Optics(educationpoint.co.cc)

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

NATURE OF LIGHT & REFLECTION Properties of Light: Under normal circumstances - i.e. traveling through a uniform medium, light travels in a straight line. Such traces along the path of light are called rays. A bunch of rays constitute a beam of light. 1.

However, the following phenomena can alter the path or nature of the light.

(a)

Absorption Light falling on an object may be absorbed, transmitted, or reflected.

(b)

Reflection Those surfaces which reflect the most light appear white, or silver. A highly polished, smooth and flat silver surface acts as a mirror, reflecting a perfect image of the world around it.

(c)

Refraction Light that is transmitted through a medium will usually be deviated somewhat from the straight path it was previously following. Refraction is an important characteristic of lenses that allows them to focus a beam of light onto a single point. Refraction occurs as light passes from a one medium to another when there is a difference in the optical density between the two materials.

2.

Wave/Particle Duality Light may be regarded as a flood of particles, called photons, or as a wave. In either case, it carries energy through a vacuum at a velocity which is a universal physical constant, and is the same for all observers and for all colors. Light frequently behaves as a particle.

3.

Speed of light. The speed of light in vacuum is a universal constant (3 x 10 8 m s-1). It is the same for all frequencies of light. The colour of light depends on the frequency of light. Different colours of light travel with different speeds in all other mediums except vacuum. The wavelength of visible light lies in the range of 4µm - 7µm.

The Reflection of Light: ¾

Incident Ray: A ray of light falling on a reflecting surface is called incident ray.

¾

Reflected ray: A ray of light reflected by the reflecting surface is called reflected ray.

STEPS____________________________________________________________ 1 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

¾

Point of Incidence: It is a point on the reflecting surface where a reflected ray and incident ray meet.

¾

Normal: A perpendicular drawn on reflecting surface at the point of incidence is called normal.

¾

Incident angle: The angle drawn in between incident ray and normal.

¾

Reflected angle: The angle drawn in between reflected ray and normal

Laws of reflection: ¾

Angle of incidence = angle of reflection

¾

The incident ray, reflected ray and Normal are coplanar.

Image: A point where an object appears to be, because the rays from any given point on the object have been bent so that they come back together and then spread out again from the image point, or spread apart as if they had originated from the image Real image: A point where an object appears to be, because the rays from any given point on the object have been bent so that they come back together and then spread out again from the new point. Virtual Image: A point where an point object appears to be, because the rays from any given point on the object have been bent so that they spread apart as if they had originated from the point. An extended object contains many points and the image of the extended object contains the images (real or virtual) of all the object points. Plane mirror: Consider an object placed a certain distance in front of a mirror, as shown in the diagram.

X

Here it is obvious that h0 = hi

(distance between parallel lines)

… (i)

∠ACB = ∠BCX = θ

(Laws of reflection)

… (ii)

∠ECD = ∠ BCX = θ

(Vert. opp. angles)

… (iii)

tan θ = h0/do = hi/di hence di = do

(From (i), (ii) & (iii)

2 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Image characteristics - Plane mirrors: Images formed by plane mirrors are •

virtual (for real object)

•

upright

•

left-right reversed

•

the same distance from the mirror as the object's distance, and

•

the same size as the object.

Lateral Inversion: The exchange of right and left side of an object and its image is called lateral inversion. Spherical mirrors: A spherical mirror is one whose reflecting surface is a part of a hollow sphere. The spherical mirrors are of two types : concave mirror and convex mirror. •

Concave mirror: is silvered from outer side and the reflection takes place at the inner (concave) surface.

•

Convex mirror: is silvered from inner side and the reflection takes place from outer (convex) surface.

SPHERICAL MIRRORS – DEFINITIONS Center of curvature (C): is the center of the sphere whose curvature would be identical to that of the mirror; therefore the center of curvature would be at the center of the sphere that the mirror would form if the mirror curved all the way around. Focal point (f): The point through which all light rays coming into the mirror parallel to the axis would reflect through, or appear to have reflected from. The mirror (principal) axis: The line which passes through the pole (vertex), the center of curvature, and the focal point. It is normal to the mirror at its pole (vertex). Pole (vertex): The physical center of the mirror surface. Aperture: The width of the reflecting surface is called aperture. Focal length: The distance of separation between the pole and the focus is called focal length.

