Light - Physics (Year 9)

Includes images, definitions and concepts.

Light

Light is a type of electromagnetic radiation (ENERGY) that allows the human eye to see or makes objects visible. Light is actually a beam of energy called a ray.

Light Energy

The energy carried by light waves.

Light Ray

A light ray refers to a narrow beam of light that travels in a straight line. It is often used as a simplified representation of the path that light follows. Light rays are used to describe the behaviour of light when it interacts with objects or travels through different mediums.

Speed of Light

In a vacuum light moves at the speed of light: 299,792,458 metres/s (approx 300,000km/s)

Transparent

Transparent materials let almost all light through. It is transmitted.

Translucent

Translucent materials only allow some of the light through. It is transmitted and refracted/bent at different angles.

Opaque

Opaque materials don’t allow any light through. It is absorbed or reflected.

Luminous

Emits/creates their own light.

Non-Luminous

Does not emit light, they will absorb or reflect light.

Angle of Incidence (i)

The angle of Incidence is when light rays approach a surface, the angle between the incident ray (incoming ray) and the normal is known as the angle of incidence. It is measured with respect to the normal line.

Angle of Reflection (r)

The angle of Reflection is when light rays bounce off a surface, the angle between the reflected ray (outgoing ray) and the normal is called the angle of reflection. According to the law of reflection, the angle of incidence is equal to the angle of reflection, both measured relative to the normal line.

Angle of Refraction (r)

The angle of Refraction is when light passes through a boundary between different transparent media, such as from air to water, the angle between the refracted ray (the ray that bends) and the normal is known as the angle of refraction. It is determined by the refractive indices of the media and is governed by Snell's law.

Critical angle

The critical angle is the angle of incidence where the angle of refraction is 90°.

The light must travel from an optically more dense medium to an optically less dense medium.

Emergent Ray

An emergent ray refers to a ray of light that has undergone transmission through a boundary between two different mediums. It is the ray that emerges or comes out from the boundary after interacting with the interface.

The emergent ray may experience changes in direction and other properties depending on the angle of incidence and the properties of the mediums involved.

Incident Ray

\n An incident ray refers to a ray of light that is approaching or striking a surface or interface between two different mediums. It is the ray of light that is incident or incoming onto the boundary.

Reflected Ray

A reflected ray refers to a ray of light that has bounced off a surface after striking it. It is the ray that reflects or bounces back from the surface. (Image shows rough vs smooth surface)

Refracted Ray

Refracted ray is the ray which has entered the second medium and bent either away from the normal or towards the normal depending on the density of the new medium in comparison to the old one.

Law of Reflection

The law of reflection is a fundamental principle in optics that describes the behaviour of light when it reflects off a surface. It states that the angle of incidence (θi) is equal to the angle of reflection (θr), both measured with respect to the normal line (a line perpendicular to the surface at the point of incidence).

θi = θr

Normal

The normal refers to a line that is perpendicular (at a 90-degree angle) to a surface at a specific point of incidence. The normal line is drawn or imagined to be extending straight out from the surface.

Plane Mirror

A plane mirror is a smooth or highly polished surface that reflects light in order to form an image. It is a flat, polished, and reflective surface that produces a virtual image of the real object. Moreover, a light ray passing through the plane mirror must make the angle of reflection equal to the angle of incidence.

Reflection

Reflection refers to the phenomenon where light or any other form of electromagnetic radiation bounces off a surface or boundary upon striking it. When a light wave encounters a reflective surface, a portion of the incident light is reflected back into the same medium from which it originated.

Refraction

Refraction refers to the bending or change in direction of a wave, such as light or sound, as it passes from one medium to another. This bending occurs due to a change in the wave's speed as it transitions from one medium to another with a different optical density.

Refractive Index

The refractive index is a measure of how much a particular material or medium can bend or refract light. It quantifies the degree to which the speed of light changes when it travels from one medium to another.

The refractive index of a medium is typically denoted by the symbol "n" and is defined as the ratio of the speed of light in a vacuum (or air) to the speed of light in the given medium.

Higher refractive index → Refracts more

Total Internal Reflection (TIR)

Complete reflection of a ray of light within a medium such as water or glass from the surrounding surfaces back into the medium. The phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle.

Wavelength

The wavelength of a wave describes how long the wave is; the distance between identical points (adjacent crests) in the adjacent cycles of a waveform signal propagated in space or along a wire.

Shorter wavelength = More dangerous, hotter, higher frequency, higher refractive index

Dispersion

When white light is passed through a glass prism it splits into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colours is termed as dispersion.

White Light/Visible Light

White light refers to the visible light that appears colourless or as a combination of all visible colours. It is the light that is typically produced by a light source such as the Sun or a light bulb.

White light is a combination of all colours in the colour spectrum. It has all the colours of the rainbow.

Electromagnetic Spectrum

The electromagnetic spectrum is the entire range of wavelengths of all known electromagnetic radiations.

• shorter wavelength → more dangerous, frequency higher (faster), higher refractive index (refract more)

• longer wavelength → safer, frequency lower (slower), lower refractive index (refract less)

Ray Diagram

A ray diagram is a representation of the possible paths light can take to get from one place to another.

This is often from a source or object to an observer or screen.

Absorbed

• turned into heat

Reflected

• bounced off

Transmitted

• passes through

Light → More Dense (what happens)

the light ray:

• bends towards the normal

• the light ray slows down

Light → Less Dense (what happens)

the light ray:

• bends away from the normal

• increases in speed

Amplitude (optional)

Amplitude is the distance between the resting position and the maximum displacement of the wave.

Frequency (optional)

Frequency is the number of waves that pass a fixed point in unit time