Category: Physics Notes PPT

Conductivity of Metals

Conduction is an important concept in Physics. It is one of the three concepts by which heat and energy can be transferred from one place to the other within a material or from one material to another. Conduction occurs by direct contact whereas Convection occurs by movement or flow of heat. Radiation on the other hand occurs with the help of electromagnetic waves. Conduction mostly occurs in metals and the conductivity of metals is different from one another. When it comes to the best metal for thermal conductivity, ideally it is Silver but Copper is used instead. The reason for it is that Silver is too expensive and is infeasible to use. Copper, on the other hand, is available in abundance, is affordable, and can be used in any application.

Law of Conduction

A list containing the conductivity of various metals is used to compare and accordingly use metal for a specific purpose. Calculating the thermal conductivity of the metal rod was one of the initial and important experiments that were carried out to measure the conductivity of metals.

The law of conduction or the Fourier’s law states that the time required for heat transfer through metal is proportional to the negative gradient in the area and temperature. This law is applicable in two forms; differential form and integral form. 

Various calculations can be made to measure the transfer of heat. Heat flux for instance is the amount of heat flow that occurs per unit area. Similarly, the opposite of thermal conductivity, which is thermal resistivity, can also be calculated. 

Thermal resistivity is the ability of a material to not conduct heat. This property is present in materials that do not conduct heat. They are used as insulators. Some of them are rubber, fabric, cork, ceramic, styrofoam, etc.

Heat transfer is bound to occur when two materials of different temperatures come in contact with each other. Thermal insulation is done to minimize the heat that is transferred from one material to the other. This process also depends on the product density and the specific heat capacity of the insulating material used.

Thermal Conductivity of Metals: Uses

Based on their rate of conduction, metals are classified and used for particular applications. If a metal has high thermal conductivity, then it is used in heat sink applications. On the other hand, if the metal has low thermal conductivity, then it is used in thermal insulation applications. 

Insulation acts against heat transfer whereas heat sink favors it. If you consider some metals like steel and copper then, the thermal conductivity of steel and copper is 45 W/m-K i.e. 45 Watts per meter-Kelvin and 385 W/m-K i.e. 385 Watts per meter-Kelvin respectively.

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Solved Example

Question 1:  Consider a window of width 1.3m and height 1.7m, having a thickness of 6.3mm and a thermal conductivity value of 0.28 W/m/degree C. The temperature inside the house is 22 degrees Celsius and that outside the house -5 degrees Celsius.  Calculate the rate of heat transfer.

Answer: Surface Area of the window = length x breadth

    = (1.3 x 1.7) meter square

= 2.21 meter square

The thickness of the window = 6.3 mm

= 0.0063 m

Rate of heat transfer = (Thermal conductivity) x (Surface area) x (Difference of the temperatures) / (Thickness of the window)

= (0.28 W/m/degree C) x (2.21 meter square) x (22-(-5) degrees C) / (0.0063 m)

          ~ 2652 W

The rate of heat transfer for the given example is 2652 W.

Fun Fact

Did you know that besides metals, gases can be conductors of heat too? Yes, that’s right. In the entire periodic table, metals and gases are the two categories of elements that can conduct heat. Most of the metals and some other elements are conductors of electricity as well. Gases, however, do not conduct electricity.

Even though they can conduct heat, their rate of conductivity is much less than the rate of conductivity of metals. Due to this, gases are instead used as insulators. Thermal insulation or resistivity is the property of not being able to conduct heat. 

Materials such as rubber, wood, plastic, glass, etc. are used as insulators. They work opposite to the thermal conductivity of metals.

In astronomical terms, the strain that is produced on a celestial object undergoing variations in gravitational attraction, variations that are cyclic in nature as it orbits, is termed tidal friction. Tidal friction is also observed in cases when one celestial object is orbiting another such as the Earth and the moon. 

Tidal friction is usually seen occurring between sea bottoms and water tides, especially in parts where the sea is relatively shallow. It is also observed between parts of the planet’s or satellite’s solid crust that rotate against each other. 

Tidal Friction Theory

The tidal friction theory was first developed after 1879 in mathematical terms. George Darwin, the English astronomer and son of Charles Darwin is credited with having formulated the mathematical theory of tidal friction. 

