Light

For you to see anything, light must travel from an object to your eye. Objects that produce their own light, like the Sun or a light bulb, are called luminous. Most objects, like a book or the Moon, are non-luminous; we see them by the light they reflect from a luminous source. Light is a form of energy that travels in straight lines, which we represent using straight lines with arrows called rays. It is a transverse wave and, uniquely, can travel through a vacuum (empty space). In a vacuum, light travels at the fastest possible speed in the universe: 300,000,000 metres per second (3 x 10^8 m/s).

Reflection is when light bounces off a surface. A smooth, shiny surface like a plane (flat) mirror reflects light in a predictable way. To describe this, we imagine a line drawn at 90 degrees to the mirror's surface at the point where the light hits; this line is called the normal. The incoming ray is the incident ray, and the outgoing ray is the reflected ray. The Law of Reflection states that the angle of incidence (the angle between the incident ray and the normal) is equal to the angle of reflection (the angle between the reflected ray and the normal). The image you see in a plane mirror is virtual (it cannot be projected onto a screen), upright, laterally inverted (left and right are swapped), the same size as the object, and appears to be as far behind the mirror as the object is in front of it.

Refraction is the bending of light as it passes from one medium to another, for example, from air into glass. This happens because light travels at different speeds in different materials. A material that slows light down more is called optically denser. When light enters an optically denser medium at an angle, it bends towards the normal. When it enters a less dense medium, it bends away from the normal. The amount of bending is described by the material's refractive index (n). A higher refractive index means more bending. Snell's Law relates the angle of incidence(i) and the angle of refraction(r) with the refractive indices of the two materials.

When light travels from a denser medium to a less dense medium (e.g., from glass to air), it bends away from the normal. As the angle of incidence increases, the angle of refraction also increases. At a specific angle of incidence, called the critical angle (c), the angle of refraction becomes 90°. If the angle of incidence is increased beyond the critical angle, the light does not exit the medium at all. Instead, it is completely reflected back into the denser medium. This phenomenon is called Total Internal Reflection (TIR). For TIR to occur, two conditions must be met: 1. Light must be travelling from a more optically dense medium to a less optically dense one. 2. The angle of incidence must be greater than the critical angle.

A lens is a piece of transparent material shaped to refract light in a specific way. A convex or converging lens is thicker in the middle than at the edges. It causes parallel rays of light to converge and meet at a single point called the principal focus or focal point (F). The distance from the centre of the lens to the principal focus is the focal length (f). The type of image formed by a converging lens depends on the object's distance from the lens. If the object is placed further away than the focal length, the lens produces a real, inverted image. This is how cameras and projectors work. If the object is placed closer than the focal length, the lens produces a virtual, upright, and magnified image. This is how a magnifying glass works.

Visible light is just one small part of a much larger family of waves called the electromagnetic (EM) spectrum. All EM waves are transverse waves, can travel through a vacuum, and travel at the same speed in a vacuum (the speed of light, 3 x 10^8 m/s). They differ only in their wavelength and frequency. The spectrum, in order of increasing wavelength (and decreasing frequency/energy), is: Gamma rays, X-rays, Ultraviolet, Visible light, Infrared, Microwaves, Radio waves. Each part of the spectrum has different properties and uses. For example, radio waves are used for broadcasting, microwaves for cooking and communication, infrared for remote controls and thermal imaging, and X-rays for medical imaging.