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Refractive index
Theoretical introduction:
Refractive index is a dimensionless physical quantity, which is specific
for a certain medium, and its value characterizes the speed of light in this
medium. We distinguish between the relative and absolute refractive index.
The absolute refractive index is defined as a ratio of the speed of light
in vacuum and in selected medium. Generally, the refractive index depends
on the wavelength of incident light.
Relative refractive index is defined as a ratio of speeds of light in two
different media. Usually it characterizes properties of an interface between
these media.
If light impacts the interface between two media, it is partially reflected
and partially refracted. Snell’s law describes the relation between the angles
of incidence and refraction.
n12 =
sin α v1 n2
=
=
sin β v 2 n1
Comparing two media, the medium with smaller refractive index is
called optically sparser while the medium with higher refractive index is called
optically denser.
When light is refracted into the optically sparser medium, the refractive
angle can be 90 degrees for certain incident angle. This incident angle is
called critical and for angles of incidence above the critical angle all light is
reflected. Such effect is called the total reflection. The value of the critical
angle depends on refractive indexes.
sin ϕ =
n2
n1
Equipment:
1)
2)
3)
4)
5)
Halogen lamp
Aperture (1 and 2 slits)
Protractor
Plan convex lens
AC power supply
Tasks:
1) Find the refractive index for light incidence from air to glass (both
absolute and relative).
2) Find the value of the critical angle for light incidence from glass to
air.
Execution:
1) Find the refractive index for light incidence from air to
glass
1) Adjust the halogen lamp with the protractor (use the light source with
rectangular shape of the output)
2) Put the aperture with 1 slit on the lamp output.
3) Adjust the plan convex lens on the protractor as is shown on figure.
4) Aim the light ray from the lamp directly to the center of the lensunder
certain angles of incidence. Repeat the experiment for the angles of
incidence 20, 30, 40, 50 and 60 degree.
5) Find the corresponding refractive angles and write down the values.
Note that the reflection also occurs.
6) Calculate the refractive index n12 for the light incidence from air to
glass (relative). Perform the calculation for each pair of incident and
refractive angles and calculate the average value as well.
n12 =
sin α v1 n2
=
=
sin β v 2 n1
7) Calculate the refractive index n2of glass (absolute) (refractive index of
air at normal atmospheric conditions is n1=1.00026)
8) Write down all the values and make the conclusions.
2) Find the value of the critical angle for light incidence
from glass to air.
1) Adjust the halogen lamp with the protractor (use the light source with
rectangular shape of the output)
2) Put the aperture with 1 slit on the lamp output.
3) Adjust the plan convex lens on the protractor along one axis,
symmetrically to the second axis. The convex part of the lens is located
on the light source side.
4) The light is refracted on the planar interface of the lens.
5) Increase the angle of incidence from 10 degree and find the critical
angle, when the refractive angle is 90 degree.
6) Write down the value and verify it using the equation for the calculation
of the critical angle. Use the values of refractive indexes from the
previous task. Make the conclusion.
Questions:
1) What is the refractive index?
2) What is the critical angle?
3) Where the total reflection is used?
Defects of vision
Theoretical introduction:
An optical system of health human eye provides a sharp image on its retina. If
the image is distorted, we are talking about defects of human vision. The most
common defects are hyperopia (farsighted eye) or myopia (nearsighted eye).
When the eye is nearsighted, the light coming from a distant object is focused
in front of the retina rather than directly on it. This causes distant objects out of focus,
but close objects in focus.
On the other hand, farsighted eye focuses the light behind the retina, resulting
in out of focus close objects.
Equipment:
1.
2.
3.
4.
5.
Halogen lamp
Aperture (3 and 5 slits)
Model of human eye
Lens
AC power supply
Task:
1) Find which kind of vision defect has the model of human eye and
suggest proper correction.
Execution:
1. Put the halogen lamp in front of the model eye (use the light source with
rectangular shape of the output)
2. Employ the aperture with 3 slits on the lamp output.
3. Place the lens on the model eye (as is shown if figure).
4. Adjust the middle light ray parallel to the horizontal axis (as is shown in figure).
5. Observe the place where the rays are focused in the case of health and
defected eye. Describe the defects of vision for all model eyes. Write down the
results and suggest proper correction.
Question:
1) What is diopter?
Additive color mixing
Theoretical introduction:
Color is a perception, which is created by the incidence of visible light
on human eye retina. Colorful vision is mediated by retinal cones, which are
sensitive to three basic colors (red, green, blue).Various mixing of these three
basic colors creates additional colors.
There are two types of color mixing: additive and subtractive. In the
case of additive mixing theindividualcomponents are summed, which results is
the light with higher intensity. The additive mixing is working with three basic
colors (red, green, blue (RGB)) and is used in display technology (monitors,
televisions, digital cameras).
On the other hand, the intensity is decreased with every additional
color in substantive color mixing. The basic colors in substantive color mixing
are cyan, magenta and yellow. This type of color mixing is used in printers.
Equipment:
1)
2)
3)
4)
5)
three color lamps (RGB)
mounts for optical bench
screen
optical bench
power supply
Task:
1) Experience additive color mixing
Execution:
1.
Install lamps into the holders and adjust them on the optical
bench. Adjust the screen on the optical bench (see figure bellow).
2.
Connect the lamps to the power supply.
3.
The lamps will create three light spots of basic colors on the
screen. Adjust the lamps to partially overlap the light spots in the center
of the screen.
4.
Write down which colors will be created by mixing of two or
three basic colors.
Questions:
1) How different colors on the PC monitor are created?
2) How the human eye senses colors?