P12 Waves

How do waves transfer energy?

Waves transfer energy without transferring matter; through particles that oscillate about a fixed point. 

What is a wavefront?

an imaginary surface that connects all the points in a wave that are in the same phase.

What is the amplitude?

the distance from the equilibrium position to the maximum displacement

What is the wavelength?

the distance between two consecutive points on a wave that are in the same phase

What is the frequency?

the number of complete waves that pass a single point per second. Measured in Hertz (Hz) so 1 Hz equals to 1 wave per second

What is the equation for frequency?

Frequency (Hz) = 1 / time (seconds)

What is the wave speed? 

the distance travelled by a wave each second

What is the equation for the wave speed?

velocity (m/s) = frequency (Hz) x wavelength (m)

V=fλ

What is the time period?

the time taken in seconds for one complete wave to pass a point

What is the equation for the time period?

Time period (seconds) = 1 / frequency (Hz)

What are the two types of waves?

Transverse and longitudinal waves

What are transverse waves? + oscillations

A type of wave where the particles of the medium move perpendicular to the direction of the wave.

-oscillations are up and down

What are some examples of transverse waves?

Electromagnetic waves (light waves, microwaves, etc), Seismic S-waves

Label this transverse wave

What are longitudinal waves? + oscillations

A type of wave where the particles of the medium move parallel to the direction of the wave.

-oscillations are side to side

What are some examples of longitudinal waves?

Sound waves, Ultrasound waves, Seismic P-waves

Label this longitudinal wave

What are sound waves?

Sound waves are longitudinal waves created by vibrating sources. A medium is needed to transmit sound waves (such as air).

What is the link between amplitude and sound waves?

The greater the amplitude of a sound wave, the louder it is.

What is the link between frequency and sound waves?

The greater the frequency of a sound wave, the higher its pitch.

What is the range of audible frequencies for a healthy human ear?

20 Hz to 20000 Hz/20 kHz

What are the two methods of measuring sound waves?

1)Microsecond Timer

The timer starts when sound is detected by the closest microphone and stops when sound is detected by the further microphone.

Speed = distance between mics / time taken between travel

2)Using echoes

make a noise at a known, large distance from a solid wall and record the time for the echo to be heard

Speed = distance / time

Why does sound travel faster through a solid?

The sound wave is passed on by collisions between particles, so the speed the wave moves depends on the density of the particles. Sound travels fastest through a solid because the particles are closer together. This means vibrations are more easily passed from one particle to another. 

What is ultrasound?

sound with a frequency higher than 20 kHz

What are the uses of ultrasound?

Nondestructive testing of materials, Medical scanning of soft tissue tissues without using ionising radiation, Sonar to measure water depth or locate underwater objects

Explain nondestructive testing of materials

The waves reflect from boundaries between materials or defects, identifying issues. Example: cracks in metal or plastic

Explain medical scanning of soft tissue tissues without using ionising radiation

The waves reflect at boundaries between different tissues, forming an image. Example: organs or a fetus during pregnancy

Explain sonar to measure water depth or locate underwater objects

A pulse of ultrasound is sent underwater and the time taken for the echo to return is measured. Example: locating submarines, fish

Depth Equation: Depth or Distance = (Speed of Sound x Time taken for echo) / 2

What is diffraction?

the bending and spreading of waves as they pass through a gap or around an obstacle

Does frequency, wavelength or speed change in diffraction?

No

How to increase diffraction?

The narrower the gap or the greater the wavelength, the more the diffraction

What is reflection?

the bouncing back of light waves when they hit a surface and change direction, staying in the same medium.

How do waves reflect off smooth vs rough surfaces?

-Waves reflect off smooth, plane surfaces rather than getting absorbed

-Rough surfaces scatter the light in all directions, so they appear matte and unreflective

Does frequency, wavelength or speed change in reflection?

No

Label this diagram

What is the normal?

A line perpendicular to the reflecting surface at the point where the light ray strikes.

What is the angle pf incidence?

The angle between the incident ray and the normal

What is the angle of reflection?

The angle between the reflected ray and the normal

What are the characteristics of the image formed by a plane mirror?

• Same size as the object.

• Same distance behind the mirror as the object is in front.

• Virtual: The image cannot be projected onto a screen.

• Laterally inverted: Left and right are reversed.

What are the three main things that can happen when light strikes a material?

• Transmission – light passes through the material

• Absorption – light energy is taken in by the material

• Reflection – light bounces off the surface

What determines whether a material is transparent, translucent, or opaque?

