TALEEM: Refraction of Light and Optical Instruments

Substance	Index Of Refraction	Substance	Index Of Refraction
Vacuum	1.0000	Glycerine	1.47
Air	1.0003	Crown glass	1.52
Water	1.331	Flint glass	1.62
Ethyl Alcohol	1.36	Ruby	1.54
Quartz	1.461	Diamond	2.42
Sodium Chloride	1.53	Sulphuric acid	1.43
Benzene	1.501

Position of the Object	The Object Lies at Infinity
Position of the image:	The image will form at focus.
Nature of the image:	The image will be real and inverted.
Size of the Image:	Highly diminished.

Q.25: Define the magnification of lenses and write down its formula?

Ans:
Magnification: The linear or lateral magnification for lenses is determined by the following formula:

M = \frac{\text{Height of image} (h_1)}{\text{Height of object} (h_0)}

M = \frac{h_1}{h_0} \tag{Equation i}

M = \frac{\text{Distance between image and lens (q)}}{\text{Distance between object and lens (p)}}

M = \frac{q}{p} \tag{Equation ii}

Now, by comparing Equation (i) and Equation (ii), we get:

\frac{h_1}{h_0} = \frac{q}{p}

Q.26: What are optical instruments? Give some examples.

Ans: Optical Instruments: Those instruments in which lenses and mirrors both or any one of them is used as a main and essential part are known as optical instruments.

Examples:

Cameras
Telescopes
Microscopes
Eye glasses
Periscopes
Projectors

Q.27: Define camera, its construction, and working.

Ans: Camera: It is an optical device for obtaining still photographs or for exposing cinematic film.

Construction: It consists of a light-proof box blackened inside to absorb stray light. The box contains a lens at one end and a plate or film at the other end. The ordinary camera has a convex lens but in costly cameras, a combination of lenses is used.

Working: The amount of light entering the camera is controlled with the help of a diaphragm and a shutter. A shutter of variable speed and a diaphragm with a variable aperture controls the length of time and quality of light entering through the lens. Normally the shutter is closed. It opens only for a fraction of a second when the button is pressed to take the photograph. To get a clear and sharp image, the lens is moved in and out with the help of a mechanical mount carrying the lens.

Q.28: Draw a neat and labeled diagram of the human eye and show the important parts of an eye.

Ans: The image in the document shows the Internal Structure of the Human Eye with parts labeled:

Sclera
Choroid
Retina
Optic Nerve
Cornea
Iris
Pupil
Lens
Vitreous Humor
Aqueous Humor
Optic Disc

There is also an Internal Structure of Camera showing:

Object
Convex Lens
Image formation

Q.29: Explain the functions of different parts of a human eye. OR Describe the structure of a human eye.

Ans: A Human Eye: The eye is shaped like a ball, with a slight bulge at the front. Each part of the human eye has a special function.

The Eyeball: It is almost spherical in shape. The eyeball is nearly spherical in structure with a diameter of about 2.5 cm. The whole structure of the eye is pulled into various positions by muscles attached around the eyeball. The eyeball consists of three layers:

Sclera
Choroid
Retina

Sclera: The outermost layer of the eyeball is called the sclera. This is a thin but very tough protective shell that is commonly called the white of the eye. The sclera is covered by a transparent membrane which protects the eye from becoming dry.

Cornea: The front part of the sclera is known as the cornea. It is a transparent eye structure that is a powerful refracting surface. Similar to lenses used in cameras, the cornea is shaped like a dome of glass. This property of eye structure is critical to its focusing function. A dome shape also gives the cornea properties of reflection.

Ciliary Body: Ciliary muscles in the ciliary body control the focusing of the lens automatically.

Choroid: Choroid forms the vascular layer of the eye, supplying nutrition to the eye structures.

Iris: It forms the rear boundary of the anterior chamber of the eye. The iris contains muscles that control the size of the pupil, which is the dark-colored area in the center of the iris. The muscles of the iris regulate how much light enters the interior of the eye. Depending on the amount of light, the pupil responds by getting bigger (dilating) or smaller (constricting). In this respect, the iris functions much like the aperture of a camera.

The diagram shown in the image depicts the Internal Structure of the Human Eye with labels such as:

Retina
Iris
Cornea
Pupil
Aqueous Humor
Vitreous Humor
Optic Nerve
Macula

Lens: The lens of the eye is located directly behind the iris. The lens is a flexible eye structure. This unique characteristic enables us to focus on things both up close and far away. The lens changes its shape through a process called accommodation. This is a mechanism by which the lens changes its optical power in order to maintain focus on an object as its distance from us changes.

