Summary of what I learnt (Refraction)

When light strikes an opaque reflecting surface, though most of the light is reflected, some of it is absorbed by the surface too.

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

When light is partially reflected off the surface, it follows the laws of reflection (Refer to my previous post) while the light that is transmitted through the medium travels from one optical medium into another medium. This bending effect of light is also known as refraction.

The amount of refraction will depend on the optical density of the medium it strikes on. The higher the optical density of a medium, the lower the transmittance and the slower the light travels, thus the greater is the refraction of light in it. For example, if a light ray were to enter glass and water separately from air, assuming all other parameters to be kept constant, the ray would bend more in glass than water, as glass is an optically denser medium than water.

When a light ray travels from an optically less dense medium into an optically denser medium at an angle, it will always refract towards the normal as light travels slower in the optically denser medium.

When a light ray travels from an optically denser medium into an optically less dense medium at an angle, it will always refract away from the normal as light travels slower in the optically denser medium.

Critical angle: The angle of incidence in the optically denser medium when the angle of refraction in the optically less dense medium in 90 degree. The critical angle is dependent on the optical density of a material. The greater the optical density of a material, the smaller the critical will be.

Total internal reflection: A phenomenon whereby there is no refraction as all the light is reflected at the interface of the medium. It occurs only when the incident ray is in the optically denser medium and the angle of incidence is greater than the critical angle.

Note:

-Bending of light only occurs at the interface. Within the same medium, light still travels at a straight line

- There would be no refraction or reflection if a light ray strikes perpendicularly to the surface of an optical medium. However, the speed of the ray will decrease

Applications of total internal reflection:

Optical Fibres:

An optical fibre is a long thin strand of glass that normally has an outer plastic coating that helps to prevent the glass surface from being scratched, as a damaged glass surface allow light to escape through its side. When a light ray is shone at one end of the fibre such that the angle of incidence is greater than the critical angle, total internal reflection occurs. If the fibre were straight, the light ray will undergo multiple reflections and eventually emerge at the other end of the fibre.

Optical fibre could be used in telecommunication, where it transmits information over long distances. For example, a piece of information could be sent using electromagnetic waves or light signals. As such, large amount of information are transmitted through electronic devices such as the telephone, television, fax and Internet.

Periscope:

A periscope is formed using 2 right-angled prisms, and is normally used by people in a submarine to see activities above sea level or to see over the heads of the people in a crowd. Glass prims are used in periscope as it does not produce multiple reflections, and it do not have exposed silvered surfaces which can be easily damaged.

Effects of Refraction:

An object will appear to be nearer to the water surface when it is fully submerged in water. A partially immersed object will appear “bent” in water too.

Steps to draw ray diagram of refraction:

1) Draw the virtual image using dotted lines. Take note that the virtual image have to be the same size and orientation.

2) Draw 2 rays from 1 image point to the eye. Take note that dotted lines have to be used once below the water level, and solid lines have to be used once above water level.

3) Draw 2 rays from the corresponding object point to the points of refracted rays at the interface.

4) Double check that arrowheads are labelled clearly to indicate the direction of the light rays, and a normal is drawn at each point of incidence to check that the light rays are correctly refracted.