Optics

Question 1

Light properties

Answer 1

• a form of energy
• travels at 3x108 m/s
• universal speed limit; c in E = mc2
• travels in straight lines
• has wavelike properties

Question 2

Equation for light

Answer 2

• v = f x ƛ
• v = speed = (cm/s) or (m/s)
• ƛ = wavelength = Greek letter lambda = (cm/cycle) or (m/cycle)
• f = frequency of waves passing a point = (cycle/second) or (Hz)

Question 3

Visible light

Answer 3

• visible light is the portion of the electromagnetic spectrum with wavelengths between 400nm and 700nm
• white light is actually composed of ROYGBIV
• neighbouring types of electromagnetic radiation with longer wavelengths are infra-red, microwaves and radio waves
• electromagnetic radiation with shorter wavelengths (and more energy) includes ultraviolet, X-rays and Gamma Rays

Question 4

Radio stations

Answer 4

• FM radio stations broadcast between 88 and 108 MHz

* the station numbers indicate the frequency

Question 5

Colours

Answer 5

• each colour has a characteristic wavelength and frequency
• red = long, slow wave (low energy)
• violet = short, fast wave (high energy)
• objects appear coloured due to the wavelengths of light that they reflect
• in a vacuum, all wavelengths in white light travel together and are indistinguishable
• when light enters and exits a prism, each wavelength bends a different amount separating the different wavelengths (colours)

Question 6

Heat

Answer 6

• a different form of energy than light

Question 7

Conduction

Answer 7

• transfer of heat by fast particles hitting slower ones, speeding them up
• requires a medium

Question 8

Convection

Answer 8

• as a fluid is heated its particles push further apart and it expands
• this reduces the fluids density so it rises, conversely, cooler fluids sink by the opposite process
• requires a medium

Question 9

Radiation

Answer 9

• causes fluid movement because fluid density changes with thermal expansion
• transmits electromagnetic energy
• doesn’t require a medium and can travel through a vacuum

Question 10

The Ray Model of Light

Answer 10

• represents light by a few straight light rays drawn as arrows
• light rays travel in straight lines until they pass through a material or bounce off

Question 11

Transparent

Answer 11

Allows light to pass through freely, giving a clear image

Question 12

Translucent

Answer 12

Only allows some light rays to pass through, giving a blurry image

Question 13

Opaque

Answer 13

Doesn’t light light rays pass through

Question 14

Ray Diagrams

Answer 14

• show the path of a few key light rays
• θi = angle of incidence
• θr = angle of reflection
• a normal is an imaginary line perpendicular to the point of reflection
• when measuring angles, measure from the normal to the light ray

Question 15

Laws of reflection

Answer 15

• the angle of incidence equals the angle of reflection

* the incident ray, reflected ray, and normal are all in the same plane

Question 16

Regular/specular reflection

Answer 16

• occurs when light bounces off a polished surface

* individual light rays obey the law of reflection, and since the normals are parallel, so are the reflected rays

Question 17

Diffuse reflection

Answer 17

• occurs when light bounces off an unpolished surface

* individual light rays obey the law of reflection, but since the normals aren’t parallel, the reflected rays scatter

Question 18

Convex/diverging mirrors

Answer 18

• reflected rays diverge, but their extensions converge on the far side focal point
• ray 1 = parallel to the principal axis, then extend from the mirror surface through the far focus
• ray 2 = straight to the far focus, then reflect back from the mirror, parallel to the principal axis, extend reflected ray behind the mirror
• image is always upright, smaller, and virtual
• examples = passenger mirror, security mirror

Question 19

Concave/converging mirrors

Answer 19

• reflected rays converge on the same side focal point
• ray 1 = parallel to the principal axis, then reflect from the mirror surface through the near focus
• straight through the near focus, then reflect back from the mirror, parallel to the principal axis
• there are several possibilities on the outcome of the image
• examples = make-up mirror, flashlight

Question 20

Refraction

Answer 20

• refraction = bending of light as it passes from one medium to another
• the speed of light is constant in any given medium, but changes when light crosses into a different medium
• refraction makes partially submerged solid objects look bent or disjointed
• rainbows are caused by dispersion, the refraction of white light into separate wavelengths

Question 21

Rules for refraction

Answer 21

• the incident ray, the refracted ray, and the normal are all in the same plane
• the incident ray and the refracted ray are on opposite sides of the line that separates the two media
• moving from a fast to slow medium, light bends toward the normal (FST)
• moving from a slow to fast medium, light bends away from the normal (SFA)

Question 22

Index of Refraction

Answer 22

• a number never less than 1.00
• measures how much light bends when passing from a vacuum to a medium
• a larger refractive index means the medium decreases the speed of light more
• light travels fastest in a vacuum and is assigned a value of 1.00
• since all other media slow light down, they all have a higher index of refraction

Question 23

Equation for Index of Refraction

Answer 23

• n = c/v
• n = the refractive index (no unit)
• c = speed of light in a vacuum (3.00 x 108 m/s)
• v = speed of light in the medium

Question 24

Partial Reflection and Refraction

Answer 24

• refraction of light is often accompanied by reflection
• example = windows are make to be transparent, but we often see reflections of ourselves in them at the same time
• incident rays striking below the critical angle will reflect and also refract

Question 25

Total internal reflection

Answer 25

• at angles of incidence exceeding the critical angle, light will completely reflect back into the slower medium, and no refraction occurs
• jewellery = with a very high index of refraction, diamond has the lowest critical angle that gives diamonds the glittery sparkle
• fiber optics = sending light through thin transparent fibers is a much faster way to transmit digital information than electrons through copper and doesn’t corrode

Question 26

Snell’s Law

Answer 26

• n1sinθi = n2sinθr
• n1 = index of refraction of first material
• θi = angle of incidence
• n2 = index of refraction of second material
• θr = angle of refraction
• calculation for critical angle = θr is 90 degrees

Question 27

Concave/diverging Lenses

Answer 27

• always forms a smaller, upright, virtual image
• the focal length is positive
• example = peephole
• ray 1 = ray extends to the middle of the lens, refracts away from the principal axis, and its extension passes through the closer focal point
• ray 2 = through the center

Question 28

Convex/converging Lenses

Answer 28

• forms different images depending on the location of the object
• the focal length is negative
• example = magnifying glass
• ray 1 = ray extends to the middle of the lens, refracts towards the principal axis passing through the far focal point
• ray 2 = through the center