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Tidal Forces

The moon’s gravitational force and the tidal force of the sun cause the tides in the oceans. The inverse square law states that a net stretching force is produced as a result of the force being greater on the nearer side as compared to the force on the far side. This is the principle behind tidal forces stretching the earth in the direction of the body that produces the tide. Tidal stretching, while more prominent in the oceans is also observed in the case of landmasses.  

The character of the orbit of an orbiting body is gradually changed by the tidal forces acting on it. For example, in the case of an orbiting moon which is also undergoing rotation about an axis that is perpendicular to the plane of the orbit, the tidal force acting stretches the moon along the line that joins it with the planet. At the same time, the stretching relaxes as the diameter is rotating away from the line. The stretching is subjected to frictional resistance and in the stretching and relaxation of the deformation energy is dissipated as heat which eventually takes energy away from the system. 

Tidal Friction Between the Earth and the Moon

The tidal bulge caused on the Earth’s crust and seas by the pull of the moon is prevented by tidal friction. The rotation of the earth, instead, carries out the bulge from under the moon directly. The earth spins in its orbit a total of 30 times for every revolution carried out by the moon in its orbit. There exists a mutual attraction between bulge material and the moon which increases the tendency of the moon to accelerate in its orbit. This causes the moon to move farther from the earth by about 1.2 inches or 3 centimetres every year and also slows down the daily rotation of the earth per year by a small fraction of a second.

Impacts of Tidal Friction

The moon’s rotational angular momentum is diminished by the torque exerted by gravity as the moon rotates away and eventually, its rotation rate slows down. Over a prolonged period, this braking effect brings the rotation rate of the moon to zero, relative to the connecting line. This causes the rotation period to approach the orbital period and causing the same part of the surface to face the planet constantly.

Scientists have attributed the bygone effects of the moon’s tidal friction as the main reason behind that only one of its surfaces is constantly turned towards the earth. Similarly, the tidal friction is the cause behind the same side of Mercury always faces the sun so much that one side is scorchingly hot and the other side is always cold. 

The tidal force of the moon on the Earth so that tidal friction causes energy to dissipate. The Earth’s tidal deformation, in turn, causes it to rotate away from the connecting line. Additionally, the elongated shape of the earth provides an asymmetry causing it to slow down due to the braking torque. These effects, in the event of a million years from the present time, may cause the Earth‘s rotation to become 50 times longer or perhaps equal to a month’s duration of that time. The earth might also always keep the same side facing a distant moon. These conditions are, however, unlikely to be stable owing to the Sun’s tidal effects on the moon-earth system. 

Conclusion

On this page, we have provided an insight into tidal friction and its resulting impacts. Tidal friction has been long studied by scientists to understand the mechanisms by which the rotational energy of the earth is dissipated and the origin of the moon. 

Introduction

In this experiment, you can determine the effective length of pendulum with the help of graphs. You should plot two types of graphs, such as L-T and L-T2 graphs.

All you need is a simple pendulum graph. Calculation of effective length is a very significant and sensitive experiment. Students should go for the error-free values to determine the right parameter at the end.

This article is completely based on simple pendulum experiment readings and graph. Students can gain a brief idea about this experiment after reading the article.

Materials or Apparatus Required

Some of the materials that are listed below for the second’s pendulum experiment:

Principle of Simple Pendulum Experiment

This experiment has the objective to find the time of a simple pendulum. Also, the length of the second’ pendulum can be calculated within the same operation. Simple harmonic motion on a simple pendulum exhibits an acceleration. 

As you know, the acceleration of the pendulum’s bob is directly proportional to the displacement from the mean position. The time is the value that we need to determine for the simple pendulum.

The relation of time is given below:

T = 2π √(L/G)

Procedure

1. Keep the clamp stand over the table. Attach the pendulum bob with a hook. The length of the string is about 150 cm. another string is two half-pieces of a split cork.

2. Attach the cork tightly with the clamp. This allows a better line of separation between the two pieces of the split cork at right angles to the line OA. When the pendulum oscillates, you can notice the change.

3. Identify the edge of the table that the pendulum is reaching. Mark the lines that lie beyond the edge of the table (about 2 cm above the floor). Mark the line that comes just behind the vertical thread OA.