STEPS____________________________________________________________ 3 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Rules for drawing ray diagrams: •

Light incident parallel to the axis will be reflected through the focal point.

C

•

•

F • F

•

• C

The reverse of this is true, also. Light passing through the focal point will be reflected parallel to the axis.

•

•

F

• F

Finally, light striking the vertex will be reflected at an equal angle to the axis.

θ θ

θ θ

RAY DIAGRAM FOR CONCAVE MIRROR (i)

Position of object at ∞ :

Nature of the image: Image is real point sized at focus. (ii)

Position of object; Beyond C

4 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Nature of image: Image is inverted, small and in between C and F. (iii) Position of object at C

Nature of image: Image is inverted, same sized at C (iv)

Position of object in between C and F

Nature of image: Large in size, inverted, beyond C (v)

Position of object; At F

Nature of image: Image is at infinity. Highly enlarged and real

STEPS____________________________________________________________ 5 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

(vi)

Position of object; In between P and F

Nature of image: Image is virtual, erect and enlarged CONVEX MIRRORS Object at infinity parallel rays converge at the principal focus:

• F

• C

All other cases: Image formed is virtual, upright and reduced in size

θ θ

New Cartesian Sign Convention: •

The object will always be placed on the left of the mirror/lens

•

Distances measured along principal axis is measured from pole

•

Distance measured along the direction of the incident ray is positive and opposite to it is negative.

•

Heights measure upwards from principal axis is positive and downward relative to • principal axis is negative.

6 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

THE MIRROR FORMULA Assumptions for mirror formula (a)

The rays are close to the principal axis.

(b)

The aperture of the mirror is small.

-u M ho

(1) –hi

α θ

θ (4) α

(3)

ho O

(2) N

-v

hi

-f

Applying Cartesian sign conventions •

image distance = – v

•

object distance = – u

•

focal length = – f

•

height of object = ho

•

height of image = –hi

from assumption (a) arc MON can be approximated as a straight line. Then from ∆1 and ∆2 tan θ =

ho … − u − (− f )

tan θ =

− hi f

from ∆2

Combing we get ho f

tan α = hi v−f

⇒

=

hi ho

from ∆1

ho f −u

=

− hi f

… from ∆3

⇒

hi ho

tan α =

=

f u−f

− hi − v − (− f )

… (i) …from ∆4

ho f v−f f

=

… (ii)

From (i) & (ii) f u−f

=

v−f f

⇒ f2 = (u – f) (v – f) ⇒ f2 = uv – fv – fu + f2 ⇒ uv = fv + fu dividing throughout by uvf 1 1 1 = + f u v

STEPS____________________________________________________________ 7 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Magnification (m): It is defined as the ratio of the size of the image (h2) to the size of the object (h1) M=

size of image (hi ) size of object (ho)

For a concave mirror f is positive. For a convex mirror, f is negative. M=

−ν u

Some important Points: (i)

Positive magnification means both the object and image are upright (i.e., erect)

(ii)

Negative magnification means, object and image have different orientations. If object is upright (i.e., erect), then image is inverted (i.e., in the downward direction with respect to the principal axis).

USES OF CONCAVE AND CONVEX MIRROR Concave mirror is used (i)

as a reflector to concentrate light

(ii)

as shaving mirror

(iii)

in ophthalmoscope to examine throat and ear of a person generally, parabolic (concave) mirrors are used.

(iv)

as a reflecting type astronomical telescope

(v)

in search light, head light of automobiles etc.

Convex mirror are used (i)

in automobile to see large traffic at the back.

(ii)

as a device to check theft in shops

(iii)

to view the corners which are not directly reachable.

8 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

REFRACTION OF LIGHT Laws of refraction of light: (i)

The incident ray, the refracted ray and the normal are co-planar.

