Light
| Light is a form of energy which controls the sense of vision. |
| Reflection of Light from Curved Mirrors |
| Difference between Concave and Convex Mirrors |
| Distinguish between concave and convex mirrors |
| Concave mirror is a spherical mirror whose reflecting surface is curved inwards. A Good |
| example is the driving mirror of a car. |
| Convex mirror is a spherical mirror whose reflective surface is curved outwards. A good |
| example of a convex mirror is a shaving mirror. |
| General demonstrations of convex and concave mirrors (curved mirrors: |
| The Terms Principle, Axis, Pole, Principle Focus and Radius of Curvature as |
| Applied to Curved Mirrors |
| Explain the terms principle, axis, pole, principle focus and radius of curvature as applied to |
| Terms used in studying curved mirrors |
| the centre of the sphere of which a mirror is a part of. |
| : the radius of sphere of which a mirror is a part of. |
| : the central point of the reflecting surface of spherical mirror ( curved or convex |
| the straight line joining the centre of curvature (C) and the pole (P). |
| the point o the principal axis where light rays tend to intersect. This |
| point is between centre of curvature and the pole. |
| the straight line joining the centre of curvature (C) and the pole (P). |
| the point on the principal axis wher e light rays tend to intersect. This |
| point is between centre of curvature and the pole. |
| The Images Formed by a Curved Mirror |
| Locate the images formed by a curved mirror |
| When a beam of light parallel and very close to the principal axis, CL, is reflected from a |
| concave mirror, it converges to a point, F, on the principal axis called the principal focus. |
| When a ray passes through the principal focus, F, it is reflected parallel to the principal axis. |
| When a ray passes through the centr e of curvature, C, which therefore strikes the mirror at |
| normal incidence, it is reflected back along its original path. |
| : Concave mirrors have a real focus because light passes through the focus. |
| The formation of images by concave mirror tends to change as the position of object changes. |
| Case 1: Image ( I) formed by a concave mirror when the object is beyond C. |
| Properties of images formed: |
| The image is between C and F |
| The image is smaller than the object |
| The image is inverted (upside down) |
| Case 2: The object is placed at C |
| The image has the same size as object |
| The image is inverted (upside down) |
| Case 3: The object is placed between C and F |
| Properties of image formed |
| The image is large than object |
| The image is formed beyond C |
| The image is inverted (upside down) |
| The image is large than object |
| The image is formed beyond |
| The image is inverted (upside down) |
| Case 4:The object is placed at F |
| The image is formed at infinity (x) |
| The image is formed beyond C |
| The image is large than object |
| Case 5:The object is placed between F and P. |
| Properties of image formed: |
| The image is formed behind the mirror |
| The image is large than the object |
| Formation of images in a convex mirror: |
| Obviously,there isonly one kind of image formed when an object is placed at any position. |
| Properties of image formed by convex mirror: |
| The image is smaller than object (diminished) |
| The image is formed behind the mirror. |
| An object 2cm long is erected 8cm infront of a concave mirror of radius of curvatur e 10cm. By |
| using a scale drawing, determine the position, size and nature of image formed. |
| Height of object, Ho = 2cm |
| Radius of curvature, r = 10cm |
| Height of object, Ho = 2cm |
| The Focal Length of a Concave Mirror |
| Determine practically the focal length of a concave mirror |
| Focal length ( f) is the distance between the principal focus and the pole. |
| Convex and Concave Mirrors in Daily Life |
| Use Convex and concave mirrors in daily life |
| Curved mirrors are used as: |
| Why is convex mirror used as driving mirror? |
| The convex mirror is used as driving mirror because it provides the wider field of view. |
| Why concave mirror used as shaving mirror? |
| Concave mirrors are used as shaving mirrors because they form an enlarged image when held close up. |
| The Concept of Refraction of Light |
| Explain the concept of refraction of light |
| Refraction of light refers tothe bending of light as it passes through two different medium |
| because the speed of light tends to change when travelling from one medium to another. |
| Figure showing refraction of light as it passes from air to glass. |
| The Angle of Incidence and Angle of Refraction |
| Measure the angle of incidence and angle of refraction |
| The angle of incidence (i) |
| is the angle between the incident ray of light and the normal at the |
| The angle of Refraction (r) |
| is the angle between the refracted ray and the normal at the point of |
| State the laws of refraction |