The balance between absorption and transmission

How does light react to transparent materials?

Transparent materials allow most light to be transmitted through them with little absorption or scattering. Examples: clear glass, clean water, air
Transmission

• Light passes through almost unchanged in direction

• Rays remain mostly parallel

• Objects can be seen clearly through the material

Absorption

• Very little light energy is absorbed

• The material does not significantly heat up

How does light react to translucent materials?

Translucent materials allow some light to be transmitted, but the light is scattered in many directions. Examples: frosted glass, tracing paper, thin fabric

Transmission

• Light passes through partially

• Rays are scattered, so they lose their original direction

• Objects appear blurred or unclear

Absorption

• Some light energy is absorbed

• Less light exits than enters

How does light react to opaque materials?

Opaque materials do not transmit light. Examples: wood, stone, metal

Transmission

• No light passes through the material

Absorption

• Light is mostly absorbed, converting light energy into thermal energy 

• Dark materials absorb more light than light-colored materials

Reflection

• Some light may be reflected

• Shiny opaque materials reflect more 

• Dull opaque materials absorb more

What is refraction?

the bending of waves as they pass from one medium to another due to a change in speed

What happens a wave enters a more optically dense medium?

its speed decreases and it bends towards the normal

What happens a wave enters a less optically dense medium?

its speed increases and it bends away from the normal

Does frequency, wavelength or speed change in refraction?

frequency stays the same but the wavelength changes

What does the refractive index measure?

a measure of how much the wave slows down

What does a substance with a large refractive index mean?

it will slow down the ray more and cause the ray to bend through a larger angle.

What are the two definitions of refractive index?

The ratio between the speed of light in a vacuum and the speed of light in the medium.

The ratio of the speeds of a wave in two regions of different optical densities.

What are the three equations of refractive index?

Refractive index = speed of light in a vacuum / speed of light in the medium

n = c/v

Refractive index = (sin x incident angle) / (sin x refracted angle)

n = sini / sinr 

Refractive Index of medium 1 x (sin x incident angle) = Refractive Index of medium 2 x (sin x refracted angle)

n₁sinθ₁ = n₂sinθ₂

What are some possible errors in a refraction practical?

• Thick pencil lines leads to

  • Uncertainty in where the ray actually is

  • Slightly different angle readings each time

  • Scatter in points on the sin⁡i/sinr graph

• Human error when reading the protractor

  • Small variations in measured angles

  • Less precise gradient and refractive index

• Protractor consistently misaligned with the normal

  • All angle measurements too large or too small

  • Gradient gives an incorrect refractive index

• Centre of curved edge marked inaccurately

  • Light does not always enter at the true centre

  • Refraction angles are consistently wrong

What are some possible improvements in a refraction practical?

• Use a thin pencil or laser ray to reduce uncertainty in ray position

• Carefully align the protractor with the normal for every measurement

• Take multiple readings at each angle and calculate a mean to reduce random error

• Increase the number of readings beyond seven to improve the reliability of the line of best fit

What are some possible extensions in a refraction practical?

• Repeat the experiment with different transparent materials and compare refractive indices

• Use different wavelengths (colours) of light to investigate how refractive index depends on wavelength

What is dispersion?

When white light is passed through a glass prism, it splits up into its constituent colours. This happens because the different colours travel at different speeds in the glass as they have different wavelengths, so they refract by different amounts.

What are the seven colours in order of decreasing wavelength?

red, orange, yellow, green, blue, indigo and violet 

How is wavelength related to refractive index?

The shorter the wavelength, the slower the speed in glass and the greater the refractive index.

Red is refracted the least and violet the most

What does monochromatic mean?

light of a single frequency

What is the critical angle?

the minimum angle of incidence at which total internal reflection occurs in a boundary between two different optical media. It is the angle of incidence that produces a 90⁰ angle of refraction

What is the equation for the critical angle?

n = 1 / sinc

refractive index = 1/sin(critical angle)

What is total internal reflection?

a specific case of internal reflection that occurs when:

• light travels from a denser medium to a less dense medium

• all the light is reflected back inside the denser medium

• no light is refracted out

• happens when the angle of incidence is greater than the critical angle

Why can light entering a denser material not be totally reflected?

it bends towards the normal, not away from the normal

What is total internal reflection used for?

