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LIGHT SHADOW

INTRODUCTION

Light is a invisible form of energy that causes the sensation of vision.

SOURCES OF LIGHT

Those bodies which emit light in all directions are called sources of light. The sources can be point one or extended one. The sources of light are of two types :

(I) Lumininous sources :

Those bodies which emit light in all directions are called sources of light. The sources can be point one or extended one. The sources of light are of two types :

Eg : Sun and stars (natural luminous sources), electric lamps, candles and lanterns (artificial luminous sources).

(ii) Non - luminous sources : Those objects which do not emit light but become visible only when light from luminous objects falls on them. They are called non - luminous sources.

Eg. : Moon, planets (natural non - luminous sources), wood, table (artificial non - luminous sources) etc.

· Bioluminescence: The production or emission of light by a living organism as a result of some chemical energy getting converted into light energy is called bioluminescence. The name originates from the Greek word bios for 'living and the Latin word lumen for 'light'. Fireflies are insects that give off a pale, greenish yellow light that flashes or glows in the dark . Some of the fish, such as angler fish, living deep under the sea are also bioluminescent.

MEDIUM OF LIGHT

Substances through which light propagates or tends to propagate is called a medium of light.

According to the medium of light objects are divided into three parts :

(i) Transparent object :

Bodies that allow light to pass through them i.e. transmit light through them, are called transport bodies.

Eg.: Glass, water, air etc.

(ii) Translucent object : Bodies that can transmit only a part of light through them are called translucent object.

Eg.: Frosted or ground, greased paper, paraffin wax etc.

(iii) Opaque object : Bodies that do not allow light to pass through them at all called opaque objects.

Eg.: Chair, desk etc.

RECTILINEAR PROPAGATION OF LIGHT

Light travels in a straight line. In vacuum or air, light travels with velocity of 3 × 108 m/s.

Activity : If we see at a lighted candle through a straight pipe as shown in figure, we are able to see the candle but if we see the candle through a bent pipe we are not able to see the candle flame.

This activity showed that light travels along straight lines.

(a) we can see the candle through the straight pipe.

(b) we cannot see the candle through bent pipe.

CHARACTERISTICS OF LIGHT

Some common characteristics of light are given below :

(i) Light is invisible form of energy.

(ii) Light does not require material medium for its propagation i.e., light can travel through vacuum.

(iii) The speed of light in free space (vacuum) is 3 × 108 m/s. Its speed is marginally less in air. Its speed decreases.

(iv) Light undergoes reflection from polished surfaces such as mirrors etc.

(v) When light goes from one medium to another, it changes its path. This phenomena is called refraction.

SOME DEFINATION RELATED TO THE LIGHT

(A) Ray of Light :

The path along which light energy travels in a given direction is called ray of light. A ray of light is represented as a straight line. The arrow head on it gives the direction of light.

(B) Beam of Light :

A collection of rays of light is called beam of light. However, if the number of rays is too small then such a collection of rays is - called Pencil of light.

(C) Parallel Beam :

When the rays of light travel parallel to each other, then the collection of such rays is called parallel beam of light. For example, sun rays entering into a room through a ventilator constitute a parallel beam.

The opaque objects do not allow light to pass through them, therefore they give rise to the formation of shadow of the opaque object on the opposite side of the source of light.

(a) The cause of formation of shadows is

Rectilinear propagation of light (light travels on straight lines)

(b) Essential of a shadow:

(i) Source of light

(ii) Presence of opaque object in the path of light

(iii) Screen on which shadow is formed

(c) Location of a shadow :

Shadow fills the space between the opaque object and the screen. It is the volume and not area on the screen.

(d) Formation of shadow : Sunlight is parallel, so its shadows remain the same size as the object’s profile. A desk lamp or a street light also casts shadows because the light is streaming out in all directions away from the source. However, since Its rays are not parallel, shadows get bigger farther away from the object.

Shadow sticks :

A shadow stick is a vertical pole placed in the ground. Sunlight casts its shadow on to a level surface below.(e.g. a sheet of card or just level ground)

As the Sun moves from A to C, the shadow shortens and then lengthen accordingly

The length and position of the shadow then depends on both the time of year and the time of day. Local noon can be found from the time when the shadow is shortest. At this time the Sun is highest in the sky and crossing the meridian.