4. After the setup, you can measure the length of the simple pendulum.

5. Students must consider some of the effective length L along the x-axis. Also, consider the y-axis of value T2 (or T) for plotting the graph. Some of the experiential values from the table are taken into consideration. 

6. Choose convenient scales on these axes to present L and T2 (or T). A graph that is plotted between L and T2 and also between L and has the impact.

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Observation

From the above setup, students should verify the details and answer the following parameters to get appropriate outputs. The following points are given for considering them:

• Length of the hook = __ cm

• The pendulum’s radius of the bob = __ cm

• The least count of the stopwatch = __ s

• The least count of the meter scale = __ mm

How to Plot the Graphs?

The simple pendulum graph is here as shown:

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i) L Vs T Graph

You can plot the graph between L~T to observe different outputs. A table can be helpful to note down all outputs. You can take L along the x-axis and T along the Y-axis. From the figure, you can notice that the graph will give you a curve line.

ii) L Vs T2 Graph

You can plot the graph between L and T3. All of the results can help you further if you keep them in a tabular form. The output of the relation will give you a straight line, as shown in the figure.

iii) We can find out the effective length of the second’s pendulum for T2 = 4s2 obtained from the second graph.

The Output of the Experiment

From the above experiment, you can obtain three output which is given below:

A curve that comes out of the L-T graph gives rise to convex upwards.

A straight line is obtained from the L – T2 graph.

Viva Questions

1. How Do You Define the Connection Between Frequency and Time?

Answer: The connection between frequency and time period is given by the following expression,

 f = 1/T

2. How Do You Define the Restoring Force?

Answer: Restoring force is a force that helps to bring back the vibrating body towards the mean position.  This force acts as the resistance force for the body that vibrates.

3. Is it Useful to Use a Cricket Ball Instead of the Bob in a Pendulum?

Answer: No, this is not that good. The bob must be lighter and smaller. A cricket ball can’t be the possible reason for the replacement period.

4. Why Don’t We Use a Rubber Band by Replacing the Thread?

Answer: No, this is not the way we should experiment. The rubber band is not inextensible like a thread. So, we keep the thread on a priority basis.

The optical phenomenon of total internal reflection occurs when light is entirely reflected at the interface between two media.

The effect happens when the incidence angle exceeds a predetermined limiting angle, referred to as the critical angle.

Let’s take an example for more clarity.

When an incident light ray hits the interface, it is reflected and/or refracted.

A ray of light travels from a medium of water to a medium of air, and vice versa. Light Rays will be refracted at the interface that separates the two media.

The refracted light ray bends away from the normal as it passes from a medium with a higher refractive index to one with a lower refractive index.

The incident ray of light is refracted in such a way that it passes down the water’s surface at a given angle of incidence known as the critical angle.

The refraction angle is now 90 degrees and the incident light can reflect on the medium if the angle of incidence is greater than the critical angle. 

As a result, this phenomenon demonstrates to be a total internal reflection.

What is Total Internal Reflection?

Total internal reflection happens at a time when a light ray that travels from a denser to a rarer medium. The ray is incident at an angle of incidence that is greater than the critical angle. After that, the light rays are reflected in another denser medium. It is the same medium before reflection. The entire process is known as Total internal reflection.

When light travels between two surfaces made of transparent materials, then it gets refracted. This is also called as the bending of light. Light or rays that come towards or drive away from the normal have interfered. This happens due to a change of one medium to another medium. This gives the simple definition of total internal reflection.

When does Total Internal Reflection Take Place?

Take the example of two lights that incident at certain points from an optically denser medium to an optically rarer medium. 

The bending of light, i.e. from the normal happens under the phenomena called refraction of light. This is a special condition where the refracted angle is more than the incident angle.

The above statement explains that the increase in the angle of incident results in the increment of the angle of refraction.

A point still exists where the angle of refraction becomes perpendicular. When this happens, the refracted ray will become parallel to the interface. 

The incident ray angle of the denser medium corresponding with the refracted ray angle of the rarer medium is 90o. This is called the Total internal reflection critical angle(ic).

That time when the ray is incident on the surface at an angle greater than the critical angle, the ray comes back to the same medium. The entire procedure of returning a light ray away from the denser medium is known as Total Internal Reflection.

