Energy and Change Light Electricity Energy
Introduction
Questions to
think about
How we see
Reflection
Refraction
What is refraction?
Exploring prisms
Lenses & images
Refraction
Gems
Summary
Colour
Conclusion
Light > Refraction > Gems PD  |  Assessment  |  Home

Activity 4: Gems*** Overall management

If your best friend wanted to buy you a sapphire ring, how could you be sure it was a real sapphire? How could you tell the difference between sapphire and glass? How can you be sure a ruby is real and not glass?

A gemmologist is a person who studies gems and can identify gem stones. Gemmologists test the authenticity of gem stones by measuring their powers of refraction.

Normal and angles
The 'normal' ray is a ray drawn perpendicular to the surface at the point where the light ray enters the surface.

The 'angle of incidence' is the angle between the incoming light ray and the normal ray.
Optical density and refraction
The path of a refracted ray of light
The 'angle of refraction' is the angle between the light ray leaving the surface of the material and the normal ray after the light has been refracted.

The greater the optical density of the material, the greater the amount of bending or refraction of the light.

A    Investigation 1
A gemmologist has found that a set of six different unlabelled gem stones has been mixed up. In order to identify them, a refractometer is used to measure the amount of refraction each gem produces.

Hints The refractometer shines a ray of light at an angle of incidence of 45° onto one facet (face) of a gem. The angle of refraction is then measured three times for each gem. The results are listed in the following table.
Measurments of the angle of refraction through some gems

  1. Calculate the average of the three readings for each gem. (Add the readings and divide by three.)
  2. Suggest reasons why the angle of refraction was measured three times and an average calculated.
  3. Optical density is the property of a gem stone which determines the amount of refraction that occurs. Diamond is known to have the highest optical density, and thus causes the most refraction. Which gem could be the diamond?
  4. Sapphires and rubies have the same optical density and thus cause the same amount of refraction. Which gems could be the ruby and the sapphire?
  5. Quartz crystal has a very low optical density and causes the least refraction. Which gem could be the quartz?
  6. Spinel has an optical density just a little less than sapphire. Which gem could be the spinel?
  7. Topaz has an optical density between quartz and spinel. Which gem could be the topaz?
B    Investigation 2
Books about gems and gemmology (the study of gems) refer to the refractive index of a gem stone. This is the term used when identifying gems. It is another way of describing the amount of refraction that will occur when a ray of light enters a substance.

The refractive index of a material is a number which ranges between 1.00 and 2.42. Diamond has the highest refractive index of all: 2.42.

Look at the definition of the refractive index.

Refractive index
The term 'refractive index' of a material is a number used to describe the amount of refraction (or bending of light rays) that can occur when a ray of light passes into that material.

The higher the refractive index of the material, the greater the amount of refraction that will occur.

The refractive index of a material is calculated by the formula:

Refractive index = velocity (speed) of light in vacuum/velocity (speed) of light in a material.

The velocity of light in a vacuum is equal to 300,000,000 metres per second (m/s).

Example 1

Velocity of light in diamond is known to be 124,000,000 m/s.

Therefore, the refractive index of diamond = 300,000,000/124,000,000 = 2.42.

Example 2

Refractive index of quartz is known to be 1.54.

Therefore, the 1.54 = 300,000,000/velocity of light in quartz.

Therefore, the velocity of light in quartz = 300,000,000/1.54 = 195,000,000 m/s.

The refractive indices of some common gems are shown in the table.
The refractive indices of some gems
  1. Using the information about the refractive index, calculate:
    1. the velocity of light in a ruby
    2. the velocity of light in a sapphire.
  2. Which would have the higher velocity of light, topaz or spinel?

Student section


Copyright  |  Disclaimer  |  Privacy notice