(ii)

A ray of light traveling along the normal will not get refracted and will pass undeviated.

(iii)

The ratio of the sine of the angle of incidence to that of the sine of the angle of refraction is constant for a given pair of media. This constant is called the refractive index n. This is called the Snell’s Law. Thus Snell’s Law gives Sin i = n Sin r

Rules of refraction: •

When a light ray travels from a rarer medium to a denser medium, the light ray bends towards the normal

•

When a light ray travels from a denser medium to a rarer medium, the light ray bends away from the normal.

Note: As the speed of light is reduced in the slower medium, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by medium changes Optical Density: Consider two transparent media, A and B. If light travels more slowly in medium B than in medium A, we say that medium B is more (optically) dense than medium A. Refractive Index: The refractive index for light passing from medium 1 to medium 2 (written n21) is given by n21 =

v velocity of light in medium 1 = 1 velocity of light in medium 2 v2

If we consider light passing from a vacuum (or air) to a medium the ratio of the velocities is called the absolute refractive index of the medium. The velocity of light in a vacuum (or air) is c.

STEPS____________________________________________________________ 9 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

In the diagram above n2 =

c v2

n1 =

c v1

so the relative refractive index of light passing from medium 1 to medium 2 is given by n21 =

n2 n1

n1sinθ1 = n2sinθn2 (n21 means I of med. 2 w.r.t. 1 It should also be clear that n21 = (n12)–1 n32 = n31 × n12 =

n 31 n 21

Lens: A transparent optical device used to converge or diverge transmitted light to form an image Convex lens: A lens that is curved outward (convex): the ends are narrow and the middle is wide. Often referred to as a converging lens. A convex lens can produce either a real or virtual image. They are used to correct hypermetropia. Concave lens: A lens with inward-curving (concave) surfaces: the ends are wide, the middle is thin. Light passing through a concave lens is diverged. Concave lenses can be used in telescopes, microscopes, and eyeglasses. In glasses, they are used to compensate for myopia (nearsightedness).It always produces virtual images. Uses of lenses: •

One important use of lenses is as a prosthetic for the correction of visual impairments such as myopia, Hypermetropia, presbyopia, and astigmatism.

•

Another use is in imaging systems such as a monocular, binoculars, telescope, telescopic gun sight, theodolite, microscope, and camera (photographic lens).

•

A single convex lens mounted in a frame with a handle or stand is a magnifying glass.

•

Radio astronomy and radar systems often use dielectric lenses, commonly called a lens antenna to refract electromagnetic radiation into a collector antenna.

RAY DIAGRAMS FOR LENSES A ray passing through the optical centre of the lens: This ray is not deviated (this is true for both convex and concave lenses)

10 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

A ray parallel to the principal axis: For a convex lens (also called a converging lens), this ray is deviated to pass through the principal focus, F.

For a concave lens (also called a diverging lens), this ray is deviated so as to appear to be coming from the principal focus, F.

When drawing ray diagram for lenses, for simplicity we imagine that the refraction takes place at the centre of the lens. In reality, of course, the light changes direction at the surfaces. RAY TRACING Convex lens:

For an object outside the focal point, a real inverted image will be formed

STEPS____________________________________________________________ 11 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

For an object inside the focal point, a virtual erect image will be formed Convex lens:

The ray diagrams for concave lenses inside and outside the focal point give similar results: an erect virtual image smaller than the object. The image is always formed inside the focal length of the lens. The Lens Equation:

h

tan α = h/-u = H/v H/h = -v/u The ratio H/h is called the linear magnification produced by the lens. Now, tan β = h/f=H/(v-f)

or

H v − f = f h

12 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ v v − f = This gives u f LECTURE NOTES – PHYSICS

which, after a bit of rearrangement gives 1/v – 1/u = 1/f This is called the lens equation. Power of a Lens: The power of a lens is defined P (D) =

1 f (m)

Where f is the focal length (in metres) and the power is measured in diopters, or m-1. Power of a converging lens is positive and of a diverging lens is negative. Critical angle: The critical angle can be defined as the maximum angle of incidence possible in denser medium for the ray to just emerge out into rarer medium. When angle of incidence exceeds critical angle Total Internal Reflection occurs.