Explain Total Internal Reflection of Light

We have two major conditions that help us to decide the phenomenon of total internal reflection. (TIR) is based upon. The minor change in the two conditions may not give the appropriate result. 

Total internal reflection has 2 necessary conditions such as:

(i) The light incident upon the interface of two different media should choose to travel from a denser medium to another rarer medium.

(ii) The greater angle of incidence is necessary than the critical angle for these two media.

The formula and Applications of Total Internal Reflection

Formula:

Snell’s law determines the angle of refraction at the interface of two materials.

n₁⋅sin(θ) = n₂⋅sin(θ′)

Where,

n₁ and n₂  = Refractive indices of the two materials

θ = Incident angle of light

θ’ = The angle of refraction

So, now if suppose 

 θ’ is greater than θ as n₁ > n₂

 θ’ becomes π /2 (i.e. 90o) for a certain incident angle θ = θc. 

This angle is known as the critical angle, and it may be calculated using the following formula:

Θ_c = sin−1 (n₂/n₁)

Now, at an incident angle θ > θ_c, Light is completely reflected in material 1 (water) since refracted light can no longer exist.

To make this simple just imagine if you dive into a swimming pool and attempt to see above the surface of the water, you will be unable to see outside the water at a shallow angle.

Applications

1. Optical fibers

Fiber optics uses total internal reflection, which has many advantages in telecommunications. Fiber optics are light-travelling glass or plastic threads the size of a hair.

When light contacts the core-cladding boundary at an angle of incidence larger than the critical angle, it is refracted back into the core. As a result, light can travel many kilometers with little energy loss.

2. Endoscope

An endoscope is a medical device used for diagnostic and surgical procedures. It has two fiber-optic tubes in a pipe.

The light enters the patient’s organ through one of the endoscope’s fiber tubes and is then reflected by the physician’s viewing lens through the outer fiber tube thus confirming total internal reflection.

3. Prisms

A few examples of optical equipment that use right-angled prisms to reflect a light beam through 90° or 180° are as below:

Cameras, Binoculars, Periscope, and Telescope.

Apart from these instances, the total internal reflection phenomena can be applied in a variety of other contexts and with a variety of diverse applications.

Practical Applications of Total Internal Reflection

1. Optical Fibre

Total internal reflection method is used in optical fibre. The inner part of the fibre lies inside the core of the higher refractive index. All of these fibres are surrounded by another layer of glass.  They lie just beneath the lower refractive index. A plastic jacket is there to surround the fibres.

Back to back total internal reflection occurs when the light from one end of the core travels toward cladding, and the light pro
pagates through it. Optical fibres usage is quite popular among decorative table labs. They have huge applications in the medical field for endoscopy.

2. Mirage

Mirage is also known as another name called optical illusion of water. You must have seen this during the summer. The appearance of mirages is quite higher in the deserts on a hot summer day. 

Mirage occurs when a ray of light falls on earth by travelling from the top of a tree or sky; it gradually deviates away from the normal. We know that total internal reflection exists when the angle of incidence becomes greater than the critical angle. So this generates mirage in hot deserts or any open spaces during the hot summer.

An Eclectic cable is a cable used to distribute and transmit electrical power. The Electric Cable is used to transmit high voltage where overhead lines are impractical to use.

An electric cable is made of three kinds of things those are –  

• Conductor

• Dielectric

• Sheath

The conductor provides the conducting path in the Electric Cable. The insulation or dielectric withstands the service voltage and isolates the live conductor with another subject.

The sheath protects the Cable from all external influences like chemical and fire attacks. Also, it prevents moisture from harming the electric Cable.

Introduction

Electric cables are usually made of copper Because electricity moves well in copper. Copper is not as expensive as silver, making the work easier. Sometimes aluminum is used in cable making because it is cheaper than copper. A cable is made when many wires come together.

Electrical cables are shielded conducting wires, used in the generation, transmission, and distribution of electrical power. Electrical cables, in a circuit, are joined through connectors. A combination of cables and connectors is known as a cable assembly. 

The conducting wire in a cable carries electricity whereas the insulator and the sheath give protection to the cables from unintended circuit paths and chemical reactions. Cables can be classified into various categories, depending on their different uses and structures. Some types are coaxial cables, twisted pairs, optical fibers, patch cables, power cables, data cables, etc. 

