experiment to determine refractive index of glass block

3.19 practical: investigate the refractive index of glass, using a glass block

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IGCSE Prescribed Practical: Determining the Refractive Index of Glass in Physics

Keywords: IGCSE Prescribed Practical, Refractive Index, Glass, Angle of Refraction, Physics, Snell’s Law, Angle of Incidence, Light Ray, Glass Block, Optics

Introduction: In this IGCSE Prescribed Practical experiment, we will measure the angle of refraction of a ray of light passing through a glass block to calculate its refractive index. The refractive index is a measure of how much a medium slows down light compared to its speed in a vacuum. By understanding the refractive index of materials, we can explore the principles of optics and the behavior of light as it travels through different media.

Equipment Needed:

Glass block Ray box or laser pointer White paper Protractor Pencil Ruler Step by Step Method:

• Place the glass block on the white paper and trace its outline using a pencil.
• Position the ray box or laser pointer so that it shines a light ray onto one side of the glass block at an angle, ensuring that the light ray passes through the glass block and exits on the opposite side.
• Trace the path of the incident light ray and the refracted light ray on the white paper using a pencil.
• Remove the glass block and extend the traced rays using a ruler to form two lines that intersect.
• Measure the angle of incidence (i), which is the angle between the incident ray and the normal (a line perpendicular to the glass surface).
• Measure the angle of refraction (r), which is the angle between the refracted ray and the normal.
• Use Snell’s Law to calculate the refractive index (n) of the glass: n = sin(i) / sin(r)

Expected Findings and Calculations: By measuring the angle of incidence (i) and the angle of refraction (r), we can calculate the refractive index (n) of the glass using Snell’s Law. The refractive index is a property of the material that influences how light travels through it.

Snell’s Law formula: n = sin(i) / sin(r)

Conclusion: By performing this IGCSE Prescribed Practical experiment, you will be able to determine the refractive index of glass. Understanding the refractive index is essential for studying optics and the behavior of light in different media, which has numerous applications in physics and engineering.

• What is the refractive index?
• How do you measure the angle of incidence and the angle of refraction?
• What is Snell’s Law?
• Why do we use a glass block in this experiment?
• What are some applications of understanding the refractive index in real life?
• The refractive index is a measure of how much a medium slows down light compared to its speed in a vacuum.
• The angle of incidence is measured between the incident ray and the normal (a line perpendicular to the glass surface), and the angle of refraction is measured between the refracted ray and the normal.
• Snell’s Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the refractive index: n = sin(i) / sin(r).
• We use a glass block in this experiment because it is a transparent medium with a known refractive index, allowing us to easily observe the behavior of light as it passes through.
• Understanding the refractive index has numerous applications, including the design of optical devices such as lenses, prisms, and fiber optics, as well as in fields such as astronomy and telecommunications.

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• Waves And Optics

Determination of the refractive index of a glass block

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Syllabus Edition

First teaching 2014

Last exams 2024

Determining Refractive Index ( DP IB Physics: HL )

Revision note.

Physics Project Lead

Determining Refractive Index

Aim of the experiment.

• To investigate the refraction of light by a perspex block
• Ray Box - to provide a narrow beam of light to refract through the perspex box
• Protractor - to measure the light beam angles
• Sheet of paper - to mark with lines for angle measurement
• Pencil - to make perpendicular line and angle lines on paper
• Ruler - to draw straight lines on the paper
• Perspex block - to refract the light beam
• Protractor = 1°
• Ruler = 1 mm
• Dependent variable = angle of refraction , r
• Use of the same perspex block
• Width of the light beam
• Same frequency / wavelength of the light

Apparatus to investigate refraction

• Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
• Switch on the ray box and direct a beam of light at the side face of the block
• A point on the ray close to the ray box
• The point where the ray enters the block
• The point where the ray exits the block
• A point on the exit light ray which is a distance of about 5 cm away from the block
• Draw a dashed line normal (at right angles) to the outline of the block where the points are
• Remove the block and join the points marked with three straight lines
• Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
• An example of the data collection table is shown below:

Analysis of Results

• i and  r are always measured from the  normal
• For light rays entering perspex block, the light ray refracts towards the central line:
• For light rays exiting the perspex block, the light ray refracts away from the central line:
• When the angle of incidence is 90° to the perspex block, the light ray does not refract, it passes straight through the block:
• If the experiment was carried out correctly, the angles should follow the pattern, as shown below:

How to measure the angle of incidence and angle of refraction

Evaluating the Experiment

Systematic Errors:

• Use a set square to draw perpendicular lines
• If the mirror is distorted, this could affect the reflection angle, so make sure there are little to no blemishes on it

Random Errors:

• Use a sharpened pencil and mark in the middle of the beam
• Use a protractor with a higher resolution

Safety Considerations

• Run burns under cold running water for at least five minute
• Avoid looking directly at the light
• Stand behind the ray box during the experiment
• Keep all liquids away from the electrical equipment and paper

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When finding the refractive index of a glass block should the graph be forced through the origin or not? [closed]

Experiment: To measure the refractive index of a block of glass.

A block of glass is placed on a sheet of paper and the outline is drawn. A point of incidence is chosen and the normal at the point of incidence is drawn. Rays are also drawn at angles of incidence of 30, 40, 50, 60, 70 and 80 degrees. A ray box is used to produced the incident ray at the various angles. The path of the refracted rays are marked and the angle of refraction is measured for each angle of incidence. Sin(i) values are plotted on the y-axis, sin(r) values are plotted on the x-axis. A best-fit line is a straight line nearly through the origin. The refractive index of the glass block n is found from the slope of the graph.

The data I used is:

i-values = {30,40,50,60,70,80}

r-values = {19,27,32,36,40,44}

My textbook says to force the best-fit line through the origin.

But my understanding is: If the data points are fairly linear in that they have a correlation coefficient close to 1 and the line does not go through the origin it means that during the experiment the normal was off by about 1 or 2 degrees.

If this is so then taking the slope of the best-fit line not forced through the origin would take this into account.

What are peoples thoughts on this ?

If the data show good linearity and the difference between the forced and non-forced slope is say 1.46 and 1.38 which slope should be taken as the refractive index, the forced or the non-forced ?

Also is my understanding reasonable that the reason for the best-fit line missing the origin slightly is that the normal was off slightly ?

• experimental-physics
• data-analysis

• 1 $\begingroup$ What graph? What are you plotting? You should explain i.e., what is on the X and Y axes. You should also explain the objective of the experiment - it may not be immediately obvious to all who read this. $\endgroup$ –  joseph h Commented Oct 21, 2022 at 0:02

The line may not pass through the origin due to a systematic error, and in that case forcing it to pass through the origin will give a poorer fit and leave the systematic error unidentified. So in general I would not force the fit to pass through the origin.

• $\begingroup$ That's what I was thinking myself. The graph identifies a sytematic error. One must identify the source of the error and then go back and gather the data again. The best-fit line should better approach the origin. That's my understanding anyway. $\endgroup$ –  Kantura Commented Oct 21, 2022 at 10:40
• 1 $\begingroup$ Would it not have been a good thing to have obtained $r$ for $i=0°$ as one of your experimental values? $\endgroup$ –  Philip Wood Commented Oct 21, 2022 at 11:08
• $\begingroup$ @PhilipWood Well these are not my data , but yes that would have been useful. $\endgroup$ –  Kantura Commented Oct 21, 2022 at 16:43

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• Refractive Index

The aim of Experiment : 1. Finding the refractive index of Glass (í µí¼¼) by using Block glass. 2. Apply the Laws of refraction and reflection . Apparatus/tools: Refractive index of Glass Experiment ‫الدكتور‬ ‫الاستاذ‬ : ‫خلف‬ ‫اسماعيل‬ ‫م‬ . ‫م‬ / ‫الجبوري‬ ‫اسماعيل‬ ‫محمد‬

• February 2020

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