However, shadow sticks are not good clocks -the azimuth of the Sun's shadow at a given time changes throughout the year with the Sun's declination.

(i) The shape, size and other characteristics of a shadow depend upon :

(ii) Position and distance of the source of light with respect to the object.

(ii) The distance between the object and the surface on which the shadow falls.

A Shadow has two Regions :

For a non - point source of light (i.e., and extended source of light), the shadow of an object has two parts, namely, the umbra and the penumbra. The umbra is the darkness inner part of the shadow where no light falls at all. The penumbra is the part which surrounds the umbra. Some rays of light fall on this region and partially illuminate it. Thus, the penumbra is called the region of partial darkness.

(i) Umbra : The inner region of total darkness is called umbra. It is the evenly dark part of a shadow. Umbra means 'shade' in latin.

(ii) Penumbra : The outer region of partial darkness is called penumbra. It is the fuzzy part between the dark and the light . Pene means 'almost' in Latin. Umbra and penumbra are clearly formed only when the source of light Is big and opaque body is small If one is in the umbra of an object, the light source is completely obscured. If one is in the penumbra, the source is only partially obscured, to a greater or lesser degree as one moves through the penumbra.

The wider the light source, the fuzzier, is the shadow.

With a wider light source,the shadow becomes fuzzier Also, closer the object to the light, the larger and fuzzier is the shadow. In other words, the closer the object to the screen, the smaller and well defined is the shadow. Thus, the size of the umbra becomes smaller and smaller if the screen is moved farther and farther from the light source and the object.

Shadow are often fuzzy, particularly when the surface on which the shadow lies is far from the object casting the shadow. This fuzziness is because of only point light source in space. All sources have some geometrical size.Thus, light from one edge of the source is not quite parallel to light from the other edge.

The result is a larger and fainter penumbra that is hardly visible. This is the reason why we cannot see the shadow of a bird flying high up in the air.

REFLECTION OF LIGHT

When a beam of light falls on any surface, a part of it is sent back into the same medium from which it is coming. This phenomenon is known as the reflection of light.

(A) GENERAL DEFINITIONS ABOUT REFLECTION :

(i) Mirror : A smooth polished surface from which regular reflection can take place is called mirror. MM' Is the mirror as shown in figure.

(ii) Incident ray : A ray of light which travels towards the mirror is called incident ray. Ray AB is Incident ray in figure.

(iii) Point of incidence: The point on the mirror, where an incident ray strikes is called point of incidence. 'B' is the point of incidence in figure.

(iv) Reflected ray : A ray of light which bounces off the surface of a mirror, Is called reflected ray. BC is reflected ray in figure.

(v) Normal: The perpendicular drawn at the point of incidence, to the surface of mirror is called normal. BN is the normal in figure.

(vi) Angle of incidence: The angle made by the incident ray with the normal is called angle of incidence. LABN is the angle of incidence in figure. It is denoted by Li.

(vii) Angle of reflection: The angle made by the reflected ray with the normal Is called angle of reflection. LCBN is the angle of reflection In figure. It is denoted by L r.

(viii) Glance angle of incidence: The angle which the incident ray makes with the mirror is called glance angle of incidence. L MBA is the glance angle of incidence in figure.

(ix) Glance angle of reflection: The angle which the reflected ray makes with the mirror is called glance angle of reflection. LM'BC is the glance angle of reflection in figure

(B) Laws of Reflection :

The reflection of light from a surface obeys certain laws called laws of reflection. They are:

(i) Angle of incidence is equal to the angle of reflection, i.e. Ði = Ðr.

(ii) Incident ray, reflected ray and normal to the reflecting surface always lie in the same plane.

PINHOLE CAMERA

The word camera comes from the Greek kamara, a vaulted chamber.

A pinhole camera is the simplest camera possible. It consists of a light-proof box, some sort of film and a pinhole. The pinhole is simply an extremely small hole like you would make with the tip of a pin in a piece of thick aluminum foil.

A pinhole camera works on a simple principle.