Total Internal Reflection: Total internal reflection is an optical phenomenon which occurs when light propagating in a dense medium (such as glass) moves into a less dense medium, such as water. When angle of incidence exceeds critical angle refraction stops and reflection takes place. TIR only takes place when both of the following two conditions are met: •

the light is in the more dense medium and approaching the less dense medium.

•

the angle of incidence is greater than the so-called critical angle.

TIR finds use in esign of optical fibres, periscopes, binoculars, projection lanterns, reflex camera and jewellery cutting and polishing. Stars Twinkle While Planets Don’t: Stars appear as point objects in the night sky due to their large distance from earth. Moving pockets of air (having different optical densities) in the Earth's atmosphere distort the light from these stars. These pockets of air refract light in random directions before some of the light reach our eye. Hence the apparent position of star fluctuates producing the twinkling effect.

STEPS____________________________________________________________ 13 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Planets don't appear to twinkle because they are much nearer to Earth than stars. Rather than being points of light, planets are small discs. As their light is more spread out, even if some of it is absorbed by our atmosphere, some of the light still filters through, so the planet doesn't twinkle. Other related effects of atmospheric refraction: Sun is visible to us 2 minutes before actual sunrise and 2 minutes after actual sunset. The sun appears to be flattened at sunrise and sunset. Mirage:

An optical mirage is an image of an object that appears to be in a location other than the correct one, as the result of abnormal atmospheric conditions. Heat radiating from a hot earth surface, such as a desert, causes a reduction in air density just above the surface. Hence the density of the air increases from bottom to top continuously. The boundaries between the hotter and colder layers bend light rays from a distant object continuously. After some time the rays undergo total internal reflection and when the rays finally enter the observer’s eyes as shown in figure the image produced by the rays appears inverted and below the real object. Just as an image reflected in water appears when observed from a distance. This is very common on a paved road in hot weather. Such reflections are known as inferior mirages. In the case of a mirage at sea, the denser layers of air are next to the cool surface of the water, and the reflection takes place from the rarer atmosphere above. Thus the object appears distorted, elongated, and suspended in the air, producing a so-called looming effect. Mirages which cause objects to be seen over the horizon are so called superior mirages

14 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

OPTICS OF COLORS & OPTICAL INSTRUMENTS Dispersion

of

white

light:

In

optics,

dispersion is a phenomenon that causes the separation of a wave into its components with different frequencies, due to a dependence of the wave's speed on its frequency Dispersive prisms are used to break up light into its constituent spectral colours because the refractive index depends on frequency

R O Y G B I V

the white light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Refractive index n decreases with increasing wavelength λ (or decreasing frequency). Thus, blue light, with a higher refractive index, will be bent more strongly than red light, resulting in the wellknown rainbow pattern. COLORS OF OBJECTS Primary Spectral Color: A primary color is a color that cannot be created by mixing other colors. Additive primary colours are red, green, and blue. Additive mixing of red and green light, produce shades of yellow or orange. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple and magenta. Mixing equal proportions of the additive primaries results in shades of grey; when all three colors are fully saturated, the result is white. Composite Color: Mixing two primary colors produces what is generally called a composite color. Composite colors are also obtained by subtracting a primary color from white light. Complementary colors: Two colors are called complementary if, when mixed in an additive system, they produce a shade of white Applications: TV works on the principle that mixing primary colors in various proportion gives us the entire spectrum of colors. Painters use primary colors of paint to form different color combinations. Subtractive colors: Subtractive color explains the theory of mixing paints, dyes, inks, and natural colorants to create colors which absorb some wavelengths of light and reflect

