12PHX 2.3 Waves

what are the three properties of electromagnetic waves?

travels through a vacuum, all transverse, speed is 3×10⁸ m/s

reflection rule

angle of incidence = angle of reflection, always from ray to normal

when they say describe the nature of the image what do they want?

size, orientation, real or virtual

whats the difference between a real and virtual image

real images can be projected onto a screen and have real light rays pass through them whereas virtual images cannot be projected and we have to backtrack the reflected rays to find the image (no light rays pass through)

what do concave mirrors do

converge rays (together)

what do convex mirrors do

diverge rays (apart)

how far away is the centre of curvature from the principle point (mirror)

twice the distance of the focal point

what is the first ray you draw for mirrors

through the focal point then reflected straight/parallel to the floor

what is the second ray you draw for mirrors

parallel to the axis then reflected through the focal point

what is the third ray you draw for mirrors

towards the pole of the mirror then reflected back at the same angle (>)

what is the fourth ray you draw for mirrors

through the centre of curvature and reflected back along the same line ( / )

what is the nature of the image for ALL positions with a convex mirror

upright, diminished, virtual

what is the one position for a concave mirror that there is no image

focal point

when is the one position for a concave mirror where the image is upright and virtual

between the mirror and the focal point

what is the equation linking f to do and di

1/f = 1/do + 1/di

what does it mean if the focal point is positive (real)

concave mirror

what does it mean if the focal point is negative (virtual)

convex mirror

what does it mean if di is positive

real image

what does it mean if di is negative

virtual image

what is the equation for magnification

m = di/do = hi/ho

if m>1 what does it mean

image is enlarged

if m<1 what does it mean

image is diminished

if m=1 what does it mean

image is same size

define refraction

the change in speed and direction of a light ray when it enters a different medium

how do you calculate refractive index

n = speed of light in a vacuum (c = 3×10⁸) / speed of light in medium (v)

what properties change during refraction

speed and wavelength only, not frequency

what is the refractive index of air (absolute lowest)

1

what is snells law

n1sin01 = n2sin02 : sin02/sin01 = w2/w1 = v2/v1 = n1/n2

what is total internal reflection

when a ray is completely reflected back into the incident medium and there is no refracted ray

what are the conditions for total internal reflection

the incident angle must be greater than the critical angle, and the ray must be travelling from a more optically dense to a less optically dense medium

equation for critical angle

sin0c = n2/n1

define critical angle

the incident angle at which the refracted ray travels at 90^o to the normal

what is the idea behind apparent depth

light rays speed up as they exit the water, causing them to bend away from the normal. unaware of a change in direction, our eyes assume the light has travelled in a straight line. we mentally backtrack the rays to find where the image is, so it appears to be shallower than it actually is.

what is the idea behind dispersion

the refractive index of a material is dependent on wavelengths, so shorter wavelengths like violet refract at greater angles (prism demo)

what do convex lenses do

converge rays (together)

what do concave lenses do

diverge rays (apart)

what is the first rule we use for lens diagrams

through/towards the focal point then parallel to the floor

what is the second rule we use for lens diagrams

through the origin of the graph (middle) no change in bend

what is the third rule we use for lens diagrams

parallel to the floor then through or from the focal point

what image is produced for ALL positions for a concave lens

upright, diminished, virtual

what is the one position for a convex lens where there is no image

at focal point

what is the only position for a convex lens where the image is upright and virtual

between the focal point and the lens

do lenses have C or 2f

2f

what is the minimum number of significant figures for calcs

2sf

define transverse waves

a wave that oscillates at 90 degrees to the direction of travel

define longitudinal waves

a wave that oscillates in the direction of travel

what is the period of a wave

time for a complete wavelength to pass a fixed point (s)

what is the frequency of a wave

amount of wave cycles per second (Hz)

do sound waves need a medium

yes

does sound travel faster in gas or solid

solid

define diffraction

the bending of a wave as it passes around a barrier or through a gap

do larger or shorter wavelengths bend more

larger

how much diffraction is there when the gap is a lot bigger than the wavelength

not much spreading and a sharp shadow

how much diffraction is there when the gap size is equal to the wavelength

lots

when waves travel from deep to shallow water, what happens to speed and wavelength

both decrease

when travelling from deep to shallow water, how do waves bend?

towards the normal

when changing speed, why does a wave not change frequency

when the speed of the wave decreases, the wavelength decreases and vice versa. the frequency remains constant because it depends on the frequency of the disturbance causing the wave. additionally waves are not created or destroyed at the boundary so the number of waves per second (f) does not change

what is superposition

the ability of waves to add their displacements and energy as they pass

if two peaks or two troughs meet what happens (constructive interference)

the amplitude combines to make a tall peak or deep trough

if a trough and peak meet what happens (destructive interference)

they cancel out and there is little or no amplitude

what are the other names for constructive interference lines

maximum disturbance, maxima, antinodes, antinodal lines

what are the other names for destructive interference

minimum disturbance, minima, nodes, nodal lines

how do you describe constructive interference

the two waves arrive in phase and combine to create tall peaks and deep troughs

how do you describe destructive interference

the two waves arrive out of phase by 180 degrees and cancel out to produce no waves

what is path difference

the difference in distance travelled by the two waves from their sources

how do you calculate path difference for antinodal lines

number of waves x wavelength (where n=0,1,2,3…)

how do you calculate path difference for nodal lines

(number of waves - 0.5) x wavelength (where n=1,2,3…)

paragraph for young’s slit experiment

1. light waves pass through the narrow slits and diffract

2. diffracted waves overlap and interfere constructively and destructively

3. bright bands are where light waves have arrived in phase and constructively interfere with a path difference of nwavelengths

4. dark bands are where light waves have arrived out of phase by 180 and destructively interfere with a path difference of (n-0.5)wavelengths

what happens when a wave hits a fixed end on a spring

the wave is reflected and inverted

what happens when a wave hits a loose end on a spring

the wave is reflected and upright

what happens to the wave when it passes from a light spring to a heavy spring

the wave is partially transferred forward and partially reflected back inverted. the reflected wave has the same wavelength but a smaller amplitude. the transferred wave has a smaller wavelength and a smaller speed.

what happens to the wave when it passes from a heavy spring to a light spring

the wave is partially transferred forward and partially reflected back upright. the reflected wave has the same wavelength but a smaller amplitude. the transferred wave has a longer wavelength and a faster speed