ACTIVITY :

Imagine you are inside a large, dark, room - sized box containing a pinhole, Imagine that outside the room is a friend with a flashlight, and he is shining the flashlight at different angles through the pinhole. When you look at the wall opposite the pinhole, what you will see is a small dot created by the flashlight's beam shining through the pinhole. The small dot will move as your friend moves his flashlight. The smaller the pinhole. The small dot will move as your friend moves his flashlight. The smaller the pinhole (within limits), the smaller and sharper the point of light that the flashlight creates.

Rays scatter off objects selects only the rays travelling in all directions. The pinhole in the front of the camera selects only the rays travelling in the direction of the pinhole. These rays pass into the camera and strike the back surface. The front and side wall shade that point on the back from rays coming from different points of origin.

A pinhole camera casts an inverted image on the back.

This image will also be slightly blurry, because light coming from the object is spread out over the back of the camera according to the size of the pinhole. Each disk of light cast onto the back overlaps and blurs with disks from nearby atoms.

One can reduce the blur by making the pinhole smaller. However, then the image becomes darker and may not be visible or recordable on film.

OBJECTIVE QUESTIONS

1. Light causes the sensation of :

(A) Vision (B) Sight

2. Light is :

(A) an electromagnetic radiation (B) form of energy

3. Which of the following is a natural luminous source of light ?

(A) Sun (B) Wood

4. Light shows :

(A) Random propagation

(B) Curvilinear propagation

(D) None of these

5. Which of the following is reflector of light ?

(A) Sun (B) Star

6. Wood is an example of :

(A) Translucent (B) Transparent

7. Sending of light back in same medium by

object is -

(A) Reflection (B) Refraction

8. When light enters in a medium and changes its path, then this phenomenon is called

(A) Reflection (B) Refraction

9. The colour of a shadow is

(A) always black and white

(B) only black

(D) same as that of object

10. Objects that do not allow any light to pass through them are called

(A) transparent object

(B) translucent object

(D) None of these

11. Object that allow the entire light to pass through then are called

(A) transparent object

(B) translucent object

(D) None of these

12. Objects that allow the partial passage of light through them are called

(A) transparent object

(B) translucent object

(D) None of these

13. The image formed by pinhole camera is

(A) real and inverted

(B) virtual and erect

(D) All of these

14. Which of the following is a luminous object?

(A) Sun (B) Electric bulb

15. Which of the following is a non-luminous object ?

(A) Chair (B) Tube light

16. Speed of light is -

(A) 3 × 109 ms-1 (B) 3 × 108 ms-1

17. Light travels in

(A) straight line (B) Curved line

SUBJECTIVE QUESTIONS

1. With the help of an activity show that light travels along straight lines.

2. State the characteristics of image formed by a pinhole camera.

3. Why Shadows of very far objects from the earth's surface are not visible?

4. What is pinhole camera? Discuss the shadow formation of object by point source of light.

ANSWER KEY

1. C 2. D 3. A 4. C 5. D 6. D 7. A 8. B 9. B 10.C

11.A 12.B 13.A 14.D 15.A 16.B 17.A

1. Which of the following materal allows light to pass through it?
(A) Copper (B) Wood
(C) Rubber (D) Glass

2. The sun in the early morning can cause a building to form a shadow. This shadow will be
(A) Fat (B) Long
(C) thin (D) Short

3. We can see objects in a bright room be- cause
(A) The objects give off light to the air
(B) the Objects reflect the light falling on them.
(C) The objects send light away from our eyes.
(D) Our eyes give off light to the objects

4. WHich of the following characteristics is not exhibited by the shadow of an object?
(A) Right side up
(B) Same colour as objects
(C) Can be formed on a screen
(D) Can be bigger than the objects

5. The object that does not give out light on its own is the
(A) Star
(B) Lighted matchstick
(C) Light Bulb
(D) Shining mirror

6. Which of the following is not a characteristic of a virtual image formed in a plane mirror?
(A) Cannot be formed on a screen
(B) its is inverted
(C) It is same size as the object
(D) It is laterally inverted

7. Sunlight can pass through
(A) Walls (B) Bushy trees
(C) Rocks (D) Water

8. WHich of the following are translucent mate rials?
(i) Air (ii) Ground glass
(iii) Clouds (iv) Aluminium sheet
(A) (i) and (ii) (B) (ii) and (iii)
(C) (i) and (iii) (D) (ii) and (iv)

