In an experiment to verify Snell's law, what is plotted and what does the gradient of the graph give?

The sine of the angle of incidence is plotted on the vertical axis against the sine of the angle of refraction on the horizontal axis, and the gradient of the straight line through the origin is the refractive index of the block.

How can optical pins be used instead of a ray box in a refraction experiment?

Two pins are placed to define the incident ray and two more are placed on the far side so that all four appear in line when viewed through the block, and the lines joining each pair mark the incident and emergent rays.

Why is the experiment repeated over a wide range of angles of incidence rather than done once?

A range of readings allows a graph to be plotted, and the gradient of that graph averages out the random errors in individual angle measurements far better than a single pair of values could.

Why does a straight line through the origin on a graph of sin i against sin r verify Snell's law?

It shows that sin i is directly proportional to sin r, which is exactly the statement of Snell's law, and the constant of proportionality is the refractive index.

State the main source of uncertainty in a refraction experiment with a glass block and how it can be reduced.

The main uncertainty is in measuring the angles with a protractor, made worse by the finite width of the ray, and it is reduced by using a very narrow ray, marking the ray with a sharp pencil and using large angles of incidence.

CCEA A-Level Physics 1210

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Refraction

Measuring refractive index

3 things to know
Unit AS 2

Overview

Recall questions on the ray-box and optical-pin methods with a rectangular block, what is plotted and what the gradient gives, why a wide range of angles is used and why very small angles are avoided, the main source of uncertainty, and the semicircular block method for the critical angle.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe an experiment to verify Snell's law using a rectangular glass block.

Key term

In a refraction experiment with a glass block, what precaution stops the results being spoiled by the block moving?

Key term

Describe how a semicircular block is used to measure the critical angle of glass.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe an experiment to verify Snell's law using a rectangular glass block.
Model answer
The block is drawn round on paper and a narrow ray from a ray box is directed at one face at a measured angle of incidence, the incident and emergent rays are marked, the block is removed and the refracted path drawn in, and the angles of incidence and refraction are measured from the normal for a range of angles.
2
In an experiment to verify Snell's law, what is plotted and what does the gradient of the graph give?
Model answer
The sine of the angle of incidence is plotted on the vertical axis against the sine of the angle of refraction on the horizontal axis, and the gradient of the straight line through the origin is the refractive index of the block.
3
How can optical pins be used instead of a ray box in a refraction experiment?
Model answer
Two pins are placed to define the incident ray and two more are placed on the far side so that all four appear in line when viewed through the block, and the lines joining each pair mark the incident and emergent rays.
4
Why is the experiment repeated over a wide range of angles of incidence rather than done once?
Model answer
A range of readings allows a graph to be plotted, and the gradient of that graph averages out the random errors in individual angle measurements far better than a single pair of values could.
5
Why does a straight line through the origin on a graph of sin i against sin r verify Snell's law?
Model answer
It shows that sin i is directly proportional to sin r, which is exactly the statement of Snell's law, and the constant of proportionality is the refractive index.
6
State the main source of uncertainty in a refraction experiment with a glass block and how it can be reduced.
Model answer
The main uncertainty is in measuring the angles with a protractor, made worse by the finite width of the ray, and it is reduced by using a very narrow ray, marking the ray with a sharp pencil and using large angles of incidence.

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