Optics

angle of incidence

The angle between the incident ray and the normal at the interface of two different media.

angle of reflection

The angle between the reflected ray and the normal at the interface of two different media.

angle of refraction

Angle of refraction is the angle between the refracted ray and the normal.

concave lens, diverging lens

Concave lenses are lenses with the central parts concave inwards. They are usually made of glass or other transparent materials. Concave lenses diverge light beams. Therefore they are also called diverging lenses. Spectacles for shortsighted are concave lenses.

concave mirror

Concave mirrors are curved mirrors with the reflecting surfaces concave inwards. Concave mirrors converge light beams. Therefore they are also called converging mirrors. Concave mirrors are used in reflecting telescopes.

convex lens, converging lens

Convex lenses are lenses with the central parts convex outwards. Convex lenses converge parallel light beams. Therefore they are also called converging lenses. Magnifying glasses and spectacles for longsighted are convex lenses.

convex mirror

Convex mirrors are curved mirrors with the reflecting surfaces convex outwards. Convex mirrors diverge light beams. Therefore they are also called diverging mirrors. Convex mirrors may be used as rear view driving mirrors.

focal length

The distance between the focus and the centre of a mirror or a lens.

focus

Incident light rays parallel to the principal axis will converge to one point after reflection by concave mirrors or refraction by converging lenses. This point is called the focus of the optical instrument. Incident light rays parallel to the principal axis will diverge apparently from one point after reflection by convex mirrors or refraction by diverging lenses. We also call this point the focus of the optical instrument. The plane passing through the focus and perpendicular to the principal axis is called the focal plane.

image

The light rays coming from a point light source will change their directions of propagation after reflection or refraction. If these rays seem to be coming out from one point, we call this point the image of the light source. The image of an extended object consists of the images of the point sources which makes up the object.

law of reflection

When a wave is reflected, the angle of reflection equals the angle of incident; the normal, the incident ray and the reflected ray lie on the same plane.

light ray

A narrow beam of light. In optical experiments, we use the rays produced by ray box to examine the properties of light. We also draw ray diagrams in the analysis of geometrical optics.

magnification

Magnification is the linear sizes of image to object ratio for optical instruments. For example, the magnification of plane mirror is one, i.e., the image and the object are of the same size. If the magnification is greater than one, then the image is larger than the object.

normal

The straight line perpendicular to the interface of reflection (or refraction), passing through the point of incidence.

principal axis

A straight line passing through the centre of lenses or mirrors and perpendicular to the their plane.

real image

If the light rays really pass through the position of the image, we call the image a real image. The image can be projected on a screen at its image.

reflection

When a wave meets the interface of two media, part of it will travel back to the first medium This phenomenon is called reflection. For example, light is reflected by a plane mirror.

refraction

When a wave travels from one medium into another medium, the wave speed changes, while the frequency remains unchanged. the direction of propagation changes due to the change of wave speed in a different medium.

refractive index

Refractive index is a property of an optical medium. It describes the degree of refraction of waves in a medium. The refractive index of an electromagnetic wave in a medium is the ratio of its speed in vacuum to that in the medium: . In experiments, we can measure the angle of incident and the angle of refraction . By Snell's law, , hence we can calculate the ratio of refractive indexes between the two media (This ratio can also be defined as the refractive index of medium 2 relative to medium 1). Note that electromagnetic waves with different wavelengths have different refractive indexes in the same medium. Therefore they are refracted by different degrees. This is the working principle of colour dispersion by a prism.

Snell's law

When waves propagates from medium 1 into medium 2, they will be refracted. The angle of incident and the angle of refraction would obey the following relation: , where and are the refractive indexes of medium 1 and medium 2 respectively.

total internal reflection

Consider light rays travel from medium 1 to another medium 2. If the refractive index of medium 1 is greater than that of medium 2 , and the angle of incident is larger than the critical angle defined as , then all the light rays will be reflected back to the first medium according to the law of reflection. This is called total internal reflection. Total internal reflection is the working principle of optical fibre.

virtual image

If the light rays do not pass through the position of the image, we call the image a virtual image. The image cannot be projected on a screen at the position of the image. The image produced by a plane mirror is an example of a virtual image.