15 - Transmission and reflection of waves

Question 1

Reflection

Answer 1

• all reflection follows law of reflection

- the angle between the incidence ray and the normal is the same as the angle between the normal and the reflected ray

Question 2

Refraction

Answer 2

• wave front = a line or surface in a wave along which all points are in phase, one wavelength apart
• wave fronts in water waves can be slowed by making the water shallower. As the wave has a fixed frequency, the wavelength must reduce so the wave fronts have to move closer together
• EM waves like light are slowed by collisions with the atoms of a medium, so density will affect it. As it slows the ray of light will bend towards the normal

Question 3

Refractive index

Answer 3

• speed of light in first medium/speed of light in second medium = refractive index of second medium
• alt. sin(angle of incidence)/sin(angle of refraction)
• useful alternative for Snell’s law - refractive index of first mediumsin(angle of incidence) = refractive index of second mediumsin(angle of refraction)

Question 4

Total Internal Reflection

Answer 4

• when travelling from glass to air, the critical angle for light to be travelling at in order to be internally reflected is found with the formula sin(critical angle) = 1/refractive index of the glass
• can work with light ray travelling from a more to less dense material, with refractive index of second material/refractive index of first material = sin(c)
• if angle of incidence > critical angle, no reflection
• if i = C, light travels parallel to the boundary of the medium
• if i < C, TIR occurs

Question 5

Lenses

Answer 5

• convex lenses bring rays together
• concave lenses spread rays out
• power of a lens = 1/focal length
• power of multiple lens’ = P1 + P2 + … + Pn
• focal length = distance from lens to focal point where all light converges
• power of multiple lenses = sum of each individual power

Question 6

Drawing diagrams

Answer 6

1- draw a ray from the top of the object parallel to the x-axis
2- when it meets the lens, draw a line from that point through the focal point
3- draw a ray from the top of the object going straight through the middle of the lens and crossing the line through the focal point
4- if the two lines cross, draw the object again there, if they don’t expand them backwards and see if they cross there

Question 7

Magnification

Answer 7

• if object >2F, image will be inverted, real (projected onto a screen), smaller and between the lens and F
• if object between 2F and F, image will be be >2F, magnified, real and inverted
• if object is at F then no image will be made
• if object is at 2F, image will be real, stay the same size and be at 2F
• if object is between F and lens, image will be behind object, magnified, virtual (not projected) and not inverted
• magnification = image height/object height

Question 8

Eyesight

Answer 8

• concave lenses fix nearsightedness

- convex lenses fix longsightedness

Question 9

Thin lens equation

Answer 9

1/object distance + 1/image distance = 1/focal length

Question 10

Polarisation

Answer 10

• EM waves oscillate in any plane of 360 degrees, thin filters can prevent all but a few planes of oscillation passing through
• water reflects all light back into one plane of oscillation (plane polarisation) at certain angles, so polarising lenses that block that plane allow you to remove that glare completely. many other reflective substances plane polarise light at specific angles too
• some complex molecules like sugar solutions plane polarise light at different angles of rotation depending on their concentration, so viewing them through a polarising lens and rotating them allows you to calculate their concentrations
• long chain polymers also rotate the plane of polarisation as stress is applied to create multi-coloured stress patterns

Question 11

pulse-echo

Answer 11

• sonar, radar and ultrasound all rely on emitting a sound, timing its return and calculating the distance between the object it reflected off and the detector
• ultrasound is partially reflected at every change in medium, it creates lower quality images than x-ray but causes no damage
• higher wavelengths provide more accurate images, increasing frequency provides higher resolution images but lower frequencies are use to image organs deeper in the body as effective range is about 200 wavelengths