| The First Law of refraction states that "the incident ray, the refracted ray and the normal at the |
| point of incident are located in the same plane.” |
| Second Law of ref raction states that “when a light ray passes from one medium into another |
| medium, the angle of incidence (i) and corresponding angle of refraction( r) are such that the |
| ratio of sine of the angle of incidence to the sine of the angle of refraction (sini/sinr) is a constant |
| value called the ref ractive index." |
| : The Second Law of Refraction is called Snell's Law in honour of a Dutch scientist named |
| Snell (1591 – 1626) who first described it. |
| The Refraction Index of a Material |
| Determine the refraction index of a material |
| Refractive index (n) is the ratio of the sine of the angle of incidence to the sine of the angle of |
| Refractive index (n) is the ratio of the velocity of light in air to the velocity of light in glass. |
| n = Velocity of light in air (Va)/Velocity of light in glass (Vg) |
| Refractive index, n is the constant number which expresses how many times or to what extent a |
| light ray bends when passing through different medium. |
| Absolute refractive index (n |
| ) is the refractive index between vacuum or air and any other |
| The refractive indices between air and some common media is given below: |
| The refractive index for light passing from air to water is equal to 1.333 find the refractive index |
| for light travelling from water to air. |
| Required: To find refractive index from water to air |
| is the actual height measured without taking account any refraction of light |
| is the virtual height measured when viewed by observer. |
| The Concept of Critical Angle and Total Internal Reflection of Light |
| Explain the concept of critical angle and total internal reflection of light |
| Critical angleis the angle of incidence (i) for which the angle of refraction (r) is equal to 90º . It is |
| obtained when light rays moves from a dense medium to a less dense medium. |
| Total Internal Refraction |
| This occurs when a light ray from a less dense medium is reflected into the denser medium at the |
| boundary separating the two media. |
| Conditions for total internal reflection to occur include the following: |
| Light must be travelling from a more dense to less dense medium. |
| Light must incident at the boundar y at an angle gr eater than the critical angle (C). |
| These are very thin tubes of plastic or glass and because they are so thin they can bend without |
| breaking, so they can carry light around the corners. |
| Used in telecommunications to carry telephone calls over vast distance, without loss of |
| intensity and without interfer ence. |
| Used in endoscope to view inside a patient body for example inside stomach. Light is |
| carried into the stomach through a bunch of fibres and is reflected into small camera, which then |
| displays a pictur e on a screen. |
| Explain the occurrence of mirage |
| This is the phenomenon inwhich an object appears to be at an incorrect position due to the |
| bending of light rays from the object. |
| Mirages occur during hot days. |
| Refraction of Light by Rectangular Prism |
| The Passage of Light through a Triangular Prism |
| Trace the passage of light through a triangular prism |
| Deviation of light in a prism is the changing in direction of the incident ray when it enters/hits a |
| is the angle of incidence |
| s is the angle of deviation |
| The minimum angle of deviation ( qm) |
| In order to determine the minimum angle deviating (Qm) then we must set triangular Glass prism |
| The Dispersion of White Light |
| Demonstrate the dispersion of white light |
| Dispersion of light is the splitting up of light beam (white light) into its seven components of |
| Spectrum is the patch or band of colours which comprise / constitute seven component of white |
| Pure section is the patch or band of colours in which the colours are clearly separated. |
| In order to produce pure spectrum then we must use two converging lenses (convex lenses). |
| When colours of spectrum are combined, they for m white light. |
| In order to combine colours of the spectrum, weneed two triangular glass prisms and one lens. |
| the band/patch of colours which overlap and are not seen clearly. |
| a bow-shaped spectrum of seven colours of white light formed when white light |
| undergoes dispersion within the rain drops because water is denser than air, so has a large |
| A rainbow can be demonstrated as follows: |
| Spray some water into the air in a direction opposite to that of the sun. |
| Look at the water shower while you face away from the sun. You will see the colour of the |
| spectrum of white light in the falling drops of water. The spectrum so formed hasthe shape like a |
| bow. So it is called rainbow. |
| There are two main types of rainbow: |
| This is formed when light undergoes one or single total internal reflection in the water droplets. |