To see things up close, muscles squeeze the lens, which causes its curvature to increase. To see things far away, accommodation is not necessary. Instead, the muscles relax, which has the effect of flattening, or reducing the curvature of, the lens.

Anterior Chamber: The area of the eye structure bounded by the cornea, iris, pupil, and lens is called the anterior chamber of the eye. The anterior chamber is filled with a watery substance called the aqueous humor. This special fluid is filtered from blood, circulates through the anterior chamber, and is returned to the bloodstream.

Posterior And Vitreous Chambers: The posterior chamber of the eye is a space in the eye behind the iris and in front of the lens. It is filled with aqueous humor. Produced by a structure alongside the lens called the ciliary body, the aqueous humor passes first into the posterior chamber and then flows forward through the pupil into the anterior chamber of the eye. Light enters the eye through the cornea, passes through the lens, and is focused onto the retina. But before light strikes the retina, it must first pass through the entire inside of the eye known as the vitreous chamber. It constitutes almost 70% of total volume in the eyeball and is filled with a transparent gelatinous material called the vitreous humor.

Retina: It is the innermost layer of the eye and serves a function much like film in a camera. The retina is a soft, transparent layer of nervous tissue made up of millions of light receptors. The retina is connected to the brain via the optic nerve.

Q.30: What is the least distance of distinct vision?

Ans: The shortest distance from which an object can be seen clearly is called the least distance of distinct vision. For a child, it is at 5cm and for a normal eye, it is at 25cm.

Q.31: What is meant by the power of accommodation?

Ans: This is the ability of the eye to change the focal length of its lens to form a clear image of an object on the retina, called its power of accommodation.

Q.32: Define power of the lens? Write down its formula and unit.

Ans: Power of the Lens: It is equal to the reciprocal of the focal length of a lens. Its unit is dioptre.

$\text{{Power (in dioptre)}} = \frac{1}{{\text{{Focal Length (in meter)}}}}$

Q.33: Explain the defects of vision, their causes, and how they can be corrected?

Ans: Defects of Vision: There are four main defects in the human eyes:

Short sightedness (Myopia)
Long sightedness (Hypermetropia)
Astigmatism
Lack of accommodation (Presbyopia)

Short Sightedness (Myopia): It is a defect in which a person can see near objects clearly but distant objects are not seen clearly.

Causes: The reason for this defect is either the focal length is too short or the eyeball is too elongated.

Effects: The light rays from a distant object are focused in front of the retina.

Correction: To correct this defect, a concave lens of suitable focal length is used in front of the cornea to diverge the rays of light so that the eye lens can focus the image clearly on the retina.

Long Sightedness (Hypermetropia): It is a defect in which a person can see distant objects clearly but cannot see near objects clearly.

Causes: The reason for this defect is either the focal length is too long or the eyeball is too short.

Effects: The light rays from near objects are focused behind the retina.

Correction: To correct this defect, a convex lens of suitable focal length is used in front of the cornea to converge the rays of light so that the eye lens can focus the image clearly on the retina.

Astigmatism: It is a defect in which a person can't see objects clearly.

Causes: The reason for this defect is either the cornea or the surface of the eye is not perfectly spherical.

Effects: The eye has different focal points in different planes, and an object is not focused clearly on the retina.

Correction: It is corrected by asymmetrical lenses which have different radii of curvature in different planes.

Lack of Accommodation (Presbyopia): It is a defect of long-sightedness (or in some cases short-sightedness) in old age.

Causes: At old age, the eye lens loses its elasticity and ability to change its shape, and the ciliary muscles weaken, resulting in a lack of accommodation.

Effects: An old person can't see objects clearly.

Correction: This defect can be corrected by convex lenses. However, for looking at distant objects, one will have to use concave lenses.

This is the reason that old people use spectacles with bi-focal lenses, i.e., convex part in the lower side to see near objects and concave part in the upper side to see distant objects.

Image of Bifocal spectacles.

Q.34: What is a magnifying glass or simple microscope? Explain its construction, working, and magnification?

Ans: Magnifying Glass or Simple Microscope: It is simply a single biconvex (converging) lens.

Construction and Working: It consists of a convex lens of short focal length. The object whose magnified image is to be seen is placed within the focal length of the lens. This produces an enlarged, virtual, and erect image towards the object itself.