9. Anushka put some tea into four cups made of paper, glass, metal and porcelain, respectively. WHich one of these cups will allow her to see the level of the tea clealy?
(A) Paper cup (B) Metal cup
(C) Glass Cup (D) Porcelain Cup

10. Which of these things cannot give off its own light?
(A) A torch (B) A lamp
(C) The Moon (D) The sun

11. As the sun rise in the sky, the shadow of a building
(A) Lengthens (B) Shortens
(C) Widens (D) Darkens

12. The Plane mirror forms a
(A) Virtua image (B) Real Image
(C) Inverted Image (D) Magnified image

13. Which of tghe followig will produce diffuse reflection of light?
(i) Plane Mirror (ii) Piece of paper
(iii) Still water in lake (iv) Leather bag
(A) (i) and (ii) (B) (ii) and (iii)
(C) (i) and (iii) (D) (ii) and (iv)

14. A device containing two plane mirrors whcih gives us a higher view than normal is
(A) Stethoscope (B) Microscope
(C) Periscope (D) Telescope

15. There is light on the earth even on a dark, cloudy day. This is becasue clouds are
(A) Opaque (B) Transparet
(C) Manslucent (D) Luminous

ANSWER KEY

1. D 2. B 3. B 4. B 5. D 6. B 7. D 8. B 9. C 10. C

11. B 12. A 13. D 14. C 15. C

1. A fish looking up through the water sees the outside world contained in a circular horizon. If the refractive index of water is 4/3 and the fish is 12 cm below the surface, the radius of the circle is

(A) 12 × 3 × cm (B) 12 × 3 × cm (C) 12 × cm (D) 12 × cm

2. When white light enters a transparent medium such as glass

(A) all wavelength components travel with same speed.

(B) the large wavelength component travels with maximum velocity.

(D) there is no relation between wavelength and speed.

3. Total internal reflection takes place when light is incident

(A) on a concave mirror.

(B) from air on a plane glass surface at any angle.

(D) from inside a glass cube placed in water at a certain given angle.

4. When monochromatic light passes from vacuum to a material medium and vice versa; which of the following characteristics of light beam does not change?

(A) velocity (B) intensity (C) wavelength (D) frequency

5. If there were no atmosphere on earth, the duration of day light will

(A) decrease (B) increase (C) remain unchanged (D) become infinite

6. A piece of glass when immersed in a transparent solution of refractive index 3.1 becomes almost invisible. The refractive index of glass used is

(A) zero (B) 3.1 (C) 1.48 (D) infinite

7. A transparent rectangular block 5.0 cm thick is placed on a black dot. The dot when viewed from above is seen 2.0 cm from the bottom of the block. The refractive index of the material of the block is

(A) (B) (C) (D)

8. When light transmitted through one medium is incident on the surface of another medium

I. No light is reflected at the boundary.

II. Some of the light is absorbed by the second medium.

III.The speed is reduced if the second medium is rarer relative to the first.

IV. The speed is reduced if the second medium is denser relative to the first.

(A) I and II only are correct (B) II and III only are correct

9. At night a lamp is moved up and down above the surface of a pond containing water of refractive index 4/3. When the lamp is 2 m above its surface, its image by refraction appears to coincide with the bottom of the pond. The depth of the pond is

(A) 8/3 m (B) 2 m (C) 1.5 m (D) 1 m

10. A light source is placed at the bottom of a water tank 1 metre deep. It is found that a circle of illuminated surface is formed at the top of the tank. The radius of this circle is

(A) 1.0 m (B) 1.13 m (C) 1.25 m (D) 1.51 m

11. The minimum distance between an object and its real image formed by a convex lens of focal length f is :-

(A) f (B) 2f (C) 3f (D) 4f

12. The double convex shaped air bubble in water will behave as a

(A) convex lens (B) concave lens (C) cylindrical lens (D) plane mirror

13. A lens of power +2D is put in contact with a lens of power – D, the combination will serve as

(A) a converging lens of focal length 50 cm.

(B) a converging lens of focal length 100 cm.

(D) a diverging lens of focal length 100 cm.