| In this type of the rainbow the violet colour is on the inside of the bow while the red colour is on |
| The Angles of Deviation and Minimum Deviation |
| Determine the angles of deviation and minimum deviation |
| Finding the refractive index (n) of glass by using the deviation of light in a prism |
| Refracting angle of prism is A |
| The total angle of deviation (s) is the angle between the direction orf the incident ray and the |
| Again from the Geometry Q is given by: |
| When the deviation is a minimum (Sm) the passage of light through the prism will be |
| Refractive index, n = Sin (A + Smin)/2 |
| A = Apex angle ( angle of prism) |
| Smin – The angle of minimum deviation |
| Construct a simple prism binocular |
| The Component of White Light |
| Explain the component of white light |
| There are two types of colour of light |
| Secondary colour of light |
| These are basic ( fundamental ) Colour of light to which the eye is most sensitive.Primary Colour |
| of light Include the following |
| Secondary colours of light |
| These are colour of light obtained after mixing primary colours of light |
| Colour mixing by Addition |
| This is the process of combining primary colours of light without loss any colour to form |
| secondary colours of light. |
| Primary color Secondary color |
| Recombine colours of white light |
| When all white light ( Red , Blue and Green)Combineforms WHITE LIGHT. |
| Complementary colours of light |
| : These are the colours which produce white light when |
| Red + Blue+ Green - White light |
| Blue + Yellow - White light |
| Green + Magenta - White light |
| The Appearances of Coloured Object under White Light |
| Explain the appearances of coloured object under white light |
| There are two types of coloured paints ( pigments) which Include the following |
| Primary coloured pigment (paints) |
| Secondary coloured pigment (paints) |
| Primary, Secondary and Complementary Colours of Light |
| Identify primary, secondary and complementary colours of light |
| Primary Coloured pigments |
| These are basic coloured pigments which form secondary coloured pigment when combined. |
| The primary coloured pigments include:Yellow, Cyan and Magenta |
| Secondary colour pigments |
| These are coloured pigments which are formed when two primary colours combine, whichis |
| always accompanied with the removal of other colours. |
| Difference between Additive and Subtractive Combination of Colours |
| Distinguish between additive and subtractive combination of colours |
| Colour Mixing by Substration |
| Is the process of mixing two primary coloured paints ( pigments) to f orm secondary colour |
| Magenta = ( Blue) + ( Red) |
| The colour which is common to Blue will appear while red and green disappear. |
| The colour which is common to both red will appear while blue and green will disappear. |
| The colour which is common to both green will appearwhile Blue and Red will disappear |
| Refraction of Light by Lenses |
| Difference between Convex and Concave Lenses |
| Distinguish between convex and concave lenses |
| A lens is a transparent medium bounded by two surfaces of regular shape. There are two major |
| categories of lenses which include: |
| The Terms Focal Length, Principle Focus, Principle Axis and Optical Centre |
| Explain the terms focal length, principle focus, principle axis and optical centre as applied to |
| is a geometric center of a lens. |
| is the center of the sphere in |
| is an imaginary line which passes through the optical center |
| of the lens at right angle to the lens. |
| is a point through which all rays traveling |
| close and parallel to the principal axis pass through. |
| The Focal Length of a Lens |
| Determine practically the focal length of a lens |
| Focal length is a distance between between optical centre and the principal focus. It is important |
| to note that the the principal focus is not the halfway between the optical centre and the centre of |
| curvature in lenses as it is in mirrors. The plane through the principal focus which is at right |
| angles with the principal axis is called the focal plane. |
| An object is 2 cm high and placed 24cm from a convex lens. An image formed 72 cm. find the |
| focal length of the lens. |
| The Immage Formed by a Lens |
| Locate the image formed by a lens |
| Rays diagrams are normally used toillustratesthe f ormation of images by lenses. |
| A ray parallel to the principal axis passes through or appears to diverge from the principal |
| A ray of light passing through the principal focus of a lens is refracted parallel to the |
| principal axis of the lens. |
| A ray of light through the optical center of the lens continues throughundeviated(Not |
| The position, Size and Nature of the Image formed by Lens |
| Determine the position, size and nature of the image formed by lens |