STEPS____________________________________________________________ 15 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

others. The color that an object appears to have is based on what parts of the electromagnetic spectrum are reflected by it, or conversely by what parts of the spectrum are not absorbed. Anything that is not additive color is subtractive color. No other color mixing systems are known. In color printing, the primary inks used are cyan, magenta, and yellow. Cyan is the opposite of red, meaning that cyan acts like a filter that absorbs red. The amount of cyan applied to a paper will control how much red will show. Magenta is the opposite of green, and yellow the opposite of blue. With this knowledge an infinite number of color combinations are possible. The color subtracted from white light is said to be supplementary to the color that the pigment reflects. OPTICAL INSTRUMENTS The Eye: We are able to see things because light waves bounce off from objects into our eye. When the light reaches our eyes several things happen. Light rays pass through a transparent part of the eye called the cornea. The cornea is curved and bends light inward. The light is bent even further as it passes through a liquid behind the corena. Next, light reaches the black opening in the middle of the coloured part of each eye. This part of the eye is called the pupil. The coloured part is called the iris, and it makes the

pupil

depending

change on

the

size

different

amounts of light. For example when the light is bright, the iris makes the pupil small. Once light passes through the pupil, the light enters a part of each eye known as the lens. Ciliary Muscles on the sides of the lens change its shape so it can alter the focal length of the lens and focus the rays on the retina. This is called accomodation After the light passes through the lens the light is bent still further. This bending takes place in a jelly like matter inside each eye called vitreous humour. This is also what helps your eyes to keep their shape. Finally the light forms an image on part of the inside wall of each eye. This wall is called the retina and is made up of millions of special cells. These cells send message about the image to your brain. There are two types of Light sensitive cells on the retina. Rods, are sensitive to intensity of light, and are concentrated on the sides of the retina. Cones are used for color vision, and are concentrated in the center of the retina. Vision is most acute in the area of the retina known as the fovea centralis. 16 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

These messages travel along a nerve called the optic nerve. The point where optical nerves enter the retina is called the blind spot which due to absence of rod cells cannot sense images formed on it. Notes: Least Distance of distinct vision (D): The closest distance that the 'normal' human eye can observe clearly; without any strain to the eye is called the least distance of distinct vision. It is also called near point. In connection with optical instruments the object would be considered to be held whenever feasible at the least distance of distinct vision for determining its apparent size even if the actual position of the object may not be at (D). Usually D = 25 cm. Far Point: The far point of the eye is the location of the farthest object on which the fully relaxed eye can focus. Persistence of vision: The retina captures and holds an image for one-sixteenth of a second before processing the next image. If images are flashed before the eye at least 16 frames per second, the brain thinks it is seeing a single moving image. This is also the principle of cinematography. In modern motion pictures 24 or more frames per second are used. Power of accommodation: Power of accommodation is the maximum variation in the power of eyes while focusing on near and far object. The value of power of accommodation for normal vision in young adult human being is approx. 4 D EYE PROBLEMS Presbyopia: Accommodation is the process by which the lens changes shape to allow the eye to focus at different distances. With age, accommodation becomes more difficult. This is called presbyopia. Hence the person develops problem of nearsightedness and far sightedness simultaneously. Bifocal lens are used to correct this defect. Myopia: Nearsightedness corrected by a negative lens to reduce refractive power.

Excess refractive power focuses rays before they reach the retina.

STEPS____________________________________________________________ 17 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Nearsightedness may be caused by various things. For instance, genetically, a person’s eyeball may be to long, causing the rays to focus too far in front of the retina. Nearsightedness, or myopia, is when the Light rays converge too far in front of the retina. Corrective lenses for myopia are diverging lenses. The diverging lens will cause the Light rays to bend outwards enough, so that when the rays hit the lens of your eye, they will be far enough apart to focus correctly onto the retina. Hypermetropia: Insufficient refractive power to focus rays on retina.

Farsightedness corrected by a positive lens to add refractive power.

On the other hand, farsightedness can be caused by a person’s eyeball being too short, which will cause the rays to focus in back of the retina. Converging lenses are needed to correct hypermetropia. The converging lens will cause the rays to come in enough before they hit the lens, so that they will focus onto the retina properly.