14. An object is placed at a distance equal to 2f from a concave lens, the image formed will be at

(A) 2f (B) (C) f (D)

15. In an experiment to measure the focal length of a converging lens, object distance u and corresponding image distance v are measured. 1/u is plotted against 1/v to obtain the graph below. The focal length of the lens can be determined as

(A) the slope of the graph.

(B) the reciprocal of the slope.

(D) the reciprocal of the intercept on either axis.

16. The nature of the u-v graph for a converging lens is part of a

(A) circle (B) parabola (C) hyperbola (D) straight line

17. A convex lens of focal length 15 cm is placed in contact with a plane mirror and a candle is placed at the focal plane of the lens. The image produced will be

(A) real and 15 cm in front of the mirror. (B) virtual and 15 cm in front of the mirror.

18. A convex lens has a focal length of 10 cm. When it is immersed in water it will behave as

(A) a convex lens of 10 cm focal length.

(B) a concave lens of 10 cm focal length.

(D) a convex lens of focal length less than 10 cm.

19. A convex lens forms a virtual image of an object placed at 15 cm from the lens. The focal length of the lens can be :

(A) 2 cm (B) 8 cm (C) 15 cm (D) 18 cm

20. Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one of the lenses be 20 cm, the power of the other lens is

(A) 1.66 dioptre (B) 4.00 dioptre (C) –1.00 dioptre (D) –3.75 dioptre

21. A locality is photographed from an aeroplane flying at a height 2000 m with a camera having an objective of focal length 50 cm. The size of the film in the camera is 18 cm x 18 cm. The area of ground that can be photographed at one time will be

(A) 0.52 km2 (B) 1.0 km2

22. Astigmatism is corrected with the help of

(A) concave spherical lens (B) convex spherical lens

23. A person is said to be short-sighted if

(A) the person cannot see things clearly when they are placed near the eye.

(B) the rays from a distant object are focussed before they reach the retina.

(D) the retina does not work efficiently.

24. In case of a compound microscope

(A) the image produced by the eyepiece is real.

(B) the image produced by the objective lies inside the focal length of the eyepiece.

(D) the magnification which can be produced is unlimited.

25. Which of the following is NOT paired correctly?

(A) Solar furnace-concave mirror (B) Rear-view mirror-convex mirror

26. Which of the following statements about defects of vision is/are correct?

I. For a long-sighted person, close objects appear blurred.

II. For a short-sighted person, distant objects appear blurred.

III.Short-sight is corrected by converging lenses.

(A) I only (B) II only (C) I and II only (D) II and III only

27. In case of astronomical telescope

I. the focal length of the object lens is greater.

II. the focal of the object lens and the eye-piece coincide at the time of normal adjustment.

III.the distance between the lenses cannot be adjusted.

(A) I and II only (B) I and III only (C) II and III only (D) I only

28. A short-sighted person cannot see distinctly beyond 50 cm from his eye. The power in dioptre of spectacle lenses which will enable him to see distant objects clearly is

(A) +50 (B) –50 (C) +2 (D) –2

29. A simple telescope consisting of an objective of focal length 60 cm and a single eye lens of focal length 5 cm is focussed on a distant object in such a way that the parallel rays emerge from the eye lens. If the object subtends an angle of 2° at the objective, the angular width of the image is

(A) 50° (B) 24° (C) 10° (D) 1/6°

30. A poster has a red letter P on a white background. When it is viewed through a blue transparent screen, an observer would see a

(A) Magenta P on a blue background. (B) Magenta P on a blue-green background.

31. A red and a green pencil are taken in a room illuminated with green light. In the room.

(A) both pencils will appear dark.

(B) pencils will appear as red and green respectively.

(D) red pencil will appear red and green pencil dark.

32. The sky is blue because

(A) there is more blue light in the sunlight.

(B) of the scattering of sunlight by air molecules in the atmosphere.

(D) other colours are absorbed by heavenly bodies.

33. The sky is blue because

(A) solar radiation is predominantly blue. (B) air absorbs all light except blue light.

34. The colour of light is determined by its

(A) wavelength (B) frequency (C) velocity (D) amplitude

35. Which of the following is not common in sound and light?

(A) diffraction (B) refraction (C) polarisation (D) interference

36. Through a soap film different colours are seen by white light because of the phenomenon of

(A) interference (B) dispersion (C) diffraction (D) reflection

37. Two separate sources giving out light of same frequency do not produce interference because

(A) the amplitudes of the waves from the sources are different.