| The nature, position and size of the image formed by a lens depends on the position of the object |
| in relation to the type of lens. For example in converging lens when the object is between the |
| lens and principal focus the image will be formed at the same side as the object but further from |
| the lens. It is virtual, erect, and magnified. The image by concave lens is erect, virtual and |
| Take a convex lens. Find its approximate focal length in a way described in Activity 11. |
| Draw five parallel straight lines, using chalk, on a long Table such that the distance |
| between the successive lines is equal to the focal length of the lens. |
| Place the lens on a lens stand. Place it on the central line such that the optical centre of |
| the lens lies just over the line. |
| The two lines on either side of the lens correspond to F and 2F of the lens respectively. |
| Mark them with appropriate letters such as 2F |
| Place a burning candle, far beyond 2F |
| to the left. Obtain a clear sharp image on a screen |
| on the opposite side of the lens. |
| Note down the nature, position and relative size of the image. |
| Repeat this Activity by placing object just behind 2F |
| and O. Note down and tabulate your observations. |
| The nature, position and relative size of the image formed by convex lens for various positions of |
| the object is summarized in the table below: |
| Position of the object Position of the image Relative size of the |
| Highly diminished, point- |
| Diminished Real and inverted |
| Same size Real and inverted |
| Enlarged Real and inverted |
| Infinitely large or highly |
| enlarged Real and inverted |
| On the same side of the lens as the |
| object Enlarged Virtual and erect |
| Take a concave lens. Place it on a lens stand. |
| Place a burning candle on one side of the lens. |
| Look through the lens from the other side and observe the image. Try to get the image on |
| a screen, if possible. If not, observe the image directly through the lens. |
| Note down the nature, relative size and approximate position of the image. |
| Move the candle away from the lens. Note the change in the size of the image. What |
| happens to the size of the image when the candle is placed too far away from the lens. |
| Nature, position and relative size of the image formed by a concave lens for various positions of |
| Position of the object Position of the image Relative size of the |
| Highly diminished, point- |
| Between infinity and optical centre O |
| centre O Diminished Virtual and erect |
| The Magnification of the Lens Camera |
| Determine the magnification of the lens camera |
| As we have a formula for spherical mirrors, we also have formula for spherical lenses. This |
| formula gives the relationship between object distance (u), image-distance ( ) and the focal |
| length (f ). The lens formula is expressed as1/ - 1/u = 1/f(8) |
| The lens formula given above is general and is valid in all situations for any spherical lens. Take |
| proper care of the signs of different quantities, while putting numerical values for solving |
| problems relating to lenses. |
| The magnification produced by a lens, similar to that for spherical mirrors, is defined as the ratio |
| of the height of the image and the height of the object. It is represented by the letter m. If h is the |
| height of the object and h' is the height of the image given by a lens, then the magnification |
| produced by the lens is given by,m = Height of the Image / Height of the object = h' / h(9) |
| Magnification produced by a lens is also related to the object-distance u, and the image-distance |
| . This relationship is given byMagnification (m ) = h' / h = / u(10) |
| A concave lens has focal length of 15 cm. At what distance should the object from the lens be |
| placed so that it forms an image at 10 cm from the lens? Also, find the magnification produced |
| A concave lens always forms a virtual, erect image on the same side of the object. |
| Image-distance v = –10 cm; |
| Since, 1 /v - 1 / u = 1 / f |
| or, 1 / u = 1 / v - 1 / f |
| 1 / u = 1 / -10 - 1 / (-15) = - 1 / 10 + 1 / 15 |
| 1 / u = (-3+2) / 30 = 1 / (-30) |
| Thus, the object-distance is 30 cm. |
| m = -10 cm / -30 cm = 1 / 3 = +0.33 |
| The positive sign shows that the image is erect and virtual. The image is one-third of the size of |
| The Relationship between Focal Length (f) Object Distance (u) and Image |
| Distance (v) as Applied to Lenses |
| Determine the relationship between focal length (f) object distance (u) and image distance (v) as |
| The lens equation is given as 1/f =1/u + 1/v , if sign convection is used for u, v and f the equation |
| applies to both converging and diverging lenses for all cases of object and image. |
| An object is placed 12 cm from converging lens of focal length 18 cm. Find the position of the |
| Since the lens is converging f = +18 cm. 1/v = 1/18 -1/12, v = -36. |
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