Oftentimes,

hypermetropia is latent, or hidden, because the eye can accommodate enough to make up for the hypermetropia. Astigmatism: Another error that the eye can have is astigmatism. An irregularly shaped cornea or lens will cause astigmatism. It makes objects appear to tilt. Cylindrical lenses are needed to correct astigmatism. The cylindrical lens causes certain Light rays to bend more than others. This will counteract the irregularities of your cornea or lens Cataracts: When the inner lens of the eye becomes darkened or opaque, the condition is called a cataract. The lens may be surgically replaced with a plastic lens.

18 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

OPTICAL INSTRUMENTS Resolution of an Optical Instrument: Resolution is the smallest separation for which two closely-spaced points are distinguishable.

Essential components in the design of optical instruments •

•

Resolution

Magnification

Simple Microscope:

Final image at infinity

Final image at near point

The simple magnifier achieves angular magnification by permitting the placement of the object closer to the eye than the eye could normally focus. A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type. When an object is placed nearer such a lens than its principal focus, i.e., within its focal length, an image is produced that is erect and larger than the original object. The image is also virtual; i.e., it cannot be projected on a screen as can a real image.. its magnification is limited to 10. M=

D f

This represents a lower magnification than when the image is at the near point because D is a negative quantity (it is a virtual image distance). This real image is then viewed through the eyepiece which acts as a simple magnifier to further enlarge the image. Compound microscope: A compound microscope uses a very short focal length objective lens to form a greatly enlarged image. This image is then viewed with a short focal length eyepiece (ocular) used as a simple magnifier. The image should be formed at infinity to minimize eyestrain.

STEPS____________________________________________________________ 19 Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T C Y P r og r am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

Note: Though both the eyepiece and objective have short focal length to keep the instrument compact, comparatively the eyepiece is of large aperture as well as focal length. Rays from distant object Fo •

Fe

•

Final Image

The objective forms a real and inverted image between the two lenses. The eyepiece is adjusted so that this image just falls within its focus. So this image would act as the object for eyepiece and a final virtual magnified image is obtained by the eyepiece. This image is inverted with respect to the original object. The general assumption is that the length of the tube L is large compared to either fo or fe so that the following relationships hold. The linear magnification provided by the objective lens is Mo = -L/fo The eyepiece acts as a simple magnifier, so its magnification is Me = D/fe The total magnification of a microscope is M = Mo × Me = -L.D/ fo.fe The Astronomical Telescope: The astronomical telescope makes use of two positive lenses: the objective, which forms the image of a distant object at its focal length, and the eyepiece, which acts as a simple magnifier with which to view the image formed by the objective.

Rays from distant object Fo F e • Final Image at infinity

Rays from distant object Fo Fe • • Final Image

20 STEPS____________________________________________________________ Get free chapterwise tests for Class Xth on www.tcyonline.com

STEPS … A T CY Pr ogr am _______________________________________________________________________________________ LECTURE NOTES – PHYSICS

A telescope must gather large amounts of light from a dim, distant object; therefore, it needs a large objective lens to gather as much light as possible and bring it to a bright focus. Because the objective lens is large, it brings the image of the object to a focus at some distance away, which is why telescopes are much longer than microscopes. The eyepiece of the telescope then magnifies that image as it brings it to your eye. The final image is inverted with respect to object. The astronomical telescope can be used for terrestrial viewing, but seeing the image upside down is a definite inconvenience. Viewing celestial objects upside down is no problem as they are generally spherical. Another inconvenience for terrestrial viewing is the length of the astronomical telescope, equal to the sum of the focal lengths of the objective and eyepiece lenses. Terrestrial telescopes use an additional erecting lens. Normal Adjustment: This is an adjustment where the foci of both the lenses coincide and final image is produced at infinity. In normal adjustment M= fo/fe Tube Length is the distance between objective and eye piece and has a magnitude of (fo + fe)

STEPS____________________________________________________________ 21 Get free chapterwise tests for Class Xth on www.tcyonline.com