Grade 8 Science Unit 3 Review: Light, Reflection, and Lenses

Vocabulary and key concepts from the Grade 8 Science Unit 3 Review on light properties, reflection, and lenses.

Amplitude

The distance between the highest point (crest) on a wave and the rest position; also, the distance between the lowest point (trough) and the rest position.

Hertz ($Hz$)

Cycles per second; unit used to measure frequency of a wave.

Specular reflection

Type of reflection that produces an image of the surrounding.

Crest

The highest part of a wave.

Infrared Waves

Electromagnetic waves having wavelengths between $3 \times 10^{-4}\,m$ and $7 \times 10^{-7}\,m$.

Trough

The lowest part of a wave.

Diffuse reflection

Reflection of light from a rough surface that does not produce an image but instead makes it possible to see what is on the surface of the reflecting material.

Microwaves

Electromagnetic waves having wavelengths between $0.3\,cm$ and $30\,cm$.

Ultraviolet waves

Electromagnetic waves with wavelengths between $4 \times 10^{-7}\,m$ and $4 \times 10^{-9}\,m$.

Dispersion

Formation of rainbow as light separates into its constituents colours.

Radiant energy

Energy, such as light, that spreads out from a source in all directions.

Visible light

A wave that you can see.

Electromagnetic radiation

A repetitive movement, not of particles, but of electric and magnetic energy through space.

Radio waves

Electromagnetic waves having wavelengths from a few centimetres to several kilometres long.

Wave

A disturbance or movement that transfers energy through matter or space, without causing any permanent displacement.

Transverse Wave

Matter in the medium moves up and down perpendicular to the direction the wave travels.

Compression Waves

Matter in the medium moves back and forth along the same direction that the wave travels.

Electromagnetic spectrum

Visible light energy and all the invisible forms of radiant energy exist on the electromagnetic spectrum.

Rectilinear propagation

Describes a property of light: light travels in straight lines.

Wave Model of Light

A model in which light is a type of wave that travels through space and transfers energy from one place to another.

Frequency

The number of repetitive motions, or vibrations, of a wave that occur in a given amount of time; measured in cycles per second or Hertz ($Hz$).

Reflection

The process in which light or another type of wave interacts with a surface and is sent back from the surface.

Wavelength

Distance from one wave crest to the next; length of one unit of a wave that repeats itself.

Gamma Rays

The portion of the electromagnetic spectrum with the highest energy, highest frequency, and lowest wavelength; result from nuclear reactions; used in cancer therapy.

Refraction

The changing of direction or bending of light or other type of wave when travelling from one medium to another medium having a different density.

X-rays

Electromagnetic waves with very short wavelengths, between about $3 \times 10^{-9}\,m$ and about $1 \times 10^{-12}\,m$.

Incident light ray

A ray that strikes a reflecting or refracting surface.

Convex Mirror

Curved mirror for which the centre is behind the outer edges; the mirrored surface is curved inward.

Reflected light ray

The ray leaving a reflecting surface after an incoming ray has struck the surface.

Law of Reflection

States that the angle of reflection of a light ray is equal to the angle of incidence, or the angle at which the ray strikes the reflecting surface.

Refracted ray

A ray that describes the direction of light after passing from one medium to another of different density.

Normal

An imaginary line perpendicular to a surface such as a mirror or lens.

Object

The initial object facing the mirror (if you look in the mirror, you are the object).

Angle of refraction ($R$)

The angle between the normal and the refracted ray.

Angle of incidence ($i$)

The angle between the incident ray and the normal.

Image

The likeness, or appearance, of an object as seen in a mirror or through a lens.

Focal length

The distance from the centre of a lens to the focal point of the lens.

Angle of reflection ($r$)

The angle between the reflected ray and the normal.

Real Image

The image formed when reflected or refracted rays meet; when a screen is placed at the image position, the image can be seen on the screen.

Lens

A curved piece of transparent material, such as glass or plastic, that refracts light in a predictable way; can be used to focus an image.

Concave Mirror

Curved mirror for which the centre is behind the outer edges; the mirrored surface is curved inward.

Virtual Image

Image that appears to be the source of light rays, though no rays are actually coming from it; if you place a screen at the position of a virtual image, no image will be visible.

Convex Lens

A lens that bulges outward on both sides; thicker in the centre than at the edges; causes light rays to converge.

Focal Point

The point at which converging light rays meet or from which light rays diverge.

Concave Lens

Lens that is indented in the centre; thinner in the centre than at the edges; causes light rays to diverge/spread apart, making the image appear smaller.

Plane Mirror

A flat smooth mirror, or reflecting surface.

Principal Axis

A line that is normal, or perpendicular, to the centre of a mirror or lens.

Near sighted

Vision impairment where nearby objects are seen clearly but distant objects appear blurry; light rays converge before they reach the retina.

Far sighted

Vision impairment where distant objects are clearly visible but nearby objects appear fuzzy; light rays converge behind the retina.

Pythagoras

Believed beams of light were made of tiny particles and that the eyes detected these particles to see the object.

Michaelson

First person to measure speed of light accurately ($3 \times 10^8\,m/s$) using two spinning mirrors that were $35\,km$ apart.

Galileo

Believed to be first to attempt to measure speed of light using lanterns on hilltops about $1\,km$ apart.

Speed of light

$300,000,000\,m/s$; it would circle Earth $7$ and $1/2$ times in one second.

Speed of sound

$1,200\,km/h$; it would take $32\,hrs$ to travel around the world once.

Microscope

An optical device used to magnify extremely small objects, made of a combination of lenses.

Telescope

An optical device used to magnify distant objects, composed of a combination of lenses or lenses and mirrors.