The Refraction Of Light Waves And Laws Of Refraction
In this article, we shall discuss the refraction of the light waves, refraction definition, and the change in the direction of light waves as they travel from one medium to another, for example, from air to glass or vice versa. Additionally, the article will discuss the refraction of light through a rectangular glass block, triangular prism, converging and diverging lenses, and also the dispersion of white light by a triangular glass prism.
At the end of the article, you should be able to:
- Explain how the direction of light changes as it travels from one medium to another.
- Deduce a value of the refractive index of a given material from measurements of angles of incidence and refraction.
- Explain the meanings of critical angle and total internal reflection, stating the conditions under which they occur.
- Establish the relationship between critical angle and refractive index and apply it to the solution of simple problems.
- Trace light rays through a triangular prism and obtain graphically the value of the angle of minimum deviation.
- Obtain the spectrum of white light.
- Trace rays of light through converging and diverging lenses and obtain images due to the rays.
- Derive and use the lens formula to solve simple numerical problems on lenses.
Introduction Definitions: Refraction of Light Waves
When a ray of light travels from one transparent medium to another of different densities, its direction is abruptly changed at the surface separating the two media. This is known as the refraction of the light ray. Thus a light ray appears to bend as it crosses the boundary of two different media.
Refraction is due to the difference in the speed of light in the different media.
Refraction is the bending of a light ray as it crosses the boundary between two media of different densities, thus causing a change in its direction.
The phenomenon of refraction is responsible for the following common observations:
- The bottom of a clear river or pond appears shallower than it really is.
- A rod or spoon appears bent or broken when it is partially immersed in water or any liquid.
- Letters in print seem to be nearer when we place a thick block of glass over them.
Let us consider a ray of light traveling from a medium A to another medium B as it is on the law.
The incident ray PO is the path along which the light travels in the first medium.
The angle of incidence (i) is the angle which the incident ray makes with the normal to the surface at
- The incident ray is refracted as it enters the second medium, and it travels along a path OR. OR is known as the refracted ray. The angle of refraction (r) is the angle between the refracted ray and the normal to the surface at 0.
Observations show that when the light travels from a medium A to a denser medium B, the refracted ray bends towards the normal, and the angle of refraction (r) is, therefore, less than the angle of incidence. Conversely if light travels from a denser to a less dense medium, the refracted ray bends away from the normal and the angle of refraction is more than the angle of incidence.
Refraction Through Rectangular Glass Block
We can study the refraction of light at the air-glass boundary using a rectangular block. A thick rectangular glass block, ABCD, is placed horizontally on a thick sheet of paper and the outline of the glass block is drawn with a pencil. A light ray from a ray-box is shone onto the glass surface AB along a line PO inclined at an acute angle to the normal ON. The ray is refracted in the glass slab and emerges from the side CD, along a line RS. The glass block is then removed and the line OR representing the path of the light ray in the glass is drawn. The line RS is found to be parallel to the line PO.
PO is known as the incident ray OR is the refracted ray and RS is the emergent ray. The angle (i) between the incident ray and the normal at O is the angle of incidence. The angle (r) between the refracted ray OR and the normal at O is the angle of refraction. The angle (e) between the emergent ray RS and the normal at R is the angle of emergence.
If the incident ray is along the normal NO, that is, at an angle of 90° to the side AB, (i = 0) it passes straight through the glass emerging on the face CD without change of direction.
Laws of Refraction
There are two laws governing the refraction of light.
- The incident ray, the refracted ray, and the normal at the point of incidence all lie on the same plane.
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media
The second law of refraction is known as Snell’s law:
Sin i /sin r = n, a constant for a given pair of media.
The constant n, is known as the refractive index of the second medium with respect to the first medium. It is a number that gives a measure of refraction or bending of light as it travels from one medium to another. If the light is traveling from air to glass, the refractive index of glass is given by
ang = sine of the angle of incidence in air/sine of the angle of refraction in glass
If light travels from glass to air then the refractive index gna is given by
gna = sine of the angle of incidence in glass/sine of the angle of refraction in air
From the principle of the reversibility of light we have that:
ang = 1/gna
Since refraction is due to the change in the speed of light as it travels from one medium to another, the refractive index is also given by;
ang = Speed of light in the air (vacuum)/speed of light in the glass.
To Verify Snell’s Law of refraction
We can verify Snell’s law using a rectangular glass block. The block is placed on a sheet of paper and its outline is marked by a pencil. Lines such as PO are drawn making angles of 30°, 40°, 50°, 60°, and 70° with the normal ON. A single ray of light from a ray box is directed along PO and the emergent ray ST is marked with a pencil.