrum refracted/reflected differently?

Dispersion:

• longer wavelengths of light are refracted less

• this causes white light to split into red, orange, yellow, etc

Which colour is refracted the most and the least

Most: violet

Least: red

How are different parts of the EM spectrum absorbed/transmitted/reflected/refracted differently?

It depends on wavelength and the material the wave is travelling through

• e.g. some wavelengths pass through the atmosphere and some are absorbed

Uses of radio waves

• transmitting radio broadcasts and TV programs

• controlling spacecraft from the ground

• long-distance communication

Uses of microwaves

• microwave and ovens to heat food

• comms and satellite transmissions

• phone signals

Uses of infrared radiation

• cooking

• thermal imaging

• optical fibres

• TV remotes

• short range comms

• security systems

Uses of visible light

• vision

• cameras recording images

How radio waves are produced

oscillations in electrical circuits emit radio waves

how radio waves are received

a metal rod/wire connected to a circuit will receive radio waves and cause oscillations in the electrical circuit

how the radiation emitted by an object (due to its temperature) changes as the temperature increases

• wavelength of maximum intensity decreases

• overall intensity of radiation increases

how power absorbed vs power radiated changes the temperature of an object

• power absorbed>radiated -> temp up

• absorbed=radiated -> temp constant

• absorbed temp decrease

(applies to earth absorbing incoming solar radiation, and radiating it back out as infrared)

factors that affect the energy absorbed and radiated by the earth

• greenhouse effect - atmospheric gases naturally absorbing some energy, reducing power radiated

• reflection by clouds and atmosphere - reduces power absorbed

Uses of ultraviolet rays

• security marking

• fluorescent lamps

• detecting forged banknotes

• disinfecting water

Uses of X-rays

• internal imaging of the body ("X-rays") and the skeleton

• inspecting luggage at airports

Uses of gamma rays

• sterilising food

• sterilising medical equipment

• radiotherapy for cancer

how short wavelength EM waves are affected by different substances

• X-rays: mostly transmitted by muscle and fat but bone absorbs it

• Gamma: absorbed by all of them, bone absorbs more

danger of microwaves

internal heating of cells - certain freqs of microwaves are absorbed by water molcules in our body

danger of infrared

skin burns

danger of UV

• damage to surface cells leading to skin cancer

• eye damage causing eye conditions

dangers of X-rays and gamma rays

• mutation and cancer

• damage to cells in body

what EM waves are ionising?

• UV

• X-rays

• gamma rays

how ionising radiation causes damage

• radiation can cause atoms to lose electrons and become ions

• this can cause issues such as damage to DNA, leading to mutations and potentially causing cancer

what happens when atoms absorb and emit em waves

electrons absorb and gain the EM wave energy and then move up an energy shell and when they emit em waves they lose the energy and move down an energy shell

How can radio waves be produced?

connecting an antenna to a high frequency alternating current (a.c.) power source

The oscillation of charge in the a.c. circuit produces radio waves with the same frequency of oscillation

What happens in the transmitting antenna

The charge from the alternating current oscillates up and down the antenna

This produces radio waves that can be absorbed by a similar aerial some distance away

What happens in the receiving aerial

The metal aerial absorbs the radio waves

This creates an alternating current with the same frequency as the transmitted wave

how EM radiation is produced

• changes in electrons or nuclei in atoms

• when materials are heated, changes in the way the electrons are arranged can produce IR or visible light

• changes in the nuclei of atoms can produce gamma

How does an object reach thermal equilibrium?

As an object absorbs thermal radiation it will become hotter

As it gets hotter it will also emit more thermal radiation

The temperature of a body increases when the body absorbs radiation faster than it emits radiation

Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation

At this point, the object will be in thermal equilibrium

core practical for investigating radiation (swr we never did this? its in the textbook)

Aim: Investigate which colour emits the best

• cover four or more identical boiling tubes in different coloured materials

(shiny silver, dull grey, shiny black, dull black)

• pour the same volume of hot water from a kettle into each tube

• insert bung with thermometer in each

• measure temp of water in each and start stopwatch

• record temp every 2 mins for 20 mins

Plot graphs and the one that cools the most which is black is the one that emits the most

CV:Identical flasks (except for their colour)

Same amounts of hot water

Same starting temperature of the water

Same time interval