Components of a Cable

A cable should have the following three components,

• Conductor: The conducting part is used to transmit electricity. Extensively used conductors are copper and aluminum. 

• Insulator: To keep the conductors separated from each other and prevent unintended paths for current flow (e.g. short circuit), the wires are shielded with insulating materials. Various synthetic polymers are used for this purpose.

• Sheath: It is yet another layer to give protection to the wires from chemical reactions with the atmosphere. A common material for the sheath is PVC (polyvinyl chloride).

Classification of Cables and Colors

Different colors of wires are used for different purposes. In DC circuits, the colors are,

• Red: Wires for positive current.

• Black: Wires for the negative current. 

• White or Grey: Ground wires.

For AC circuits (power 120/208/240 Volts),

• Black: Phase 1 lines are colored black. These wires are for power generation.

• Red: Phase 2 or transmission wires.

• Blue: Phase 3 or power distribution wires.

• White: Neutral wires.

• Green or Green with Yellow Stripes: Ground wires.

Various Types of Electrical Cables and Their Uses

Network cables: Network cables are used to connect different components in a networking system. Based on the size and structure of the systems, different types of network cables are used. 

These are listed below,

• Coaxial Cables: These cables (also called coax) consist of a conducting core, surrounded by another conducting layer. Both the layers are separated by a dielectric or insulator in the form of a coaxial cylinder. The entire setup is given an outer jacket for protection. Coaxial cables are used to transmit high-frequency signals with minimal energy loss in several devices like televisions, radio transmitters, antennas, and many more.  

• Twisted Pairs: Two cables are twisted together and color-coded. As compared to a pair of untwisted cables, the twisted one has less possibility of crosstalk or interference. These cables are used in ethernet networking systems. 

• Optical Fibers: Fibers have glass cores surrounded by some protective layers. These cables use total internal reflection to transmit light and therefore have almost zero energy dissipation. Optical fiber has a core inside a cladding. The cladding is optically rarer than the core. The transmitting electromagnetic wave reflects in the core-cladding interface several times. Optical fibers can be classified further into two types, which are multi-mode fiber (short-range) and single-mode fiber (long-range).    

• Internet Cables: Types of internet cables are ethernet cables (shielded and twisted). Ethernet cables connect a computer or a game console to a modem. These cables transfer data across the internet. Some cables, called “cross-over cables” can connect two devices. 

• Power Cables: Power cable assemblies are used to transmit electrical power. These can be used as permanent wiring in buildings. Power cable types are the power cord, extension cable, twisted, shielded, extensible, communication cable, and many more. These cables can be used overhead or buried underground.

• Types of Computer Cables: Types of computer cables are power cables and data cables. Power cords are used in the charging of a computer or a laptop. Data cables can be of DVI, VGA, or HDMI types. Data cables are used to transfer multimedia from one device to another.

Ribbon Electric Cables

Ribbon Electric Cable is made of multiple insulated wires running parallel with one another. It is used for the transmission of multiple data simultaneously.  For example, it says that the CPU with the motherboard is connected by the Cable and is generally used for networking devices’ interconnection.

Shielded Cables

Shielded Cable is made of 1 or 2 insulated wires covered by a woven braided shield or aluminum Mylar foil for better signal transmission and removing irregularities in radio frequency of power and external interference. These cables transmit high voltage electric current. A shield is protecting it.

Twisted Pair Cables has two or more insulated
copper wires twisted with each other and coded with color. Twisted Pair cables are especially used in telephone cables. The number of wires can measure the resistance to external interference.

Coaxial Cables are made of solid copper or steel conductor plated with copper.  The plated part is enclosed in the metallic braid and metallic tape. This entire Cable is covered with an insulated protective outer jacket.  In computer networking and audio-video networking, this kind of Cable is used.

Fibre Optics Cable transports optical data signals to the receiving device from an attached light source.  One must be pretty much aware of optical fiber and its uses in a wide variety of applications.

Did You Know?

• Copper is used as conducting wires in electrical cables because copper is a good conductor and also cheap. 

• Cable sheaths protect and hold the conductors from chemical reactions. 

