Refraction: Comparing the speed of light in glass and air
The speed of light in a vacuum (2.99 x 108 m/s.) has been described as the fastest thing in the entire universe. Light travel in form of wave and this is called propagation of light. Propagation of waves possess both direction and speed, thus velocity. The velocity of light varies with the through which light travels. Light wave can be changed in several ways and, hence, we have properties of light, which are reflection, refraction and diffraction. Reflection is the bouncing off and change of direction of light when the waves hit a shiny surface. This is commonly seen in mirrors or pool of water. Diffraction is a phenomenon that spreads out light waves, for instance, water droplets diffracting sunlight to form a rainbow. Refraction is the third characteristic of light. Refraction occur when light passes through a medium and change its path. It is simply defined as “the bending of light as it passes through different medium.” A good example is seen when a rule appears bent when placed in water. The instructions that follow will help in determining how light travels in air compared to glass and can be applied in case of other materials (media).
In this task, we will use a laser to determine refraction through different materials (media). “Laser” is a short word for “Light Amplification by Stimulated Emission of Radiation,” which in simple means firing light beams in a straight line. The aim of this task is to determine how light refract differently when it travels through varying media. The materials for the task are: pencil, ruler, sheets of paper, calculator, protractor, colored marker, rectangular glass prism (1 cm thick) and a laser pointer or laser pen.
1. Fold a clean sheet of paper into two equal parts.
2. Place the rectangular glass prism on the folded sheet of paper so the centerline of the prism is on the fold-line of the paper/
3. Trace the outline of the glass prism onto the paper with the pencil.
4. Make a small dot on the edge of the paper sheet using a colored marker. It is from that dot you will aim the laser. This dot should be on the same side as the fold, at least 1.5 inches from the fold. Marking this point will be useful. This marked point will be useful in measuring the angle of incidence of the light ray.
5. Lay the laser down on the table or countertop and adjust the beam so it enters the page at the colored dot you made and hits the object at the centerline fold.
Up to this point, the set up should look as shown in the diagram below
6. Turn the lights off in case it makes it easier to see the laser beam.
7. Observe and mark the path of the laser beam in and out of the prism using the pencil.
8. Use the protractor to measure the angle of incidence(θ1), and angle of refraction (θ2). Record the measurements and include any other observations. The angle of incidence (θ1) measures the angular distance between the center line fold (which is used as the normal line) and the line at which the laser beam hits the glass prism (which we treat as the incident ray). At this point, the medium through which light travels is air. The angle of refraction (θ2) is measured from the centerline fold (normal line) to the light path in glass.
The figure below illustrates how the angle of incidence and the angle of refraction are measured
9. Use Snell’s law, to calculate the speed of light in the air and in the glass prism. Snell’s law is given by formula below, where v1 and v2 are the velocity of light in air and glass respectively.
Light travels faster in air than in glass. This is because light slows down when passing through glass since glass has a higher refractive index than air.Snell’s law imply that the ration of sine of angle of incidence (θ1) to sine of the angle of refraction (θ2), is equal to the ratio of velocity of light in medium 1(air) to the velocity of light in medium 2 (glass). The law can be useful in determining refractive indices of materials so as to help in explaining why speed of light varies in different media. In determining refractive index, nof a material, Snell’s law can be used as follows:
Where n1 and n2 represent the refractive indices of material 1 and material 2 respectively. The refractive index is a ration and, hence, has no units. The index gives a description of how light travels through a given medium. It can also be calculated using the formula: n=where c is the velocity of light in a vacuum (2.99 x 108 m/s) and v, the velocityof light in the medium being measured.
Generally, the instructions provided above can be used in determining the speed of light in different media and help in calculating the refractive indices of various materials.