• Optical fibres used in telecommunications and medical endoscopes

• Prisms in periscopes and binoculars

• Sparkle of diamonds due to repeated total internal reflection inside the crystal

What are optical fibres?

a long thin rod of glass surrounded by cladding which uses total internal reflection to transfer information by light, even when bent

What are the uses of optical fibres?

Medicine

• used in endoscopes which allow doctors to see inside the body without major surgery

Telecommunications

• used to transmit telephone calls, internet data, and television signals

• allows data to travel long distances very quickly with little loss of signal

What are the advantages of optical fibres? 5

• Higher capacity as thinner fibres mean more can fit in a cable 

• Longer distances as there is less signal loss meaning signals can travel further without needing repeaters

• Less interference as it is immune to electromagnetic interference

• Thin and flexible so it’s easy to install and requires less space and effort

• More secure as it is more difficult to tap into compared to electrical cables

What does a concave lens look like?

Thinner in the middle, thicker at the edges

What does concave lens do to light? + drawing

• When light passes from air into glass, its speed changes

• Because the lens is thinner in the middle, light slows down less at the centre and more at the edges

• This difference in bending causes refraction away from the centre

• Parallel light rays diverge and appear to come from a virtual focal point behind the lens 

• Used to spread light

What does covex/converging lens do to light? + drawing

• When light passes from air into glass, its speed changes

• Because the lens is thicker in the middle, light slows down more at the centre than at the edges

• This difference in bending causes refraction towards the centre

• Parallel light rays converge and meet at the focal point in front of the lens

• Used to focus light

What are the 3 rules for ray diagrams?

• Rays through the centre do not refract

• Rays parallel to the principle axis refract through the focal point

• Consider rays from the top of the object

What are real images?

Light rays physically converge at a point. Image can be projected onto a screen

What are virtual images?

Light rays diverge, but appear to originate from a point behind the mirror/lens when traced backward. Image cannot be projected onto a screen. 

Label this ray diagram

Draw a ray diagram for an object between lens and F

Properties of an object between lens and F

• Virtual

• Upright

• Larger

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

Draw a ray diagram for an object at F

Properties of an object at F

No image is formed

Draw a ray diagram for an object between F and 2F

Properties of an object between F and 2F

• Real

• Inverted

• Larger

• Located beyond 2F on the opposite side of the lens

Draw a ray diagram for an object at 2F

Properties of an object at 2F

• Real

• Inverted

• Same size as the object

• Located at 2f on the opposite side of the lens

Uses of lenses for humans

• Eyeglasses

• Microscopes

• Telescopes

• Cameras

Explain eyeglasses

• Correct vision by adjusting how light is focused on the retina

  • Convex lenses help long-sightedness

  • Concave lenses help short-sightedness

• Extends the limit of human vision

Explain microscopes

• Use multiple convex lenses to magnify very small objects like cells

• The objective lens forms a magnified image of a very small object

• The eyepiece lens further magnifies this image for the eye

Explain telescopes

• Uses two convex lenses to observe distant objects like stars

• The objective lens gathers light and forms an image

• The eyepiece lens magnifies the image so it can be seen clearly

Explain cameras

• Use convex lenses to focus light onto a film or digital sensor

• Adjusting the lens position changes where the image is focused

• The focused light forms a real image on the sensor

• Help record and share visual information

What are electromagnetic waves?

● Transverse waves

● Do not need a medium

● All travel with the same speed of 3.0 x 10⁸ ms^-1 in a vacuum and approximately the same speed in air

Arrange electromagnetic waves in order of wavelength (shortest to longest)

gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves

Arrange electromagnetic waves in order of frequency (shortest to longest)

radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays

Uses for radio waves

• Used for radio, television communications, radar

• Have a long wavelength, travel long distances and are reflected by the ionosphere.

Uses for microwaves

• Used for satellite communication, microwave ovens

• Pass through the ionosphere and penetrate deep into food, heating it up

Uses for for infrared

• Used for remote controllers for television, thermal imaging

• Carries signals over short distances and detects heat difference in objects

Uses for visible

• Used for vision, photography

• Can be detected by eyes and captured by cameras to produce images

Uses for ultraviolet

• Used for detecting fake bank notes, tanning bed

• makes special security markings glow, revealing hidden patterns on fake bank notes and stimulates skin to produce vitamin D, causing tanning

Uses for xrays

• Used for medical scanning, security scanners

• Can penetrate through soft tissue easily but are absorbed by dense materials like bone or metal, creating clear images.

Uses for gamma rays

• Used for detection of cancer and its treatment

• Penetrate tissues deeply and kill cancer cells effectively