1 Nikus

Index Of Refraction Lab Conclusion Essay

 

Hanne Martine G. Ræstad 1.j Physics 25.03.2014 1

Snell’s Law

Verification of Snell

’s Law of Refraction

Apparatus

glass slab pins graph sheet compass ruler cork board pencil eyesight

Theory

When a light ray passes from one medium to another, its velocity changes with respect to the difference in refractive indices of the two media.

Snell’s law states that the ratio of the sine of the angle of

incidence (



 to the sine of angle of refraction (



 will be equal to the ratio of refractive indices of the second media to the first. (

 

)

 

The refractive index of a substance (

) is defined as the ratio of velocity of light in vacuum (

) to the velocity of light in the medium (

).

 

Procedure

The goal of the experiment itself was to find and note the refractive angles corresponding to various angles of incidence, so

that we can use these to later verify Snell’s law of refraction.

We first began with a sheet of graph paper which was to be used to record our findings. We drew a coordinate system approximately in the middle of the paper and proceeded to draw a circle of radius 10 centimetres with its centre in the point of origin using the compass. The graph paper was then placed on the cork board. The glass slab was placed so that one of the edges was in line with the x-axis of the coordinate system. Then a pin was pinned in the point of origin (shown by a small circle in the diagram). Another pin was placed on a random point of the circle which was on the positive side of the y-axis. Then we used our own eyesight to place the third and final pin, located next to the glass slab so that when seen through the glass, the pins appear to be perfectly aligned. However, what we see through the glass has already been refracted, so the pins will then indicate the refracted light ray. When the angle had been indicated on the graph sheet using a pencil and a ruler. This was repeated for seven different angles of incidence.

Introduction

This laboratory was designed to investigate the behaviour of light as it travels through a less dense into a denser medium.

Materials:

  • Ray Box with comb
  • Semicircular plastic block

Procedure:

  • Placed the semicircular plastic block on the centre of a blank sheet of paper.  Traced its outline and indicated the centre of the flat side.
  • Directed a single ray of light from the raybox to the centre of the flat side at an angle of incidence of about 0˚.  Marked the location of the incident and emergent ray.
  • Directed the raybox to the centre of the flat side at 5 different incident angles.  Marked the location of all the incident and emergent rays.
  • Removed the block and complete sets of rays were drawn.

Analysis:

Angle of Incidence (° from nearest normal)Angle of Refraction (° from nearest normal)Sine of Angle of Incidence (° from nearest normal)Sine of Angle of Refraction (° from nearest normal)
0000
15100.1736480.258819
30200.342020.5
45290.484810.707107
60350.5735760.866025
75410.6560590.965926

The sine of the angle of incidence is equal to the sine of the angle of refraction multiplied by 1.4797

Conclusion:

From the above “SIN <i vs. SIN <r” graph, a clear relationship is established:


Sine of Angle of Incidence α Sin Angle of Refraction

SIN <i α SIN <r

SIN <i = SIN <r × K

SIN <i = SIN <r × 1.4797

SIN <i × 1 = SIN <r × 1.4797

This relationship is also known as Snell’s Law.

When light passed from air into a denser medium, the ray of refraction bent towards the normal.  Also, the angles of incidence and the angles of refraction were not directly proportional in contrary to the sin of the incident and refraction angles.  Moreover, the observations and graph shows that the ratio sin i / sin R is a constant for any given medium and that although the incident and refracted ray appeared on opposite sides of the normal, but they all lie in the same plane.

Refraction of Light II

Introduction

This laboratory was designed to investigate the behaviour of light as it travels through a denser into a less dense medium.

Materials

  • Ray Box with comb
  • Semicircular plastic block

Procedure:

  • Placed the semicircular plastic block on the centre of a blank sheet of paper.  Traced its outline and indicated the centre of the flat side.
  • Directed a single ray of light from the raybox to the centre of the curved side at an angle of incidence of about 0˚.  Marked the location of the incident and emergent ray.
  • Directed the raybox to the centre of the curved side at 5 different incident angles.  Marked the location of all the incident and emergent rays.
    • Removed the block and complete sets of rays were drawn.

Analysis:

Angle of Incidence (° from nearest normal)Angle of Refraction (° from nearest normal)Sine of Angle of Incidence (° from nearest normal)Sine of Angle of Refraction (° from nearest normal)
0000
15240.4067370.258819
30500.7660440.5
35650.9063080.573576
40800.9848080.642788
45Total Internal ReflectionN/AN/A
60N/AN/A
75N/AN/A

The angles corresponding with 45°, 60°, and 75° weren’t used because they were reflected rays and not refracted.

From the above graph, it is clear that another relationship, similar to the one found in “Refraction of Light I” exists.

Sine of Angle of Incidence α Sin Angle of Refraction

SIN <i α SIN <r

SIN <i = SIN <r × K

SIN <i = SIN <r × 0.6452

SIN <i × 1.4797 = SIN <r × 1

In this case, the indexes of refraction have been reversed.

Snell’s law stats that when traveling from a denser to a less dense index, the angle of incidence will be less then the angle of refraction. Since plastic is denser then air, the incident ray will bend away from the normal when it travels through air. However, at a certain angle of incidence, the emergent ray is a reflected ray. This can be seen by using the Snell’s law. If you plug in all of the values, you’ll get that SIN > 1, and hence, it’s impossible. However, this never happens, so instead, the ray is reflected.

The conditions for total internal reflection to occur are:

i. Light must be travelling in the more refractive medium.

ii. The angle of incidence in the more refractive medium must be larger than the critical angle.

The critical angle refers to an angle of incidence that produces a corresponding emerging ray that has an angle of refraction of 90°.  It is largest possible angle of incidence that doesn’t result in total internal reflection.  In this activity, it was estimated to be 43°.

Conclusion

By doing this experiment it can be proved that there are special cases to when light travels to different mediums (high to low density). When the angle of incidence is greater than the critical angle, light doesn’t follow Snell’s Law.  Instead of refracting, the ray of light reflects. Apart from this difference in refraction, Snell’s Laws is followed throughout.

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