Reflection and Refraction of Light

Reflection and Refraction of Light

1 Introduction

Light is an electromagnetic wave. When light strikes the surface of a material, some light is usually reflected. The reflection of light rays from a plane surface is described by the law of reflection, which states that the angle of incidence θi is equal to the angle of reflection θr. These angles are measured from a line perpendicular or normal to the reflecting surface at the point of incidence. Also, the incident and reflected rays and the normal lie in the same plane.

In general, incident light rays on a plane interface will be partially reflected and partially transmitted into the second medium. The transmitted ray undergoes a change in direction because the speed of light is different for different media. The ray is said to be refracted.

The speed of light in vacuum is c (approximately 3.00 x 108 m/s), the maximum possible speed of light. For any medium, the speed of light is v where v is less than or equal to c. A quantity called index of refraction n for any medium is defined by n = c/v. The relationship between the angle of incidence θ1 and the refracted angle θ2 is known as Snell's law: n1sinθ1 = n2sinθ2. Light rays of different wavelength (colour) have different angle of refraction for identical angles of incidence. The wavelength dependence of a material's index of refraction is known as dispersion.

The index of refraction is also a measure of the optical density of a material. The greater the index of refraction, the greater the optical density and the lesser the speed of light in the material. If the second medium is less optically dense than the first medium, the refracted ray is bent away from the normal. On the other hand, if the second medium is more optically dense than the first medium, the refracted ray is bent towards the normal.

Snell's law indicates that there is a certain angle of incidence θ1 for which the angle of refraction will be θ2 = 90o. This is known as the critical angle θc. If the angle of incidence is larger than the critical angle, there will be no refracted ray and the incident ray will exhibit a total internal reflection from the boundary into the denser medium.

The purpose of this experiment is to investigate reflection of rays from a plane surface, the dependence of the angle of reflection on the angle of incidence, as well as refraction of rays from air into a transparent plastic medium and from the transparent plastic medium into air. Also, it is to determine the index of refraction of a plastic medium from direct measurements of angles of incidence and refraction of a light ray, and investigate dispersion of light and total internal reflection when light rays travel from plastic medium into air.

2 Methodology

For this experiment, there will be a light source, a ray table and a D-shaped lens. The light source is set to emit a "pencil" beam of light which crosses the exact center of the ray table. The D-shaped lens is placed on the ray table exactly entered in the marked outline.

In part A, the ray table is rotated so that ray enters the lens through the flat surface and the angle of incidence is 0o. The angle of incidence θi is varied by rotating the ray table until the incident and reflected rays can be clearly seen. The angle of incidence θi and the angle of reflection θr are recorded. The above process is repeated 3 times with 3 different angles of incidence.

In part B, the ray table is first rotated so the ray enters the lens through the flat surface. The angle of incidence θ1 is varied by rotating the ray table, in 10o increments from 10o to 80o. For each θ1, the corresponding angle of refraction θ2 is recorded. After this, the ray table is rotated so the ray enters the lens through the curved surface. The angle of incidence θ3 (now inside the lens) is varied by rotating the ray table, in 5o increments from 5o to 40o. For each θ3, the corresponding angle of refraction θ4 is recorded.

In part C, the ray table is rotated so the ray enters the lens through the curved surface and angle of incidence is 0o. A piece of white paper is held vertically near the edge of the ray table so the outgoing ray is visible on the paper. The angle of incidence is slowly increased. The angle of incidence θmin is recorded as the colour separation is first noticed in the refracted light on the paper. Angle of incidence θmax is recorded for maximum colour separation. After this, an angle of incidence θ is set as one of the angles between θmin and θmax. For this θ, the corresponding angles of refraction for red light (θred) and blue light (θblue) are recorded respectively.

In part D, the ray table is rotated such that the ray enters the lens through the curved surface and angle of incidence is 0o. The ray table is slowly rotated to increase the angle of incidence until the emerging ray (red light) just barely disappears. Next, the angle of incidence is adjusted till the emerging ray (blue light) just barely