(B) the two sources are not close to each other.

(D) the phase difference between the waves produced by two sources is changing continuously.

38. When two waves superpose each other, the algebraic addition takes place in

(A) amplitude (B) intensity (C) frequency (D) wavelength

39. Two waves of same frequency but having amplitudes 4 cm and 3 cm superpose in same direction. The ratio of the maximum intensity to the minimum intensity at various places will be

(A) 4 : 3 (B) 16 : 9 (C) 7 : 1 (D) 49 : 1

40. Sunlight filtering through a tree often makes circular patches on the ground because

(A) the sun is round.

(B) of diffraction effects.

(D) the space through which light penetrates is round.

41. A red rose is viewed in yellow light. It will appear

(A) red (B) yellow (C) orange (D) black

42. A swimming pool appears to be 2 m deep. Its actual depth is (m for water = 1.33)

(A) 2.66 m (B) 2 m (C) 2.34 m (D) 2.54 m

43. Light travels with a speed of 2 × 108 ms–1 in crown glass of refractive index 1.5. The speed of light in dense flint glass having a refractive index 1.8 is

(A) 1.33 × 108 ms–1 (B) 1.67 × 108 ms–1 (C) 2.0 × 108 ms–1 (D) 3 × 108 ms–1

44. An illuminating object is placed between the plane mirrors mutually perpendicular to each other. The number of images formed is

(A) 7 (B) 3 (C) 4 (D) 6

45. The image formed by a concave mirror is virtual, erect and magnified. The position of object must be

(A) between the mirror and its focus (B) between focus and the centre of curvature

46. An object is placed at a distance of 2f from a convex lens, the image formed will be at a distance of

(A) 2f (B) 2f/3 (C) f (D) f/3

47. An object 5 cm high is held 25 cm away from a convex lens of focal length 10 cm. The size of image will be

(A) 5/3 cm (B) 10/3 cm (C) 5 cm (D) 10 cm

48. Two lenses of focal lengths +100 cm and +5 cm are used to prepare an astronomical telescope. The minimum tube length will be

(A) 95 cm (B) 100 cm (C) 105 cm (D) 500 cm

49. The rainbow observed during the rainy season is due to

(A) reflection (B) dispersion (C) interference (D) diffraction

50. Ray optics is valid, when characteristic dimensions of the obstacle should be -

(A) Of the same order as the wavelength of light (B) Much smaller than the wavelength of light

LIGHT SHADOW

INTRODUCTION

SOURCES OF LIGHT

(I) Lumininous sources :

MEDIUM OF LIGHT

(i) Transparent object :

RECTILINEAR PROPAGATION OF LIGHT

CHARACTERISTICS OF LIGHT

SOME DEFINATION RELATED TO THE LIGHT

(A) Ray of Light :

(B) Beam of Light :

(C) Parallel Beam :

(b) Essential of a shadow:

(c) Location of a shadow :

Shadow sticks :

A Shadow has two Regions :

REFLECTION OF LIGHT

(A) GENERAL DEFINITIONS ABOUT REFLECTION :

(B) Laws of Reflection :

PINHOLE CAMERA

ACTIVITY :

OBJECTIVE QUESTIONS

SUBJECTIVE QUESTIONS

ANSWER KEY

ANSWER KEY

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LIGHT SHADOW

INTRODUCTION

SOURCES OF LIGHT

(I) Lumininous sources :

MEDIUM OF LIGHT

(i) Transparent object :

RECTILINEAR PROPAGATION OF LIGHT

CHARACTERISTICS OF LIGHT

SOME DEFINATION RELATED TO THE LIGHT

(A) Ray of Light :

(B) Beam of Light :

(C) Parallel Beam :

(b) Essential of a shadow:

(c) Location of a shadow :

Shadow sticks :

A Shadow has two Regions :

REFLECTION OF LIGHT

(A) GENERAL DEFINITIONS ABOUT REFLECTION :

(B) Laws of Reflection :

PINHOLE CAMERA

ACTIVITY :

OBJECTIVE QUESTIONS

SUBJECTIVE QUESTIONS

ANSWER KEY

ANSWER KEY

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Related Blog

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