Magnification: Its magnification is determined by:

$M = 1 + \frac{d}{f}$

Where $f$ is the focal length of the lens and $d$ is the near point distance of the object, which is about 25 cm for the normal eye.

Q.35: Explain the construction and working of a compound microscope with the help of a ray diagram?

Ans: Compound Microscope: A compound microscope is an optical instrument used to see very small objects such as germs and other microbes, etc.

Construction: It consists of two convex lenses fitted at the ends of two tubes, which can slide in and out of each other.

Objective: The convex lens near the object is called the objective. It has a short focal length and a small aperture.

Eye Piece: The second convex lens near the eye is called the eye piece. It has a long focal length and a large aperture.

Working: The object $AB$ is placed between $F$ and $2F$ of the objective. It gives a real, inverted, and magnified image $I_1$ of the object $AB$ .

Diagram: Formation of a virtual image with a simple microscope.

Q.36: Explain the construction and working of an astronomical refracting telescope with the help of ray diagram?

Ans: Astronomical Refracting Telescope: An astronomical telescope is an optical instrument used to see heavenly bodies such as the moon, stars, and planets.

Construction: It consists of two convex lenses fixed at the ends of two tubes. One of these tubes can slide into or out of the other tube so that the distance between the lenses can be changed.

Objective: The convex lens near the object is called the objective. It has a long focal length and a large aperture.

Eye-Piece: The second convex lens near the eye is called the eye-piece. It has a short focal length and a small aperture.

Working: When parallel rays of light from a distant object fall on the objective, a real, inverted, and diminished image $A'B'$ is obtained at the principal focus of the objective. This image $A'B'$ acts as the object for the eye-piece. The position of the eye-piece is such that a magnified and virtual image $A''B''$ is obtained.

Now, the position of the eye-piece is adjusted so that the final image $A''B''$ is formed at the least distance of distinct vision. This allows us to see the image clearly. The final image is virtual, inverted, and magnified as compared to what is seen with the naked eye.

Q.37: What is the difference between convex and concave lenses?

Convex	Concave
It is a converging lens.	It is a diverging lens.
It forms a real image.	It forms a virtual image.
Its focal length is positive.	Its focal length is negative.
It is used to correct long-sightedness.	It is used to correct short-sightedness.

Q.38: Write the differences between a compound microscope and an astronomical telescope?

Compound Microscope	Astronomical Telescope
It is used to see very small objects.	It is used to view far away objects.
The object is placed at the focus.	Objects are at infinity.
The lens of the eye-piece is small in size with a short focal length.	The lens of the eye-piece is large in size with a large focal length.
Its objective forms a larger image near the eye-piece.	Its objective forms a small image near the eye-piece.

Q.39: What is the difference between a camera and a human eye?

Camera	Human Eye
A real image is formed on a photographic plate.	A real image is formed on the retina.
The amount of light is controlled by the shutter.	The amount of light is controlled by the pupil.
The lens of the eye-piece is small in size with a short focal length.	The lens of the eye is large in size with a large focal length.
It has a lens with fixed focal length.	It has a lens with adjustable focal length.

Q.40: What is the difference between reflection and refraction?

Reflection	Refraction
Turning back of light after striking a surface is called reflection of light.	The bending of light and change in velocity of light as it enters from one medium to another is called refraction of light.
The angle of incidence is equal to the angle of reflection. $\angle i = \angle r$	The angle of incidence is not equal to the angle of refraction. $\angle i \neq \angle r$
It takes place mostly because of mirrors or smooth shining surfaces.	It takes place in lenses, plane glass, and prisms, etc.

Q.41: Write down the differences between short-sightedness and long-sightedness?

Short-Sightedness (Myopia)	Long-Sightedness (Hypermetropia)
It is a defect of eyes in which near objects can be seen clearly but distant objects are not seen clearly.	It is a defect of eyes in which distant objects can be seen clearly but near objects are not seen clearly.
The reason for this defect is either the focal length of the eye lens is too short or the eye ball is too elongated.	The reason for this defect is either the focal length of the eye lens is too long or the eye ball is too short.
In this defect, image of a distant object is formed in front of the retina.	In this defect, image of a near object is formed behind the retina.
It can be corrected by using concave lens spectacles or contact lenses.	It can be corrected by using convex lens spectacles or contact lenses.

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Refraction of Light and Optical Instruments (Chapter # 14) Physics 10th Question Answers