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Science project idea

Glass prism: angle of deviation against angle of incidence curve

A ray bent by a prism is deviated by an angle D that first falls and then rises as the angle of incidence increases, with a minimum in the middle. The refractive index follows from the minimum deviation and the prism angle.

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What it shows

A ray bent by a prism is deviated by an angle D that first falls and then rises as the angle of incidence increases, with a minimum in the middle. The refractive index follows from the minimum deviation and the prism angle.

The science

A prism bends a ray at both faces, and the total deviation D = i + e − A where A is the prism angle, i the angle of incidence and e the angle of emergence. D has a minimum value Dm when the ray passes symmetrically, with i = e, inside the prism parallel to its base. The refractive index is then n = sin((A + Dm) / 2) / sin(A / 2). For a 60 degree glass prism Dm is about 38 to 40 degrees.

Materials

Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • An equilateral glass prism (from a science shop)
  • A drawing board, white paper and drawing pins
  • Four pins
  • A protractor and a sharp pencil
  • A ruler
  • Graph paper

Safety

For this project: An adult must handle the pins and the glass prism, whose edges can be sharp.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Draw the outline of the prism on the paper and measure the prism angle A with the protractor to confirm it is about 60 degrees.

  2. Draw a normal at a point on one face and a line making i = 35 degrees, and fix pins P1 and P2 on this line.

  3. Looking through the other face, fix pins P3 and P4 in line with the images of P1 and P2.

  4. Remove the prism and draw the emergent ray through P3 and P4; extend the incident and emergent rays until they meet and measure the angle D between them.

  5. Repeat for i = 40, 45, 50, 55, 60 and 65 degrees, drawing a fresh sheet or fresh lines each time.

  6. Plot D (y axis) against i (x axis) and draw a smooth curve; the lowest point is Dm.

  7. Read off Dm and calculate n from the formula.

What you should see

The curve falls, reaches a minimum near 38 degrees at i of about 49 degrees, then rises again. The refractive index calculated from Dm is about 1.5.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What happens to D as i increases?

D first decreases to a minimum, then increases.

2What is the ray’s path in the prism at minimum deviation?

Symmetric, parallel to the base of the prism.

3Write the formula for the refractive index from the prism.

n = sin((A + Dm) / 2) / sin(A / 2).

4Why does a prism split white light?

Different colours have different refractive indices, so they bend by different angles.

Free ebook: Viva Questions for Science Projects

Take it further

Place coloured filters or red and blue paper over the incident ray and measure the minimum deviation for each.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

Free ebooks for this project

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