Refraction of Light and Optical Instruments
Q.1: Define refraction of light and laws of refraction?
Answer:
Refraction: When a ray of light enters from one medium into another obliquely, it undergoes a change not only in direction but in velocity as well. This change of direction and velocity of light as it enters from one medium into another is known as refraction of light.
Laws of Refraction: Refraction of light takes place under two laws, known as the laws of refraction of light:
- The incident ray, the normal, and the refracted ray at the point of incidence all lie in the same plane.
- The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant for all rays of light passing from one medium to another. This constant is called the refractive index.
This is known as Snell’s law.
Q.2: What is refractive index or refractive constant?
Answer:
Refractive Index: The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant for all rays of light passing from one medium to another. It is denoted by n.
n=sinrsiniOR
The ratio between the speed of light in the first medium to the speed of light in the second medium is called the refractive index of the medium.
n=speed of light in denser mediumspeed of light in rare mediumQ.3: With the help of an experiment, explain the refraction of light. And what conclusion can be adopted by refraction of light?
Answer: Consider a glass slab as shown in the figure. A ray CO is incident on the slab at the point O. Entering into the glass slab, the light follows the path OE in the slab. Draw a normal at the point O represented by ON.
The ray CO is called the incident ray, and the angle CON is called the angle of incidence. Ray OE is called the refracted ray, and the angle EON′ is called the angle of refraction.
Conclusion: It is deduced from the experiment that when light passes from a rarer medium to a denser medium (e.g., air to slab), it bends towards the normal. When light passes from a denser medium to a rarer medium, it bends away from the normal.
Q.4: Explain refraction of light through water?
Answer: Water is denser compared with air. So when the light ray enters from air to water, it bends towards the normal, and when it enters from water to air, it goes away from the normal. Due to this, anything lying at the bottom of water looks slightly above its actual position, and in this way, the depth of water apparently looks shallow.
Q.5: Write down the refractive indices of some common materials?
| 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 |
Q.6: Define the following terms:
Incident Ray: The light ray coming from the source and striking the boundary of another medium is called the incident ray.
Refracted Ray: The ray refracted from the point of incidence into the second medium is called the refracted ray.
Point of Incidence: The point on which the incident ray strikes the surface is called the point of incidence.
Emergent Ray: The ray that, after passing through the second medium, comes again into the first medium is called the emergent ray.
Angle of Emergent: The angle formed by the emergent ray and the normal is called the emergent angle.
Q.7: What is a prism? With the help of a diagram, trace the path of light through a prism?
Answer: A prism is a transparent refracting body bounded by three rectangular and two triangular surfaces.
Refraction Of Light Through A Prism:
The figure shows the path of a ray of light through a triangular prism.
The ray EF strikes the face AB of the prism. On entering the prism, this ray bends towards the normal OL at the point of incidence F, i.e., it bends towards the base BC of the prism. The refracted ray FG, on emerging out of the prism, further bends away from the normal PQ at the point of incidence, i.e., the emergent ray GH bends further towards the base BC of the prism. The angle EFO is the angle of incidence (∠i). Angle LFG is the angle of refraction (∠r).
Q.8: How can the refractive index of the material of a prism be found?
Answer: The refractive index of the material of the prism can be determined by the following formula:
nm=sin(2A)sin(2A+Dm)
Where:
- nm = refractive index of the prism
- A = the angle of prism
- Dm = the angle of minimum deviation
Q.9: Define the following terms:
- Angle of Deviation: The total deviation in the path of the incident ray measured in angle is called the angle of deviation.
- Angle of Minimum Deviation: The minimum value of the angle of deviation is called the angle of minimum deviation. It is denoted by Dm.
Q.10: What do you understand by critical angle?
Ans:
Critical Angle:
The angle of incidence for which the value of the angle of refraction is 90° is called the critical angle.
Q.11: Define total internal reflection. On which factors does total internal reflection depend?
Ans:
Total Internal Reflection:
When a light ray passes from a denser medium into a rarer medium and the value of the angle of incidence becomes greater than the critical angle, there is no refracted beam, but the whole ray is internally reflected back into the optically denser medium. Such reflection is called total internal reflection.
Factors or Conditions for Total Internal Reflection:
There are two factors or conditions on which total internal reflection depends:
- Light must pass from a denser medium to a rarer medium.
- The angle of incidence must be greater than the critical angle.
Q.12: Give some examples of total internal reflection?
Ans:
Some important examples of total internal reflection are as under:
- Totally reflecting prism
- Optical fibers
- Periscope
- Mirage
Q.13: What is a totally reflecting prism? Write down its working and uses?
Ans:
Totally Reflecting Prism:
It has one of its angles equal to 90° and each of the remaining two angles is equal to 45°.
Working:
If a ray of light strikes one of its faces perpendicularly, it enters the prism without any change of direction and meets the hypotenuse at an angle of 45°. As the critical angle of glass is 42°, the ray striking the hypotenuse suffers total internal reflection. The reflected ray thus strikes the other face perpendicularly and comes out of the prism without any further change of direction.
Uses:
Due to this property, a totally reflecting prism is used in periscopes as reflectors. They are also used in binoculars and projectors to change the inverted image.
Q.14: What is a periscope? Write down its construction and working?
Ans:
Periscope:
It is an instrument used in submarines with the help of which we can see over the surface of the sea.
Construction:
It consists of two totally reflecting prisms fixed at two ends of a tube and three convex lenses.
Working:
When the rays of light from the object enter into the prism without any change of direction and meet the hypotenuse at an angle of 45°, which is more than the critical angle (42°), they suffer total internal reflection and get bent at an angle of 90°. They fall over the second prism perpendicularly. They suffer total internal reflection again and emerge out perpendicularly. In this way, the rays of light coming from above the surface of the sea are able to reach inside the submarine, and an observer can see above the sea.