• Most cables have multiple insulating layers and sheaths. 

• Polyvinyl chloride, butyl rubber, polyethylene, impregnated paper, etc. compounds are used as the insulating layers in different cables.

• Multi-mode optical fibers can transmit signals of multiple wavelengths and are used for short-distance transmissions. 

• Single-mode fibers are used for the transmission of a particular frequency.

• Different kinds of glass were used as the core of the optical fiber. Nowadays, transparent plastics are used in the core.

• Power cables come in various sizes depending on the purpose of use.

• Twisted pairs have reduced energy dissipation and less interference.

• Thomas Edison invented the electric Cable.

Types of Visual Impairment

Visual or vision impairment is the decreased ability to see usually associated with age and cannot be cured by prescription glasses or other medicines and surgical processes. The term blindness is used for the condition that involves the loss of complete vision. Visual impairment ranges from low to severe. The most recurrent causes of visual impairment throughout the world are uncorrected refractive errors, cataracts, and glaucoma. There are different types of visual impairment; conditions like near and farsightedness, presbyopia, and astigmatism are caused by refractive errors. Blindness is caused by cataracts. Visual impairments are often related to the increasing age and other conditions that it causes. Other disorders, such as age-related macular degeneration, corneal clouding, and diabetic retinopathy, can also give rise to visual impairment. 

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Vision Problems

As we discussed earlier, there are various factors that give rise to different types of visual impairment. Refractive errors are caused when the shape of the eyes resists one from focusing well. It is a vital problem of a visually challenged person. Cataracts are the one that causes visual impairment for people that are old-aged, though cataracts can also harm the vision for middle-aged people as well. There are many causes of cataracts, such as cortisone medication and diabetes. Here in this section, we will discuss the two most important causes of visual impairment-

1. Cataracts- 33% of the visual impairment cases are caused by cataracts and it is one of the leading causes of blindness that doubles the prevalence with every ten years after one has reached the age of forty. It is most common among adults rather than being in the case of children. Inspire of the progress in the medical domain; cataracts remain of the leading factors in case of visual impairment.

2. Glaucoma- Glaucoma causes around 2% of all the visual impairment cases around the world. It causes visual field loss as well as damages the optic nerves. It is funny that pediatric glaucoma can be acquired after a cataract-removal surgery. 

Solution to Vision Impairment and Optical Aids

The development that we have witnessed in the medical domain over the years has given us many solutions to vision impairment. Telescopes, magnifying glasses are blessings to a visually challenged person. Some other ways are also there, such as Braille that has given a visually challenged person the ability to read and write. That uses the technology of raised dots; each set of dots represent an alphabet. People with low visual impairment are often given glasses or contact lenses according to their needs, which we will discuss later in the following section. 

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Low Vision Aids: Depending on the severity of the visual impairment, optical aids can range from reading glasses to small scale telescopes. However, low vision devices are not made for all purposes. They are used with a specific purpose in mind. People that have been contracted with low vision impairment, the specific one that is called a negative visual impairment in medical terminology, are often administered with magnifying glasses that are often mounted in spectacles that give them a wide variety. A negative visual impairment is a condition where the visually challenged person has trouble looking at a far distance. Though low vision aids help a visually challenged person to view things clearly, it does not bring back the vision that has already been lost. Various kinds of low vision aids are popular these days, and the aids have been varied throughout the centuries. In the early days, people used to fasten a rope in the spectacle and hang those in the neck, or some used to hold the spectacle in front of their eyes manually. Some used to use magnifiers or small telescopes. These low vision aids can save the remaining vision from getting lost.

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Solved Examples

1. What are the Low Vision Devices for Students?

NuEyes, DaVinci Pro, Sirecues are some of the devices that can be used for visually impaired students.

2. Name a Device Used by Blind Person.

Tape recorders, speech compressors, reading machines, optacon, Braille printers are some of essential devices that are used by a blind person. 

Did You Know?

There are almost 36 million people around the world that are blind. Around 217 million have moderate or severe distance vision impairment. Low and middle-income countries take up to 90% of the visually impaired people in the world. Cataract surgery and correction of refractive errors are one of the most cost-effective surgeries in the medical domain. 51% of all blindness is due to age-related problems or cataracts.