8 grade. 10 lab. Lesson
The title of the laboratory work: Determining the refractive index of glass
Purpose of the work:
- Determine the refractive index of glass;
- Compare the obtained refractive index value with the table value and evaluate the reliability of the result obtained.
Expected results:
- The refractive index of glass can be determined with the help of law of refraction.
Teacher’s Guide:
- Divide the students to 3 students in each group.
- Before starting laboratory work, read the safety rules:
- To download the worksheet, please follow this link:
Theoretical part
The law of light refraction: the incident beam, the refracted beam and the perpendicular to the interface between two media, restored to the point of incidence of the beam, lie in the same plane. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for the two media.
The value that shows how many times the speed of light decreased when passing from a vacuum into a transparent medium is called the absolute refractive index of the medium. It is denoted by the letter n and is determined by the formula:
Where c – is the speed of light in vacuum, v – is the speed of light in the medium.
Sources:
- Преломление света. Закон преломления света. Полное внутреннее отражение. (2018). Физика. Учебник для 8 класса общеобразовательной школы.
Experimental part
Experiment.
Procedure:
Step 1. Place a plane-parallel plate on a sheet of paper and trace it with a pencil.
Step 2.
Stick two pins into the paper, one close to the plate, the other randomly. Mark the points pierced by pins 1 and 2. Using the property of rectilinear propagation of light, determine the straight line along which the ray will go after double refraction in the plate. To do this, insert pins behind the plate so that each of them is covered by the pin closest to the observer’s eye. Mark these points 3 and 4.
Step 4: Measure the angle of incidence and angle of refraction. Determine the refractive index of glass using the formula:
Compare the results with the table value. Draw conclusions.
Research part
When deriving conclusions, answer the following questions:
- Formulate the law of light refraction.
- How is the speed of light propagation determined in a transparent medium with a known refractive index?
Conclusion
The refractive index of glass can be determined with the help of law of refraction.