Q.15: What are optical fibers? And what are its uses?
Ans:
Optical Fiber:
A very thin plastic or glass fiber of about 1001 mm across inside which light can be trapped by total internal reflection is called an optical fiber.
Explanation:
In optical fibers, light can be traveled along a curved path. The curvature of the light guide is very small. This allows total internal reflection to take place everywhere inside the fiber. Thus, light can be made to bend.
In an optical fiber, the light can travel with little loss because the light is totally reflected whenever it strikes the core cladding interface.
Uses:
- Optical fibers are used to carry telephone signals and other modern communication systems using laser beams.
- They are used by doctors and engineers to light up some inaccessible spots for inspection or photography.
Q.16: What is a lens? How many kinds of lenses are there?
Ans:
Lens:
A transparent refracting medium that is bounded by one or two spherical surfaces.
Kinds of Lenses:
There are two kinds of lenses:
- Convex lens or converging lens
- Concave lens or diverging lens
Q.17: Define a convex lens. What are its kinds?
Ans:
Convex Lens:
It is a lens that is thick in the middle and thin at the edges. It is also called a converging lens because it converges all the parallel rays after refraction at a fixed point.
Types of Convex Lenses:
There are three types of convex lenses:
- Double convex lens
- Plano-convex lens
- Concavo-convex lens
Double Convex Lens:
Both the surfaces of a double convex lens are convex.
Piano-Convex Lens:
One of the two surfaces of a piano-convex lens is plane, and the other is convex.
Concavo-Convex Lens:
One of the two surfaces of a concavo-convex lens is concave, and the other is convex.
Q.17: Define a concave lens. What are its types?
Ans:
Concave Lens:
It is a lens that is thin in the middle and thick at the edges. It is also called a diverging lens because it diverges all the parallel rays after refraction in different directions.
Types of Concave Lenses:
There are three types of concave lenses:
- Double concave lens
- Piano-concave lens
- Convex-concave lens
Double Concave Lens:
Both the surfaces of a double concave lens are concave.
Piano-Concave Lens:
One of the two surfaces of such a lens is plane, and the other is concave.
Convex-Concave Lens:
One of the two surfaces of such a lens is convex, and the other is concave.
Q.18: Define the following terms:
- Center of curvature
- Optical center
- Aperture
Ans:
Center of Curvature:
The center of the sphere from which the lens is taken out is called the center of curvature. It is denoted by 2F.
Optical Centre:
The geometric center of a lens is called the optical center.
Aperture:
The circular diameter of the lens is called aperture.
Q.19: Show the graphical construction of images by lens using ray diagram?
Ans:
A Ray Parallel to the Principal Axis:
A ray parallel to the principal axis after refraction passes, or appears to pass, through the principal focus F.A Ray Passing Through the Principal Focus:
A ray that passes or appears to pass through the principal focus F is refracted parallel to the principal axis.A Ray That Passes Through the Optical Centre:
A ray that passes through the optical centre O, goes straight without bending.
Q.20: Describe the position, nature, and size of the image formed by convex lens with the help of ray diagram, in each of the following cases:
- When the object is at infinity
- When object lies beyond the centre of curvature 2F
- When object lies at 2F
- When object lies in between 2F and F
- When object lies at F
- When object lies between F and optical centre O
Ans:
| 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. |
Position of the Object:
The Object Lies Beyond Centre of Curvature
- Position of the Image: The image will form in between F and 2F.
- Nature of the Image: The image will be real and inverted.
- Size of the Image: Diminished.
Position of the Object:
The Object Lies at 2F
- Position of the Image: The image is also formed at 2F.
- Nature of the Image: The image will be real and inverted.
- Size of the Image: Image is larger than the object.
Position of the Object:
The Object Lies in Between 2F and F
- Position of the Image: The image is formed beyond 2F.
- Nature of the Image: The image will be real and inverted.
- Size of the Image: Image will be magnified.
Position of the Object:
The Object Lies at F
- Position of the Image: The image is formed at infinity.
- Nature of the Image: The image will be real and inverted.
- Size of the Image: Highly magnified image.
Position of the Object:
The Object Lies Between F and Optical Centre O
- Position of the Image: The image is formed behind the object.
- Nature of the Image: The image will be virtual and erect.
- Size of the Image: Magnified.
Q.21: Draw the ray diagram for the image formed by a concave lens. Write down the characteristics of the image formed by a concave lens.
Ans:
Characteristics of the Image Formed by a Concave Lens:
- For all positions of the object, the image is always virtual and erect.
- The image is always smaller than the object.
- It is always located between the principal focus and the optical centre.
Q.22: Write down the characteristics of a convex lens.
Ans:
Characteristics of Convex Lens:
- It forms real and inverted image.
- It converges all the parallel rays.
- The focus of the lens is positive.
- It is thick from the middle and thin from the edges.
Q.23: Write down the characteristics of a concave lens.
Ans:
Characteristics of Concave Lens:
- It forms virtual and erect image.
- It diverges all the parallel rays of light.
- The focus of the lens is negative.
- It is thin in the middle and thick from the edges.
Q.24: What is thin lens formula?
Ans:
Thin Lens Formula:
The nature and position of the image formed by a lens can be calculated from the following formula:
f1=p1+q1
Here,
P = The distance of the object from the lens.
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:
Now, by comparing Equation (i) and Equation (ii), we get:
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.
Power (in dioptre)=Focal Length (in meter)1
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+fd
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 I1 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. ∠i=∠r | The angle of incidence is not equal to the angle of refraction. ∠i=∠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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